Final Study Exam Flashcards

Question

What happens to synapses that are not used? Why is neuron death important during development?

Answer 1

As strange as it may seem, cell death is a crucial phase of brain development, especially during embryonic stages. This developmental stage is not unique to the nervous system. Naturally occurring cell death, also called apoptosis (from the Greek apo, “away from,” and ptosis, “act of falling”), is a kind of sculpting process in the emergence of other tissues in both animals and plants. Genetically interfering with neural apoptosis in fetal mice causes them to grow brains that are too large to ﬁt in the skull (Depaepe et al., 2005), so we can see how vital it is that some cells die.

Answer 2

These cells are not dying because of a defect. Rather, it appears that these cells have “decided” to die and are ac-tively committing suicide. Your chromosomes carry death genes—genes that are expressed only when a cell undergoes apoptosis As you may have guessed, apoptosis in vertebrates is regulated by cell-cell interactions, such as the availability of synaptic targets. Reducing the size of the synaptic target invariably reduces the number of surviving nerve cells. Thus neurons compete for connections to target structures (other nerve cells or end organs, such as muscle). Neurons that make adequate synapses survive and grow; those that fail to form synaptic connections die. Apparently the neurons compete not just for synaptic sites, but also for a chemical that the target structure makes and releases. Neurons that receive enough of the chemical survive; those that do not, die. Such target-derived chemicals are called neurotrophic factors (or simply trophic factors) because they act as if they “feed” the neurons to help them survive (in Greek, trophe means “nourishment”). The neurotrophic factor that was the ﬁrst to be identiﬁed prevents the death of developing sympathetic neurons

Answer 3

Epigenetics - The study of factors that affect gene expression without making any changes in the nucleotide sequence of the genes themselves. Amblyopia - Reduced visual acuity that is not caused by optical or retinal impairments. sensitive period - The period during development in which an organism can be permanently altered by a particular experi-ence or treatment.

Answer 4

neurogenesis The mitotic division of non-neuronal cells to produce neurons. The production of nerve cells is called neurogenesis. Nerve cells themselves do not divide, but the cells that will give rise to neurons begin as a single layer of cells along the inner surface of the neural tube. These cells divide (in a process called mitosis) and gradually form a closely packed layer of cells called the ventricular zone. All neurons and glial cells are derived from cells that originate from this ventricular mitosis. Eventually, some cells leave the ventricular zone and begin transforming into either neurons or glial cells. adult neurogenesis At birth, mammals have already produced most of the neurons they will ever have. The postnatal increase of human brain weight is primarily due to growth in the size of neurons, branching of dendrites, elaboration of synapses (as we’ll see in Figure 7.6), increase in myelin, and addition of glial cells. But early reports that new neurons are added just after birth in some brain regions (Altman, 1969) have been supplemented with ﬁndings of adult neurogenesis, the generation of new neurons in adulthood, While the new neurons acquired in adulthood represent a tiny minority of neurons, there’s reason to think they are important. Enriched experience, such as learning, increases the rate of neurogenesis in adult mammals Synapse rearrangement Synapse rearrangement, or synaptic remodeling, refines synaptic connections. One influence on synaptic survival is neural activity. A neurotrophic factor may contribute.

Answer 5

Autism is a developmental disorder characterized by impaired social interactions and language and a narrow range of interests and activities. The disorder is much more common in males than females. Usually autism is discovered when apparently normal toddlers begin regressing, losing language skills, and withdrawing from family interaction. Children with autism tend to perseverate (such as by continually nodding the head or making stereotyped ﬁnger movements), actively avoid making eye contact with other people, and have a difﬁcult time judging other people’s thoughts or feelings (Senju et al., 2009). When shown photos of the faces of family members, autistic individuals display a pattern of brain activation that is different from that exhibited by controls (Pierce et al., 2001), suggesting a different brain organization for the fundamental social skill of recognizing others. Several structural differences between the brains of people with autism and controls have been reported, including a reduction in the size of the corpus callosum and certain cerebellar regions (Egaas et al., 1995). The amygdala has been associated with fear (see Chapter 15), so this ﬁnding suggests that children with autism avoid making eye contact with people because they ﬁnd it aversive. The underlying problem with autism may be an inability to empathize with others, as reﬂected in the difﬁculty that individuals with autism display in making “copycat” movements of the ﬁngers or body. When people with autism do this task, a particular part of the frontal cortex is less activated than in control subjects (Villalobos et al., 2005). The same region is also underactivated when people with autism try to mimic emotional facial expressions of others

Answer 6

A common form of intellectual disability resulting from a chromosomal abnormality is Down syndrome (FIGURE 7.20A). People with Down syndrome usually have an extra chromosome 21, for a total of three rather than the typical two copies.

Answer 7

Williams syndrome A disorder characterized by ﬂuent linguistic function but poor performance on standard IQ tests and great difﬁculty with spatial processing. Individuals with Williams syndrome speak freely and ﬂuently with a large vocabulary, yet they may be unable to draw simple images, arrange colored blocks to match an example, or tie shoelaces. The individuals are very sociable, ready to strike up conversation and smile. They may also display strong musical talent, either singing or playing an instrument. The syndrome results from the deletion of about 28 genes from one of the two chromosomes numbered leads to pixielike facial features. Several of the other missing genes are thought to lead to changes in brain development and to the behavioral features of the syndrome. Difficulty with spatial memory and drawing objects

Answer 8

Probably the most frequent cause of inherited intellectual disability is the condition fragile X syndrome (FIGURE 7.20B), which is more common in males than in females. At the end of the long arm of the X chromosome is a site that seems fragile—prone to breaking because the DNA there is unstable (Yu et al., 1991). People with this abnormality have a modiﬁed facial appearance, including elongation of the face, large prominent ears, and a prominent chin. A wide range of cognitive effects—from mild to severe impairment—are associated with the syndrome (Baumgardner et al., 1994). Cortical neurons from the brains of people with fragile X syndrome, as well as mice genetically engineered to have this syndrome, possess an excess of small, immature dendritic spines (Bagni and Greenough, 2005). These ﬁndings suggest that the syndrome affects mental development by blocking the normal elimination of synapses after birth (see Figure 7.10).

Answer 9

alcohol syndrome (FAS) (Abel, 1984). Prominent anatomical effects of fetal exposure to alcohol include distinctive changes in facial features (e.g., a sunken nasal bridge and altered shape of the nose and eyelids) and stunted growth. In some cases, the children may lack a corpus callosum (FIGURE 7.17). Few FAS children catch up in the years following birth. The most common problem associated with FAS is intellectual disability, which varies in severity. No alcohol threshold has yet been established for this syndrome, but it can occur with relatively moderate intake during pregnancy. Even when FAS is not diagnosed, prenatal exposure to alcohol is correlated with impairments in language and ﬁne motor skills The brain of an infant of the same age with FAS. This brain shows microcephaly (abnormal smallness), fewer cerebral cortical gyri, and the absence of a corpus callosum connecting the two hemispheres.

Answer 10

When control children and children with autism are asked to imitate the emotional facial expressions displayed in photographs of other people, many brain regions show activation in both groups. But the indicated region, the pars opercularis region, is less activated in children with autism than in controls. This is the region that contains “mirror neurons” (see Chapters 10 and 11). Perhaps deﬁcits in activating brain regions underlying imitation and empathy contribute to the social impairments of autism. (After Dapretto et al., 2006; courtesy of Mirella Dapretto.)

Answer 11

Scientests were looking for stem cells that could groe into nerve or brain cells They cave the mice a gene that would make them glow They expected the brain area to glow but the whole body glowed because they where in all the hair follucals Means there is a close relationship between hair and brain cells Hair folucal stem cells could help nerve cells regrow So easy to obtain

Answer 12

rare inflammatory neurological disease, characterized by frequent and severe seizures, loss of motor skills and speech, hemiparesis (paralysis on one side of the body), encephalitis (inflammation of the brain), and dementia. The illness affects a single cerebral hemisphere and generally occurs in children under the age of 15.

Answer 13

Synapse rearrangement, or synaptic remodeling, refines synaptic connections. One influence on synaptic survival is neural activity. A neurotrophic factor may contribute.

Answer 14

studies pathological effects of early exposure to toxic substances

Answer 15

Site-directed mutagenesis changes the sequence of a nucleotide in a gene. Knockout organism has a gene disabled. Transgenic has a new or altered gene

Answer 16

is a drastic failure of cognitive ability.

Answer 17

is a form of senile dementia. It begins as memory loss of recent events–brains show reduced metabolism and cortical atrophy Alzheimer’s produces cellular changes: Senile plaques form by β-amyloid buildup, also called amyloid plaques. Neurofibrillary tangles, including the tau protein, occur. Basal forebrain nuclei disappear.

Answer 18

Mechanical Touch - Contact with or deformation of body surface Pain - Tissue damage Hearing - Sound vibrations in air or water Vestibular - Head movement and orientation Joint - Position and movement Muscle - Tension Visual Seeing - Visible radiant energy Thermal Cold - Decrease in skin temperature Warmth - Increase in skin temperature Chemical Smell - Odorous substances dissolved in air or water in the nasal cavity Taste - Substances in contact with the tongue or Common chemical - Changes in CO2, pH, osmotic pressure Vomeronasal - Pheromones in air or water Electrical Electroreception - Differences in density of electrical currents

Answer 19

Today we know that the messages for the different senses—such as seeing, hearing, touching, sensing pain, and sensing temperature—all use the same type of “energy”: action potentials. But the brain recognizes the different kinds of sensation (modalities) as separate and distinct because each modality sends its action potentials along separate nerve tracts. This is the concept of labeled lines: particular neurons are, at the outset, labeled for distinctive sensory experiences. Neural activity in one line signals a sound, activity in another line signals a smell, and activity in other lines signals touch. We can even distinguish different types of touch because some lines signal light touch, others signal vibration, and yet other lines signal stretching of the skin. You can demonstrate this effect right now. If you take your ﬁnger and gently press on your eyelid, you’ll see a dark blob appear on the edge of your ﬁeld of view (it helps to look at a blank white wall). Of course, your skin also feels the touch of your ﬁnger, but why do you see a blob with your eye? The energy you applied, pressure, affected action potentials coming from your eye. Because your brain labels that line as always carrying visual information, what you experienced was a change in vision.

Answer 20

receptor potential Also called generator potential. A local change in the resting potential of a receptor cell that mediates between the impact of stimuli and the initiation of nerve impulses. The structure of a receptor determines the forms of energy to which it will respond. The steps between the arrival of energy at a receptor cell and the initiation of action potentials in a nerve ﬁber involve local changes of membrane potential called receptor potentials (or generator potentials). In most instances, the receptor potential resembles the excitatory postsynaptic potentials Pacinian corpuscle Also called lamellated corpuscle. A skin receptor cell type that detects vibration. One example of the generator potential can be studied in a receptor called the Pacinian corpuscle (or lamellated corpuscle; Loewenstein, 1971). This receptor, which detects vibration, is found throughout the body in skin and muscle.

Answer 21

Pacinian corpuscles–vibration, fast-adapting Meissner’s corpuscles– touch, fast-adapting Merkel’s discs–touch, slow-adapting Ruffini’s endings–stretch, slow-adapting

Answer 22

adaptation The progressive loss of receptor sensitivity as stimulation is maintained. adequate stimulus The frequency of action potentials progressively declines, even though the stimulus is continued (FIGURE 8.7). In terms of adaptation, there are two kinds of receptors: Tonic receptors show little or no decrease in the frequency of ac- tion potentials as stimulation is maintained; in other words, these receptors show relatively little adaptation. Phasic receptors display adaptation, rapidly decreasing the frequency of action potentials when the stimulus is maintained. Adaptation means that there is a progressive shift in neural activity away from accurate portrayal of maintained physical events. Thus, the sensory system may fail to register neural activity even though the stimulus continues. Such a striking discrepancy is no accident; sensory systems emphasize change in stimuli because changes are more likely to be signiﬁcant for sur- vival. Sensory adaptation prevents the nervous system from becoming overwhelmed by stimuli that offer very little “news” about the world.

Answer 23

Information enters the CNS through the brainstem or spinal cord and then reaches the thalamus. The thalamus shares the information with the cerebral cortex; the cortex directs the thalamus to suppress some sensations. Primary sensory cortex swaps information with nonprimary sensory cortex. This organization is present in all sensory systems except smell, which bypasses the thalamus, going directly to cortex

Answer 24

he homunculus (literally, “little man”) depicts the body surface with each area drawn in proportion to the size of its representation in the primary somatosensory cortex.

Answer 25

polymodal Involving several sensory modalities. Often the use of one sensory system inﬂuences perception derived from another sen- sory system neither sense alone may be sufﬁcient to elicit a response (B. Stein and Meredith, 1993). Similarly, humans detect a visual signal more accurately if it is accompanied by a sound from the same part of space (McDonald et al., 2000). Many sensory areas in the brain—so-called associa- tion areas—do not represent exclusively a single modality but show a mixture of inputs from different modalities. Perhaps loss of input from one modal- ity allows these cells to analyze input from the remaining senses better, as happens, for example, in cases of people who become blind early in life and are better than sight- ed people at localizing auditory stimuli (Gougoux et al., 2005). The normal stimulus convergence on such polymo- dal cells provides a mechanism for intersensory interac- tions (B. E. Stein and Stanford, 2008). For a few people, a stimulus in one modality may evoke an additional per- ception in another modality,

Answer 26

Association cortex is the cerebral cortex outside the primary areas (Figure 1). It is essential for mental functions that are more complex than detecting basic dimensions of sensory stimulation, for which primary sensory areas appear to be necessary. In humans the association areas are by far the most developed part of the cerebral cortex, and the brain in general. These areas are necessary for perceptual activities, like recognizing objects (toasters, horses, trees, words, etc), rather than simple contours, edges or sensory qualities like color or pitch. Sensory association areas combine this kind of information to represent complex objects. For example, the visual association area on the lower part of the temporal lobe plays a primary role in your ability to recognize faces, dogs, cars, trees, etc., whereas the primary visual cortex is required for detecting basic features of the visual world: edges, light and dark, location, etc. the auditory association cortex (around the primary auditory cortex on the top of the temporal lobe) the somatosensory association cortex (on the parietal lobe behind the primary somatosensory cortex)

Answer 27

The question of how the brain understands which individual attributes blend together into a single object, when these different features are processed by dif- ferent regions in the brain.

Answer 28

The Dorsal Column System Carries Somatosensory Information from the Skin to the Brain The touch receptors that we have described (Pacinian corpuscles, Merkel’s discs, Meissner’s corpuscles, and Rufﬁni’s endings) send their axons to the spinal cord, where they enter the dorsal horn and turn upward, traveling to the brain along the spinal cord’s dorsal column of white matter, which is why this is called the dorsal column system. These axons go all the way up to the brainstem, where they synapse on neurons of the dorsal column nuclei in the medulla (FIGURE 8.15). The axons of these medullary neurons then cross the midline to the opposite side and ascend to a group of nuclei of the thalamus. Outputs of the thalamus are directed to primary somatosensory cortex (S1).

Answer 29

The skin surface can be divided into bands corresponding to the spinal nerves that carry the axons from the different regions (FIGURE 8.16A). A dermatome (from the Greek derma, “skin,” and tome, “part” or “segment”) is the region of skin innervated by a particular spinal nerve. The pattern of dermatomes is hard to understand in an upright human, but remember that our erect posture is a recent evolutionary development. The dermatomes overlap a modest amount

Answer 30

Now, however, we know that cortical maps can change with experience (Merzenich and Jenkins, 1993).In one experiment, the receptive ﬁeld of a monkey’s hand was mapped in detail in the somatosensory cortex (FIGURE 8.17A). Then the middle ﬁnger was surgically removed. The region of cortex that responded to each remaining, adjacent ﬁnger expanded, so the region that had formerly responded to the removed ﬁnger now responded to the neighboring ﬁngers (FIGURE 8.17B). In another experiment, a monkey was trained to rest two ﬁngers on a rotating disk in order to obtain food rewards. After several weeks of training, the hand area was mapped again, and the stimulated ﬁngers were found to have considerably enlarged representations compared with their previous areas (FIGURE 8.17C).Similar ﬁndings were noted in rats exposed to differential tactile experiences (Xerri et al., 1996). Professional musicians who play stringed instruments have expanded cortical representations of their left ﬁngers,Brain imaging also reveals cortical reorganization in people who lose a hand in adulthood

Answer 31

1 Damaged cells release substances that excite free nerve endings that function as nociceptors. 2 Action potentials generated in the periphery can reflexively excite blood vessels and other cells to produce inflammation. 3 Information enters through the dorsal root and synapses on neurons in the dorsal horn. 4 Pain fibers release glutamate as a transmitter and substance P as a neuromodulator in the spinal cord. The dorsal horn cells then send information across the midline and up to the thalamus.

Answer 32

Pain ascends the spinothalamic system to reach the brain Sensation of pain travels from its origin to the brain via the spinothalamic system, crossing the midline in the spinal cord. anterolateral system or spinothalamic system A somatosensory system that carries most of the pa In the central nervous system, special pathways mediate pain and temperature information. Earlier we discussed the dorsal column system that carries touch information to the brain (see Figure 8.15). The sensations of pain and temperature are transmitted separately by the anterolateral, or spinothalamic, system. Morphine provides analgesia by stimulating the opioid receptors in this descending pain control system, in both the brainstem

Answer 33

A potent antagonist of opiates that is often administered to people who have taken drug overdoses. It blocks receptors for endogenous opioids.

Answer 34

Congenital insensitivity to pain–an inherited syndrome Pain helps us to withdraw from its source, engage in recuperative actions, and to signal others. Such people show extensive scarring from injuries to ﬁngers, hands, and legs (Manfredi et al., 1981), and they tend to die young of injuries, suggesting that pain guides adaptive behavior by signaling harm to our bodies.

Answer 35

cingulate cortex - Also called cingulum. A region of medial cerebral cortex that lies dorsal to the corpus callosum.

Answer 36

“acute“ - meaning new or sudden. Acute pain often originates from nociception - the nervous systems way of detecting, transmitting and processing potential damage. This potential damage is detected by nociceptors - a kind of nerve ending (sensory receptor) found all over your body. 2 Once nociceptors are activated, they send signals through peripheral nerves to the spinal cord, which then travel to the brain. 3 The signals are processed at each stage of transmission, with the brain arguably playing the largest role in how pain is consciously experienced. 4

Answer 37

``` PSYCHOGENIC Cognitive (learning, coping strategies) May activate endorphin-mediated pain control system. Limited usefulness for severe pain ``` PHARMACOLOGICAL Opiates Bind to opioid receptors in periaqueductal gray and spinal cord. Severe side effects due to binding in other brain regions. STIMULATION TENS/mechanical. Tactile or electrical stimulation of large fibers blocks or alters pain signal to brain. Segmental control; must be applied at site of pain ``` SURGICAL Rhizotomy (cutting dorsal root) Create physical break in pain pathway. Considerable risk of failure or return of pain ```

Answer 38

external ear The part of the ear that we readily see (the pinna) and the canal that leads to the eardrum. The shape of the external ear is also important in identifying the direction and distance of the source of a sound. pinna The external part of the ear. The pinnae funnel sound waves into the second part of the external ear: the ear canal. The “hills and valleys” of the pinna modify the character of sound that reaches the middle ear. Some frequencies of sound are enhanced; others are suppressed. ear canal A tube leading from the pinna to the middle ear.

Answer 39

middle ear The cavity between the tympanic membrane and the cochlea. tympanic membrane Also called eardrum. The partition between the external ear and the middle ear. ossicles Three small bones (incus, malleus, and stapes) that transmit sound across the middle ear, from the tympanic membrane to the oval window. oval window The opening from the middle ear to the inner ear. tensor tympani The muscle attached to the malleus that modulates mechanical linkage to protect the delicate receptor cells of the inner ear from damaging sounds. Stapedius A middle-ear muscle that is attached to the stapes.

Answer 40

inner ear The cochlea and vestibular apparatus. The complex structures of the inner ear ultimately convert sound into neural activity. In mammals the auditory portion of the inner ear is a coiled, ﬂuid-ﬁlled structure called the cochlea. cochlea A snail-shaped structure in the inner ear that contains the primary receptor cells for hearing. The cochlea is a coil of three parallel canals: (1) the scala vestibuli (vestibular canal), (2) the scala media (middle canal), and (3) the scala tympani (tympanic canal). The width of these canals decreases along the length of the spiral. round window A membrane separating the cochlear duct from the middle-ear cavity. Because these canals are ﬁlled with non-compressible ﬂuid, movement inside the cochlea in response to a push on the oval window requires a second membrane-covered window that can bulge outward a bit. This membrane is the round window, which separates the scala tympani from the middle ear. organ of Corti A structure in the inner ear that lies on the basilar membrane of the cochlea and contains the hair cells and terminations of the auditory nerve. hair cell One of the receptor cells for hearing in the cochlea. basilar membrane A membrane in the cochlea that contains the principal structures involved in auditory transduction. inner hair cell (IHC) One of the two types of receptor cells for hearing in the cochlea. outer hair cell (OHC) One of the two types of receptor cells for hearing in the cochlea. stereocilium A relatively stiff hair that protrudes from a hair cell in the auditory or vestibular system. tectorial membrane A membrane that sits atop the organ of Corti in the cochlear duct. tip link A ﬁne, threadlike ﬁber that runs along and connects the tips of stereocilia.

Answer 41

We perceive a repetitive pattern of local increases and decreases in air pressure as sound. Usually this oscillation is caused by a vibrating object, such as a loudspeaker or a person’s larynx during speaking. A single alternation of compression and expansion of air is called one cycle. Figure A illustrates the changes in pressure produced by a vibrating loudspeaker while it produces a single frequency of vibration. Such a sound, called a pure tone, is represented by a sine wave that plots the oscillation between the compressions (peaks) and expansions (troughs) of air in front of the vibrating cone of the speaker. A pure tone is described physically in terms of two measures: Amplitude, Frequency. Most sounds are more complicated than a pure tone. For example, a sound made by a musical instrument contains a fundamental frequency plus one or more integer multiples of the fundamental, called harmonics. So, middle A on the piano has a fundamental frequency of 440 Hz plus some combination of harmonics at 880, 1320, 1760, and so on. When different instruments play the same note, the notes differ in the relative intensities of the various harmonics, along with subtle qualitative differences between instruments in the way they commence, shape, and sustain the sound; these differences are what gives each instrument its characteristic sound quality, or timbre. Any complex sound can be decomposed into a sum of simple sine waves through a mathematical process called Fourier analysis. Because the ear is sensitive to a huge range of sound pressures, sound intensity (a measure of the difference between two pressures) is usually expressed in decibels (dB), a logarithmic scale. The common reference level for human hearing is 0.0002 dyn/cm2, the smallest amplitude at which an average human ear can detect a 1000-Hz tone. A faint whisper is about ten times as intense, and a jet airliner 500 feet overhead is about a million times as intense. The whisper is about 20 dB above threshold, and the jetliner is about 120 dB above threshold. Normal conversation is about 60 dB above the reference level.

Answer 42

Your auditory system detects rapid changes of sound intensity (measured in decibels, dB) and frequency (measured in cycles per second, or hertz, Hz). Your ears are so sensitive to sounds in the middle of the hearing range that one of the main jobs of your powers of attention is to ﬁlter out the constant barrage of unimportant little noises that your ears detect (see Chapter 18) Amplitude, or intensity—usually measured as sound pressure, or force per unit area, in dynes per square centimeter (dyn/cm2). Our perception of amplitude is termed loudness. Frequency, or the number of cycles per second, measured in hertz (Hz). For example, middle A on a piano has a frequency of 440 Hz. Our perception of frequency is termed pitch.

Answer 43

Sound waves in the air strike the tympanic membrane and cause it to vibrate with the same frequency as the sound. The movement of the tympanic membrane moves the chain of ossicles, concentrating the tiny mechanical forces of vibrating air particles, captured from the relatively large tympanic membrane, onto the small oval window. This arrangement vastly ampliﬁes sound pressure so that it can produce movement in the ﬂuid of the inner ear. Two muscles vary the mechanical linkage between the ossicles to improve auditory perception and protect the delicate inner ear from loud, potentially damaging sounds. The middle-ear muscles also attenuate self-made sounds; without this system, body movement, swallowing, vocalizations, and other internally produced sounds would be distractingly loud.

Answer 44

The principal components that convert sounds into neural activity, collectively known as the organ of Corti (see Figure 9.2D), consist of three main structures: (1) the sensory cells (hair cells) (FIGURE 9.2E), (2) an elaborate framework of supporting cells, and (3) the terminations of the auditory ﬁbers. The base of the organ of Corti is the basilar membrane. This ﬂexible membrane separates the scala tympani from the scala media and, most important, vibrates in response to sound. The basilar membrane is about ﬁve times wider at the apex of the cochlea than at the base, even though the cochlea itself narrows toward its apex. When the stapes moves in and out as a result of sound waves hitting the eardum, it sets up waves or ripples in the ﬂuid of the scala vestibuli, which in turn cause the basilar membrane to ripple, like shaking out a rug. Because the basilar membrane is tapered—it’s about 5 times wider at the apex than at the base—different parts of the basilar membrane show their strongest responses to different frequencies of sound (FIGURE 9.3). High frequencies have their greatest effect near the base, where the membrane is narrow, whereas low-frequency sounds produce a larger response near the apex, where the membrane is wider (Ashmore, 1994). The hair cells transduce movements of the basilar membrane into electrical signals.

Answer 45

hair cell One of the receptor cells for hearing in the cochlea. The hair cells transduce movements of the basilar membrane into electrical signals. Each human ear contains two sets of hair cells within the organ of Corti: a single row of about 3500 inner hair cells (IHCs; called inner because they are closer to the central axis of the coiled cochlea) and about 12,000 outer hair cells (OHCs) in three rows (see Figure 9.2D). From the upper end of each hair cell protrude tiny hairs that range from 2 to 6 micrometers in length (see Figure 9.2E). Each hair cell has 50–200 of these relatively stiff hairs, called stereocilia (singular stereocilium; from the Greek stereos, “solid,” and the Latin cilium, “eyelid/eyelash”) or simply cilia. The heights of the stereocilia increase progressively across the hair cell, so the tops form a slope. Atop the organ of Corti is the tectorial membrane (see Figure 9.2D). The stereocilia of the OHCs extend into indentations in the bottom of this membrane. Each IHC is associated with 16–20 auditory nerve ﬁbers; relatively few nerve ﬁbers contact the many OHCs. The OHCs don’t detect sound; they push on the tectorial membrane in response to commands from the brain via the efferent nerve ﬁbers. The OHCs change their length, thereby ﬁne-tuning the organ of Corti, inner hair cell (IHC) One of the two types of receptor cells for hearing in the cochlea. outer hair cell (OHC) One of the two types of receptor cells for hearing in the cochlea. stereocilium A relatively stiff hair that protrudes from a hair cell in the auditory or vestibular system. tectorial membrane A membrane that sits atop the organ of Corti in the cochlear duct.

Answer 46

The inner parts of the ear, is where the mechanical force of sound is transduced into neural activity. Auditory nerve ﬁbers contact the base of the hair cells. The organ of Corti has four kinds of synapses and nerve ﬁbers. Two of these are afferents that convey messages from the hair cells to the brain; the other two are efferents that convey messages from the brain to the hair cells. Different synaptic transmitters are active at each type of synapse. How do IHCs transduce sound into neural activity? As sounds induce vibrations of the basilar membrane, the vibrations bend the hair cell stereocilia that are inserted into the tectorial membrane (see Figure 9.2D). Very small displacements of hair bundles cause rapid changes in ionic channels of the stereocilia. Fine, threadlike ﬁbers called tip links run along the tips of the stereocilia. These tip links play a key role in the generation of hair cell potentials. Sounds that cause the stereocilia to sway, even only very slightly, increase the tension on the elastic tip links and pop open the ion channels to which they are attached (Hudspeth, 1997; Hudspeth et al., 2000) (FIGURE 9.5). The channels snap shut again in a fraction of a millisecond as the hair cell sways back. The ion channels of stereocilia thus resemble trapdoors or portholes and appear to consist of the channel protein TRPA1 (transient receptor potential type A1) (Corey et al., 2004) that contains a spring like component—so in a real sense, a stereocilium ion channel is spring-loaded with a hair trigger. Opening of the channels allows an inrush of potassium (K+) and calcium (Ca2+) ions and rapid depolarization of the entire hair cell. This initial depolarization leads to a rapid inﬂux of Ca2+ at the base of the IHC, which causes synaptic vesicles there to fuse with the presynaptic membrane and release their transmitter contents—probably glutamate—from the base of the hair cell, which stimulates the afferent nerve ﬁber to trigger action potentials in these afferent axons (see Figure 9.5).

Answer 47

The action potentials reach the brain via the vestibulocochlear nerve. On each side of your head, about 30,000–50,000 auditory ﬁbers from the cochlea make up the auditory part of the vestibulocochlear nerve (cranial nerve VIII). Recall that most of these afferent ﬁbers are carrying messages from the IHCs, each of which stimulates several nerve ﬁbers. Input from the auditory nerve is distributed in a complex manner to both sides of the brain, as depicted in FIGURE 9.7. Each auditory nerve ﬁber divides into two main branches as it enters the brainstem. Each branch then goes to separate groups of cells in the dorsal and ventral cochlear nuclei. The output of the cochlear nuclei also travels via multiple paths. One path from each cochlear nucleus goes to both superior olivary nuclei, so they both receive inputs from both right and left cochlear nuclei. This bilateral input is the ﬁrst stage in the CNS at which binaural (two-ear) effects are processed; as you might expect, this mechanism plays a key role in localizing sounds by comparing the two ears. Several other parallel paths converge on the inferior colliculi, which are the primary auditory centers of the midbrain. Outputs of the inferior colliculi go to the medial geniculate nuclei of the thalamus. At least two different pathways from the medial geniculate extend to several auditory cortical areas. Throughout the auditory pathways, neuronal response is frequency-sensitive, as with the vestibulocochlear nerve ﬁbers that we discussed earlier (see Figure 9.6). This ability to discriminate frequencies is even sharper at higher stations of the auditory nervous system. At the medial geniculate nucleus and the auditory cortex, not only are neurons excited by certain frequencies, but they are also inhibited by neighboring frequencies. This interplay of excitation and inhibition further sharpens the frequency responses, allowing us to discriminate very small frequency differences.

Answer 48

tonotopic organization A major organizational feature in auditory systems in which neurons are arranged as an orderly map of stimulus frequency, with cells responsive to high frequencies located at a distance from those responsive to low frequencies All levels of the auditory pathway display tonotopic organization; that is, they are spatially arranged in an orderly map according to the auditory frequencies to which they respond. This organization begins with the cochlea—remember, frequency is ordered along the length of the basilar membrane (see Figure 9.3), and because the IHC ﬁbers exiting the cochlea are organized according to their points of origin along the basilar membrane, a cross-section of the auditory projection reveals an orderly map from low to high frequencies.

Answer 49

deafness Hearing loss so profound that speech perception is lost. conduction deafness A hearing impairment that is associated with pathology of the external-ear or middle-ear cavities. Arises when disorders of the outer or middle ear prevent vibrations produced by auditory stimuli from reaching the cochlea. Surgery to free up the ossicular chain is helpful in some cases. Hearing aids, which are simply miniaturized audio ampliﬁers—a microphone picks up environmental sounds, ampliﬁes them, and plays the ampliﬁed sounds directly into the ear canal. By providing louder than usual sounds, hearing aids effectively overcome the mechanical problems inherent in conduction deafness. sensorineural deafness A hearing impairment that originates from cochlear or auditory nerve lesions. a condition in which auditory nerve ﬁbers are unable to become excited in a normal manner. This hearing disorder is usually permanent. Unfortunately, amplifying sounds can’t help as effectively in sensorineural hearing loss, where the hair cells or other neural components of the inner ear are damaged or missing. But while the hair cells may be completely destroyed, the electrical excitability of the auditory nerve often remains unchanged. So one approach for circumventing deafness due to hair cell loss involves directly stimulating the auditory nerve with electrical currents. Progress in the development of cochlear implants that deliver such electrical stimulation has been rapid (FIGURE 9.17). Cochlear implants provide only a limited range of frequencies and loudness, so they do not restore normal hearing, but they nonetheless provide stimulation that the brain can interpret as sound. central deafness A hearing impairment that is related to lesions in auditory pathways or centers, including sites in the brainstem, thalamus, or cortex

Answer 50

The auditory system accomplishes this feat by analyzing two kinds of binaural (two-ear) cues that signal the location of a sound source in the horizontal plane (this is called azimuth): intensity differences Perceived differences in loudness between the two ears, which can be used to localize a sound source. intensity differences occur because one ear is pointed more directly toward the sound source or because the head casts what is called a sound shadow (FIGURE 9.10A), blocking sounds located to one side (called off-axis sounds) from reaching both ears with equal loudness. The head shadow effect is most pronounced for higher-frequency sounds. Low-frequency sounds have longer sound waves that reach around the head more effectively, reducing the head shadow latency differences Differences between the two ears in the time of arrival of a sound, which can be employed by the nervous system to localize sound sources. They arise because one ear is always a little closer to an off-axis sound than is the other ear. Two kinds of latency differences are present in a sound: onset disparity, which is the difference between the two ears in hearing the beginning of the sound, and ongoing phase disparity, which is the continuous mismatch between the two ears in the arrival of all the peaks and troughs that make up the sound wave The structure of the external ear provides yet another sort of localization cue. As we mentioned earlier, the hills and valleys of the external ear selectively reinforce some frequencies in a complex sound and diminish others (FIGURE 9.12). This process is known as spectral ﬁltering, and the frequencies that are affected depend on where the sound originates. Unlike the binaural intensity and latency cues that localize a sound in azimuth (horizontal location), spectral cues provide critical information about elevation (vertical location). What brain systems analyze binaural cues? Both birds and mammals have highly specialized brainstem mechanisms that receive information from the two ears, and they use arrays of bipolar neurons to derive sound location from the left and right auditory signals. These bipolar neurons are capable of making very precise timing calculations by comparing the inputs to their two dendrites. The superior olivary nucleus is the primary sound localization nucleus in the mammalian brain, and its two main divisions serve different functions. The lateral superior olive (LSO) processes intensity differences. The medial superior olive (MSO) processes latency differences. Instead, sound location is encoded by the relative activity of the entire left MSO compared with the entire right MSO (Grothe, 2003; McAlpine et al., 2001). So, for example, a sound on the midline would activate the left and right MSOs equally, and the two signals would effectively cancel each other out. But a sound on the right would produce more excitation of the left MSO than the right MSO, and the converse would be true for sounds on the left. The bigger the difference between the left and right MSO is, the farther the sound source is from the midline. This disparity is passed along for further processing at other levels of the auditory system.

Answer 51

Auditory cortex analyzes complex sounds encountered in everyday life. There seem to be two main streams of auditory processing in cortex (Kaas and Hackett, 1999): a dorsal stream, involving the parietal lobe, is concerned with spatial location of sounds; a ventral stream through the temporal lobe may analyze the various components of sounds (Romanski et al., 1999), which in the human left hemisphere might include processing of speech sounds. This organization suggests the existence of separate auditory processing streams for what versus where, neatly paralleling a similar scheme that has been proposed for visual processing.

Answer 52

Human infants have diverse hearing capabilities at birth, but hearing for complex speech sounds in particular becomes more precise and rapid through exposure to the speech of their family and other people. Similarly, early experience with binaural hearing (via hearing aids), compared with equivalent monoaural hearing or no hearing, has a signiﬁcant effect on the ability of children to localize sound sources later in life. Music is also effective in shaping the responses of auditory cortex. It might not surprise you to learn that the auditory cortex of trained musicians shows a bigger response to musical sounds than does the same cortex in nonmusicians. t the extent to which a musician’s brain is extra sensitive to musical notes is correlated with the age at which she began her serious training in music. Certainly there are big differences by adulthood: the portion of primary auditory cortex where music is ﬁrst processed, called Heschl’s gyrus, is more than twice as large in professional musicians as in nonmusicians, and more than twice as strongly activated by music

Answer 53

cochlear implant An electromechanical device that detects sounds and selectively stimulates nerves in different regions of the cochlea via surgically implanted electrodes. But while the hair cells may be completely destroyed, the electrical excitability of the auditory nerve often remains unchanged. So one approach for circumventing deafness due to hair cell loss involves directly stimulating the auditory nerve with electrical currents. Progress in the development of cochlear implants that deliver such electrical stimulation has been rapid (FIGURE 9.17). Cochlear implants provide only a limited range of frequencies and loudness, so they do not restore nor-mal hearing, but they nonetheless provide stimulation that the brain can interpret as sound. Functional brain imaging indicates that the stimulation activates auditory cortex in a tonotopic manner, and it appears to be processed by auditory cortex in similar fashion to “natural” inputs

Answer 54

In humans, a sheet of cells called the olfactory epithelium (FIGURE 9.25) lines the dorsal portion of the nasal cavities and adjacent regions, including the septum that separates the left and right nasal cavities. Within the 5–10 cm2 of olfactory epithelium that we possess, three types of cells are found: supporting cells, basal cells, and about 10 million olfactory receptor cells. In many other mammals this number is an order of magnitude greater. Each olfactory receptor cell has a long, slender apical dendrite that extends to the outermost layer of the epithelium, the mucosal surface. There, numerous cilia (singular cilium) emerge from the dendritic knob and extend along the mucosal surface. At the opposite end of each bipolar olfactory receptor cell, a ﬁne, unmyelinated axon, which is among the smallest-diameter axons in the nervous system, runs to the olfactory bulb. olfactory receptor neurons can be replaced in adulthood.If the olfactory epithelium is damaged, it can be regenerated and will properly reconnect to the olfactory bulb.

Answer 55

The numerous axons of the olfactory nerve terminate in a complex structure at the anterior end of the brain called the olfactory bulb. The olfactory bulb, in relation to the rest of the brain, is much smaller in humans than in animals. The olfactory bulb is organized into many roughly spherical neural circuits called glomeruli, within which the axon terminals of olfac- tory neurons synapse on the dendrites of the specialized mitral cells of the olfactory bulb.Output from the olfactory bulb consists of the axons from mitral cells, which extend to a variety of brain regions. These include the prepyriform and entorhinal cortex (note that smell is the only sensory modality that can synapse directly in the cortex rather than having to pass through the thalamus), the amygdala, and the hypothalamus. This close relation between olfactory inputs and limbic system structures involved in memory and emotion may help explain the potent ability of odors to evoke strong nostalgic memories of childhood

Answer 56

The sense of smell is critical for appreciating the rich and complex ﬂavors of individual foods but has additional important functions as well, such as signaling the presence of prey, predators, or potential mates Our ability to respond to many odors—it is estimated that humans can detect more than 10,000 different odors and can discriminate as many as 5000 (Ressler et al., 1994)—is what produces the complex array of ﬂavors that we normally think of as tastes.

Answer 57

What are anosmia and ageusia? Anosmia is the inability to smell. Ageusia is the loss of taste functions of the tongue

Answer 58

note that smell is the only sensory modality that can synapse directly in the cortex rather than having to pass through the thalamus

Answer 59

It is a common misconception that the myriad little bumps on the tongue are taste buds, but they aren’t. They are actually papillae (singular papilla, Latin for “nipple”) (FIGURE 9.21), tiny lumps of tissue that serve to increase the surface area of the tongue. There are three kinds of papillae, distributed on the tongue as shown in FIGURE 9.22. Each of the relatively few circumvallate papillae and foliate papillae contains many taste buds in its sides. Fungiform papillae, which contain only about six taste buds each, resemble button mushrooms in shape Each papilla holds one or more taste buds, and each taste bud consists of a cluster of 50–150 taste receptor cells (FIGURE 9.22A). At the surface end of the taste bud is an opening called the taste pore (FIGURE 9.22B). The taste cells extend ﬁne cilia into the taste pore, and these come into contact with tastants (substances that can be tasted). Each taste cell is sensitive to just one of the ﬁve basic tastes, and with a life span of only 10–14 days, taste cells are constantly be- ing replaced. As our varied experience with hot drinks, frozen ﬂag poles, or spicy foods informs us, taste is not the only sensory capability of the tongue. Some of the sensory cells within taste buds signal heat, pain, and touch. The ability to taste many substances is already well developed in humans at birth. Even premature infants seem to show a clear preference for sweet substances, and aversion to bitter substances (Steiner, 1974). Newborns seem to be relatively insensitive to salty tastes, but a preference for mildly salty substances develops in the ﬁrst few months. This preference does not seem to be related to experience with salty tastes; rather it probably indicates maturation of the mechanisms of salt perception papilla A small bump that projects from the surface of the tongue. Papillae contain most of the taste receptor cells. circumvallate papillae One of three types of small structures on the tongue, located in the back, that contain taste receptors. foliate papillae One of three types of small structures on the tongue, located along the sides, that contain taste receptors. fungiform papillae One of three types of small structures on the tongue, located in the front, that contain taste receptors. taste bud A cluster of 50–150 cells that detects tastes. Taste buds are found in papillae. taste pore The small aperture through which tastant molecules are able to access the sensory receptors of the taste bud. tastant A substance that can be tasted.

Answer 60

The tastes salty and sour are evoked when taste cells are stimulated by simple ions acting on ion channels in the membranes of the taste cells. The gustatory system (from the Latin gustare, “to taste”) extends from the taste receptor cells through brainstem nuclei and the thalamus to the cerebral cortex. Each taste cell transmits information to several afferent ﬁbers, and each afferent ﬁber receives information from several taste cells. The afferent ﬁbers run along three different cranial nerves—the facial (VII), glossopharyngeal (IX), and vagus (X) nerves (see Figure 2.9). The gustatory ﬁbers in each of these nerves run to the brainstem. Here they synapse with second-order gustatory neurons that project to the ventral posterior medial nucleus of the thalamus. After another synapse, third-order gustatory ﬁbers extend to the cortical taste area located in the insula, just anterior to the central sulcus. Even within the cortical taste area, a “gustatopic” map is preserved—the different tastes are represented in distinct areas within gustatory cortex

Answer 61

The sense of taste provides an immediate assessment of foods (Lindemann, 1995): sweet indicates high-calorie foods; savory tastes signal a protein source; salty and sour relate to important aspects of homeostasis; bitter warns of toxic constituents. humans detect only ﬁve basic tastes: salty, sour, sweet, bitter, and umami. The sensations uniquely aroused by an apple, a steak, or an olive are ﬂavors rather than simple tastes; they involve smell as well as taste. Block your nose, and a raw potato tastes the same as an apple.

Answer 62

Odorants enter the nasal cavity during inhalation and especially during periods of snifﬁng; they also rise to the nasal cavity from the mouth when we chew food. The direction of airﬂow in the nose is determined by complex curved surfaces called turbinates that form the nasal cavity (see Figure 9.25). Airborne molecules initially encounter the ﬂuids of the mucosal layer, which contain binding proteins that transport odorants to receptor surfaces The odorant stimulus then interacts with receptor proteins located on the surface of the olfactory cilia and the dendritic knob of the receptor cells. These receptor proteins are members of a superfamily of G protein–linked receptors (Buck and Axel, 1991). Interactions of odorants with their receptors trigger the synthesis of second messengers, such as cyclic AMP (cAMP). Cyclic AMP opens cation channels (L. J. Brunet et al., 1996), resulting in depolarization of the olfactory receptor cell, which in turn leads to the generation of action potentials.

Answer 63

The visual system responds to a band of electromagnetic radiation, measured in quanta. Each quantum has a wavelength. Quanta of light energy with visible wavelengths are called photons. When quanta within the visible spectrum enter the eye, they can evoke visual sensations. The exact nature of such sensations depends both on the wavelengths of the quanta and on the number of quanta per second.

Answer 64

The visual ﬁeld is the whole area you can see without moving your head or eyes. You may think that your vision is equally good across the visual ﬁeld, but this is an illusion. Visual acuity, the sharpness of vision, is especially ﬁne in the center of the visual ﬁeld and falls off rapidly toward the periphery. We can understand this difference in visual acuity by learning more about the retina and successive levels of the visual system. FIGURE 10.5A shows a photograph of the back of the eye seen through the pupil. The central region, called the fovea (Latin for “pit”), has a dense concentration of cones. The optic disc, to the nasal side of the fovea, is where blood vessels and ganglion cell axons leave the eye. There are no photoreceptors at the optic disc, so there is a blind spot here that we normally do not notice. To locate your blind spot and experience ﬁrsthand some of its interesting features,

Answer 65

The ganglion cells in each eye produce action potentials that are con- ducted along their axons to send visual information to the brain. These axons make up the optic nerve (also known as cranial nerve II) that brings visual information into the brain on each side, eventually reach- ing visual cortex in the occipital lobe at the back of the brain

Answer 66

The ﬁrst stages of visual-information processing occur in the retina, the receptive surface inside the back of the eye (FIGURE 10.2A). The retina is only 200–300 mi-crometers (μm) thick—not much thicker than the edge of a razor blade—but it contains several types of cells in distinct layers (FIGURE 10.2B). The receptor cells that detect light are called photoreceptors. In all vertebrates, some or all of the axons of each optic nerve cross to the opposite cerebral hemisphere. The optic nerves cross the midline at the optic chiasm(named for the Greek letter χ [chi] because of its crossover shape). In humans, axons from the half of the retina toward the nose (called the nasal hemiretina) cross over to the opposite side of the brain (see Figure 10.9A). The half of the retina toward the side of the head (the temporal hemiretina) projects its axons to its own side of the head. Having our eyes cover overlapping sections of the visual ﬁeld helps us judge the distance of objects, by (unconsciously) comparing the inputs from the two eyes. In prey species such as rabbits, natural selection has sacriﬁced some of this binocular depth perception, favoring instead the broader visual ﬁeld that results from having the eyes positioned farther apart (FIGURE 10.9B); this broader visual ﬁeld helps the rabbit detect approaching predators. In such species, most of the optic nerve ﬁbers cross the midline. After they pass the optic chiasm, the axons of the retinal ganglion cells are known collectively as the optic tract. As FIGURE 10.10 shows, most axons of the optic tract terminate on cells in the lateral geniculate nucleus (LGN), which is the visual part of the thalamus (step 4a in Figure 10.10). Axons of postsynaptic cells in the LGN form the optic radiations (step 5), which terminate in primary visual cortex (V1) of the occipital lobe at the back of the brain (step 6). The primary visual cortex is often called striate cortex because a broad stripe, or striation, is visible in anatomical sections through this region; the stripe represents layer IV of the cortex, where the optic-radiation ﬁbers arrive. Information from the two eyes converges on cells beyond layer IV of the primary visual cortex, making binocular, three-dimensional vision possible, as we discussed in Chapter 7. As Figure 10.9 shows, the visual cortex in the right cerebral hemisphere receives its input from the left half of the visual ﬁeld, and the visual cortex in the left hemisphere receives its input from the right half of the visual ﬁeld. Some retinal ganglion cells send their optic-tract axons to the superior colliculus in the midbrain (see Figure 10.10, step 4b), to help coordinate rapid movements of the eyes toward a target. In addition to the primary visual cortex (V1) shown in Figure 10.10, numerous surrounding regions of the cortex are also largely visual in function. These visual cortical areas outside the striate cortex are sometimes called extrastriate cortex. These different cortical regions work in parallel to process different aspects of visual perception, such as form, color, location, and movement, as we will discuss later in this chapter. In striate cortex, as well as most extrastriate regions, there is a topographic projection of the retina, which means there’s a topographic projection of the visual ﬁeld, discussed next.

Answer 67

The receptor cells that detect light are called photoreceptors. There are two kinds of photoreceptors: some are called rods because of their relatively long, narrow form; others are called cones(FIGURE 10.2C). There are several different types of cones, which respond differently to light of varying wavelengths, providing us with color vision, as described later in the chapter. Both rod and cone photoreceptors release neurotransmitter molecules (FIGURE 10.2D) that control the activity of the bipolar cells that synapse with them (see Figure 10.2B). The bipolar cells, in turn, connect with ganglion cells. The axons of the ganglion cells form the optic nerve, which carries information to the brain. Two different functional systems arise from the two different populations of receptors (rods and cones) in the retina. One system uses the rods and works in dim light, so it is called the scotopic system (from the Greek skotos, “darkness,” and ops, “eye”). The scotopic system has only one receptor type (rods) and therefore does not respond differentially to different colors, which is the basis for the saying “at night, all cats are gray.” There is a lot of convergence in the scotopic cell, as many rods provide information to each ganglion cell. The other system requires more light and, in some species, shows differential sensitivity to wavelengths, enabling color vision. This system uses the cones and is called the photopic system (from the Greek phos, “light”). Compared with the scotopic system, the photopic system has less convergence, as some ganglion cells report information from only a single cone.

Answer 68

Two different functional systems arise from the two different populations of receptors (rods and cones) in the retina. One system uses the rods and works in dim light, so it is called the scotopic system (from the Greek skotos, “darkness,” and ops, “eye”). The scotopic system has only one receptor type (rods) and therefore does not respond differentially to different colors, which is the basis for the saying “at night, all cats are gray.” There is a lot of convergence in the scotopic cell, as many rods provide information to each ganglion cell. The other system requires more light and, in some species, shows differential sensitivity to wavelengths, enabling color vision. This system uses the cones and is called the photopic system (from the Greek phos, “light”). Compared with the scotopic system, the photopic system has less convergence, as some ganglion cells report information from only a single cone. At moderate levels of illumination, both the rods and the cones function, and some ganglion cells receive input from both types of receptors. TABLE 10.1 summarizes the characteristics of the photopic and scotopic systems.

Answer 69

primary visual cortex (V1) or striate cortex Also called area 17. The region of the occipital cortex where most visual information ﬁrst arrives. Area V1 is only a small part of the portion of cortex that is devoted to vision. Area V1 sends axons to other visual cortical areas, including areas that appear to be involved in the perception of form: V2, V4, and the inferior temporal area (see Figure 10.19C). Some of these extrastriate areas also receive direct input from the LGN. The receptive ﬁelds of the cells in many of these extrastriate visual areas are even more complex than those of cells in area V1. Perception of Visual Motion Is Analyzed by a Special System That Includes Cortical Area V5

Answer 70

scotoma A region of blindness caused by injury to the visual pathway or brain. Because of the orderly mapping of the visual ﬁeld (known as retinotopic mapping) at the various levels of the visual system, damage to parts of the visual system can be diagnosed from defects in perception of the visual ﬁeld. If we know the site of injury in the visual pathway, we can predict the location of such a perceptual gap, or scotoma (plural scotomas or scotomata), in the visual ﬁeld. Although the word scotoma comes from the Greek skotos, meaning “darkness,” a scotoma is not perceived as a dark patch in the visual ﬁeld; rather, it is a spot where nothing can be perceived, and usually rigorous testing is required to demonstrate its existence. Within a scotoma, a person cannot consciously perceive visual cues, but some visual discrimination in this region may still be possible; this paradoxical phenomenon has been called blindsight

Answer 71

Mortimer Mishkin and Leslie Ungerleider (1982) proposed that primates also have two main cortical processing streams, both originating in primary visual cortex: a ventral processing stream responsible for visually identifying objects, and a dorsal stream responsible for appreciating the location of objects and for guiding movement toward objects (FIGURE 10.27). These processing streams were called, respectively, the what and where streams. The two streams are not completely separate, because there are normally many cross-connections between them. In the ventral stream, including regions of the occipitotemporal, inferior temporal, and inferior frontal areas, information about faces becomes more speciﬁc as one proceeds farther forward.

Answer 72

It was cold in the bathroom, so the young woman turned on a small heater before she got in the shower. She didn’t know that the heater was malfunctioning, ﬁlling the room with deadly, odorless carbon monoxide gas. Her husband found her unconscious on the ﬂoor and called for an ambulance to rush her to the emergency room. When she regained consciousness, “D.F.” seemed to have gotten off lightly, avoiding what could have been a fatal accident. She could understand the doctors’ questions and reply sensibly, move all her limbs, and perceive touch on her skin. But something was wrong with her sight. D.F. couldn’t recognize faces, even her husband’s, nor could she name any objects presented to her view. D.F. still cannot recognize objects today, more than 15 years after her accident. Yet she is not entirely blind. Show her a ﬂashlight and she can tell you that it’s made of shiny aluminum with some red plastic, but she doesn’t recognize it (“Is it a kitchen utensil?”). Without telling her what it is, if you ask her to pick it up, D.F.’s hand goes directly to the ﬂashlight and holds it exactly as one normally holds a ﬂashlight. Show D.F. a slot in a piece of plastic and she cannot tell you whether the slot is oriented vertically, horizontally, or diagonally; but if you hand her a disk and ask her to put it through the hole, D.F. invariably turns the disk so that it goes smoothly through the slot (Goodale et al., 1991). Can D.F. see or not? Discovery of these separate visual cortical streams helps us understand the case of patient D.F., described at the start of this chapter. Recall that after carbon monoxide poisoning, she lost the ability to perceive faces and objects while retaining the ability to reach and grasp under visual control. The investigators who studied her (A. D. Milner et al., 1991) hypothesized that D.F.’s ventral visual stream had been devastated but that her dorsal stream was unimpaired. An opposite kind of dissociation had already been reported: damage to the posterior parietal cortex often results in optic ataxia in which patients have difﬁculty using vision to reach for and grasp objects, yet these patients may retain the ability to correctly identify objects (Perenin and Vighetto, 1988). MRI with D.F. supports the idea that her ventral stream was damaged while her dorsal stream was relatively unaffected (T. W. James et al., 2003). High-resolution MRI of D.F.’s brain (FIGURE 10.28A) reveals damage concentrating in the ventrolateral occipital cortex. Throughout the brain, there is evidence of atrophy, indicated by shrunken gyri and enlarged sulci. FIGURE 10.28B shows the area activated in fMRI recordings when normal subjects viewed pictures of objects; it corresponds to D.F.’s lateral occipital lesion. When D.F. reached for and grasped objects, her fMRI activation in the parietal lobe was similar to that of normal subjects, indicating that D.F.’s dorsal stream is largely intact. D.F.’s intact dorsal pathway not only tells her where objects are but also guides her movements to use these objects properly. It is still puzzling that one part of D.F. knows exactly how to grasp a pencil put before her, while another part of her—the part that talks to you—has no idea whether it’s a pencil, a ruler, or a bouquet of ﬂowers. Imagining what this disjointed visual experience must be like for D.F. allows us to appreciate how effortlessly our brains usually bind together information to give us the marvelous sense of sight.

Answer 73

Within a scotoma, a person cannot consciously perceive visual cues, but some visual discrimination in this region may still be possible; this paradoxical phenomenon has been called blindsight. . In other cases, stimuli that cannot be seen within a scotoma affect judgments of stimuli outside it (Stoerig and Cowey, 1997). Blindsight may also be related to the phenomenon known as hemispatial neglect—neglect of the side opposite to an injured cerebral hemisphere.

Answer 74

prosopagnosia Also called face blindness. A condition characterized by the inability to recognize faces. Acquired prosopagnosia is caused by damage to the brain, particularly the fusiform gyrus. Developmental (or congenital) prosopagnosia is the result of brain defects present from birth. In contrast, the ability to recognize objects may be retained, and the patient may have no difﬁculty identifying familiar people by their voices. Faces simply lack meaning in the patient’s life. No disorientation or confusion accompanies this condition, nor is there evidence of diminished intellectual abilities. Visual acuity is maintained, although the majority of patients have a small visual-ﬁeld defect—that is, an area of the visual ﬁeld where they are blind. Most research indicates that the right hemisphere is more important than the left for recognizing faces. Bilateral destruction of this region leads to prosopagnosia, the inability to recognize individual faces. Still, data from split-brain patients and functional imaging tests make it clear that both hemispheres have some capacity for recognizing faces. Thus, although damage restricted to the right hemisphere can impair face processing, the most complete cases of prosopagnosia are caused by bilateral damage. The fusiform gyrus, a region of cortex on the inferior surface of the brain where the occipital and temporal cortices meet (FIGURE 19.19), is crucial, and cases of prosopagnosia following brain damage almost always involve damage here. Symptoms may be exacerbated when lesions also include a nearby occipital area that performs initial face-speciﬁc visual processing and when they include the superior temporal sulcus, which has been implicated in connecting faces with speech and facial expressions. Prosopagnosia of either type may be accompanied by additional forms of agnosia (an inability to identify items, in the absence of speciﬁc sensory impairments). Indeed, fMRI studies of healthy subjects show that the fusiform region is activated not only when people are identifying faces, but also when identifying birds, or cars (Gauthier et al., 2000). So it seems that the fusiform system may be crucial for identifying individual members of large categories (e.g., faces or birds or cars), especially when the members of the category have many things in common.

Answer 75

The skeletal system and attached muscles allow for movement. The spinal cord controls skeletal muscles. The brainstem integrates motor commands.

Answer 76

reﬂex A simple, highly stereotyped, and unlearned response to a particular stimulus (e.g., an eye blink in response to a puff of air). These studies showed that some reﬂexes involve only short pathways in the spinal cord linking dorsal and ventral roots; others involve longer loops connecting spinal cord segments to each other, or to brain regions. Spinal reflexes mediate “automatic” responses A good example of automatic control at the spinal level is the stretch reﬂex—the contraction that results when a muscle stretches.

Answer 77

central pattern generator Neural circuitry that is responsible for generating the rhythmic pattern of a behavior such as walking. Many such rhythmic movements are generated by mechanisms within the spinal cord. These endogenous rhythms are normally modulated by sensory feedback, but they can function independently of brain inﬂuences or afferents. The term central pattern generator is used to refer to the neural circuitry responsible for generating rhythmic patterns of behavior, as seen in walking. The central pattern generators for walking seem much the same in cats, birds, and humans (Dominici et al., 2011). This essential rhythm of walking generated by spinal cord mechanisms is activated and modulated by the brain.

Answer 78

Pathways from the brain control different aspects of movements Some muscles are controlled directly by the brain. The cranial motor nuclei of the brainstem send their axons to innervate muscles of the head and neck (FIGURE 11.12; see also Figure 2.9). But for all the other muscles, the brain has to send commands to the spinal cord, and then the spinal cord controls the muscles. The brain sends these commands to the spinal cord through two major pathways: the pyramidal system (which we’ll discuss next) and extrapyramidal motor system (described later in this chapter).

Answer 79

The pyramidal system (or corticospinal system) consists of neuronal cell bodies within the cerebral cortex and their axons, which pass through the brainstem, forming the pyramidal tract to the spinal cord (FIGURE 11.13A). The pyramidal tract is seen most clearly where it passes through the ﬂoor of the medulla. In a cross section of the medulla, the tract is a wedge-shaped ventral protuberance (pyramid) on each side of the midline. In the medulla the pyramidal tract from the right hemisphere crosses the midline to innervate the left spinal cord, and vice versa. Because the pyramidal tract crosses the midline (technically known as a decussation) in the medulla, the right cortex controls the left side of the body while the left cortex controls the right. Lesions of the pyramidal system deprive the patient of the ability to move individual joints and limbs. Many of the axons of the pyramidal tract originate from neurons in the primary motor cortex (M1), which consists mainly of the precentral gyrus, just anterior to the central sulcus (FIGURE 11.13B). The cell bodies of many of these large neurons are found in layer V of the primary motor cortex.

Answer 80

In addition to the corticospinal outﬂow through the pyramidal tract, many other motor tracts run from the forebrain to the brainstem and spinal cord. Because these tracts are outside the pyramids of the medulla, they and their connections are called the extrapyramidal system. The two most important components of the extrapyramidal system are the basal ganglia and the cerebellum. How do these components of the extrapyramidal system communicate with the spinal cord? Their messages are transmitted via two brainstem pathways: the reticulospinal tract, which originates in the reticular formation of the brainstem, and the rubrospinal tract, which originates from the midbrain’s red nucleus (the Latin ruber means “red”). Both tracts send axons down the spinal cord to synapse on spinal interneurons. Using these brainstem pathways, the basal ganglia and cerebellum play slightly different roles in controlling motor output, as we’ll see next.

Answer 81

behavioural The basic units of behavior are reﬂexes: simple, unvarying, and unlearned responses to sensory stimuli such as touch, pressure, and pain. The motor plan, also called the motor program, is a complex set of commands to muscles that is established before the behavior starts. Feedback from movements informs and ﬁnetunes the motor program as it unfolds, but the basic sequence of movements is planned. Control Systems One way to look at the mechanisms that regulate and control our movements employs the language of engineering. In designing machines, engineers commonly have two goals: (1) accuracy, to prevent or minimize error, and (2) speed, to complete a task quickly and efﬁciently. It is difﬁcult to accomplish both these goals at once; usually there is a trade-off between the two. Two forms of control mechanisms—closed-loop and open-loop—are commonly employed to optimize one at the expense of the other. The Neuroscience View We can distinguish several different levels of hierarchically organized motor control systems:

Answer 82

We can readily analyze movements using high-speed video, which provides an intimate frame-by-frame portrait of even the most rapid events. To deal with the large amounts of data produced by image processing, methods of simpliﬁcation or numerical analysis have been devised For example, sports trainers use detailed analyses of athletic acts based on time-lapse photographs or information derived from sensors attached at joints. Computer programs process images to help quantify the performance, enabling detailed measurement of the positions of different body parts in successive instants. Another approach to the ﬁne-grained analysis of movements is to record the electrical activity of muscles—a procedure called electromyography (EMG). Like neurons, muscles produce action potentials when they contract, as we’ll see later in this chapter. Therefore, ﬁne needle electrodes placed in a muscle, or electrodes placed on the skin over a muscle, can detect electrical indications of muscle activity (FIGURE 11.2). If electrodes are placed over several different muscles, we get a record of the timing and strength of contraction of the muscles involved in a movement (Hanakawa et al., 2003). The EMGs in Figure 11.2 show that a person pulling a knob will adjust his legs just before moving his arm—another example of motor planning.

Answer 83

acetylcholine (ACh) A neurotransmitter produced and released by parasympathetic postganglionic neurons, by motoneurons, and by neurons throughout the brain. Each axonal branch carries an action potential to its axon terminal, which then (in vertebrates) releases the neurotransmitter acetylcholine (ACh). Muscle ﬁbers respond to the ACh by producing action potentials of their own.

Answer 84

Many of the axons of the pyramidal tract originate from neurons in the primary motor cortex (M1), which consists mainly of the precentral gyrus, just anterior to the central sulcus.

Answer 85

The motor cortex representation of the muscles of the body has historically been illustrated with a homunculus, like this one, in which the sizes of the ﬁgure’s body parts are proportional to the amounts of motor cortex devoted to the corresponding muscles. This sort of mapping, however, is almost certainly an oversimpliﬁcation; the organization of the motor cortex is not as discrete as these maps imply

Answer 86

The traditional account of nonprimary motor cortex emphasizes two main regions: the supplementary motor area (SMA), which lies mainly on the medial aspect of the hemisphere, and the premotor cortex, which is anterior to the primary motor cortex supplementary motor area (SMA) A region of nonprimary motor cortex that receives input from the basal ganglia and modulates the activity of the primary motor cortex. The SMA lies mainly on the medial surface of the cerebral hemispheres. suggesting that this region initiates movement sequences

Answer 87

amyotrophic lateral sclerosis (ALS) Also called Lou Gehrig’s disease. A disease in which motoneurons and their target muscles waste away. Sometimes, for reasons that remain elusive, the motoneurons of the brainstem and spinal cord spontaneously start to die and their target muscles waste away. In this disease, called amyotrophic lateral sclerosis (ALS; sometimes called Lou Gehrig’s disease after the 1930s baseball player who lost his life to the disorder), the afﬂicted person experiences gradually worsening paralysis until most skeletal muscle ceases to function. Although premature death is the usual outcome, some people with ALS can survive for long periods; celebrated British physicist Stephen Hawking, who was diagnosed with ALS in the early 1960s, is an example. A wide range of possible causal factors are under investigation, including premature aging, neurotoxins, viruses, immune responses, and endocrine dysfunction.

Answer 88

Parkinson’s disease A degenerative neurological disorder, characterized by tremors at rest, muscular rigidity, and reduction in voluntary movement,that involves dopaminergic neurons of the substantia nigra. Another feature of what is now known as Parkinson’s disease is a loss of facial muscle tone, which gives the face a masklike appearance. Patients who suffer from Parkinson’s also show few spontaneous actions and have great difﬁculty in all motor efforts, no matter how routine. The hands may display tremors while at rest but move smoothly while performing a task. Parkinson’s disease afﬂicts almost 1% of the U.S. population age 65 and older, but it sometimes unaccountably occurs in younger people, such as actor Michael J. Fox (FIGURE 11.22). Patients with Parkinson’s show progressive degeneration of dopamine-containing cells in the substantia nigra that project to the basal ganglia, particularly the caudate nucleus and putamen. The loss of cells in this area is continual, but symptoms begin to appear only after extensive cell death

Answer 89

myasthenia gravis A disorder characterized by a profound weakness of skeletal muscles; caused by a loss of acetylcholine receptors. This disorder is characterized by a profound weakness of skeletal muscles. The disease often ﬁrst affects the muscles of the head, producing symptoms such as drooping of the eyelids, double vision, and slowing of speech. In later stages, paralysis of the muscles that control swallowing and respiration becomes life-threatening. Myasthenia gravis is an autoimmune disorder: most cases result when antibodies develop and attack a patient’s own acetylcholine receptors, disrupting neuromuscular junctions. In other cases the antibodies are directed toward other proteins that are associated with the acetylcholine receptor. Treatment often consists of prescribing drugs to suppress the immune system

Answer 90

apraxia An impairment in the ability to begin and execute skilled voluntary movements, even though there is no muscle paralysis. .apraxia (from the Greek a-, “not,” and praxis, “action”), the inability to carry out complex movements even though paralysis or weakness is not evident and language comprehension and motivation are intact. Apraxia is a symptom of a variety of disorders, including stroke, Alzheimer’s disease, and developmental disorders of children. Neurologists studying patients who have suffered strokes have discovered several different types of apraxia. ideomotor apraxia The inability to carry out a simple motor activity in response to a verbal command, even though this same activity is readily performed spontaneously. Ideomotor apraxia is characterized by the inability to carry out a simple motor activity, either in response to a verbal command (“smile” or “use this comb”) or by copying someone else’s gesture, even though this same activity is readily performed spontaneously. ideational apraxia An impairment in the ability to carry out a sequence of actions, even though each element or step can be done correctly. Ideational apraxia is an impairment in carrying out a sequence of actions, although each step can be done correctly (Zadikoff and Lang, 2005). Patients with ideational apraxia have difﬁculty carrying out instructions for a sequence of acts—“push the button, then pull the handle, then depress the switch”—but they can do each of these tasks in isolation

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