chapter 12 Flashcards
(20 cards)
stimulus
receptor
sensation
perception
Sensory input that causes some change within or outside the body is called a stimulus (plural: stimuli). The stimulus is often a form of physical energy such as heat, pressure, or sound waves, but it can also be a chemical. A receptor—a structure specialized to receive certain stimuli—accepts the stimulus and converts its energy into another form. Some receptors are evolved from the dendritic structures of a sensory neuron. They convert the stimulus into a graded potential that, if it is powerful enough, initiates an impulse within the sensory neuron. Other receptors are parts of cells that produce graded potentials and release a neurotransmitter, stimulating a nearby sensory neuron. In the end, the effect is the same—generation of an impulse in a sensory neuron. When the central nervous system (CNS) receives these impulses, we often experience a sensation, meaning that we become consciously aware of the stimulus. A sensation is different from perception, which means understanding what the sensation means. As an example, hearing the sound of thunder is a sensation, whereas the belief that a storm is approaching is a perception.
Differentiate between the following receptor types:
● Mechanoreceptors respond to forms of mechanical energy, such as waves of sound, changes in fluid pressure, physical touch or pressure, stretching, or forces generated by gravity and acceleration.
● Thermoreceptors respond to heat or cold.
● Pain receptors respond to tissue damage or excessive pressure or temperature.
● Chemoreceptors respond to the presence of chemicals in the nearby area.
● Photoreceptors respond to light
Summarize how the CNS interprets nerve impulses based on origin and frequency.
How can cells in the CNS interpret some incoming impulses as images and others as sounds? How do the cells distinguish a loud sound from a soft one and bright lights from dim? As described in the chapter on the nervous system (Chapter 11), nerve impulses (action potentials) are transmitted from receptors to specific brain areas. For example, impulses generated by visual stimuli travel in sensory neurons whose axons go directly to brain regions associated with vision. All incoming impulses traveling in these neurons are interpreted as light, regardless of how they were initiated. This is why, when you are hit in the eye, you may “see stars.” The blow to your eye triggers impulses in visual sensory neurons. When these signals arrive in the visual area of the brain, they are interpreted as light. Stronger stimuli activate more receptors and trigger a greater frequency of impulses in sensory neurons. In effect, the CNS gets all the information it needs by monitoring where impulses originate and how frequently they arrive.
What is receptor adaptation?
The CNS can ignore one sensation to concentrate on others. For example, you ignore the feel of your clothing when you are more interested in other things. Some sensory inputs are ignored after a while because of receptor adaptation, in which the sensory neuron stops sending impulses even though the original stimulus is still present. To demonstrate, try touching a few hairs on your forearm. Notice that the initial feeling of light pressure goes away within a second or so, even though you maintain the same amount of pressure. Receptors in the skin for light touch and pressure adapt rather quickly. This confers a survival advantage, because they can keep the CNS informed of changes in these stimuli, without constantly bombarding it with relatively unimportant stimuli. Olfactory (smell) receptors also adapt rapidly. Olfactory adaptation can be hazardous to people if they are continually exposed to low levels of hazardous chemicals that they can no longer smell. Other receptors—pain receptors, joint and muscle receptors that monitor the position of our limbs, and essentially all of the silent receptors involved in homeostatic feedback control loops—adapt slowly or not at all. Lack of adaptation in these receptors is important to survival. Persistent sensations such as pain alert us to possible tissue damage from illness or injury and prompt us to take appropriate action. Persistent activity of silent receptors is essential to our ability to maintain homeostasis.
What are the five special senses?
The sensations (or senses) provided by receptors are categorized as either somatic or special. The somatic sensations originate from receptors present at more than one location in the body (soma is the Greek word for “body”). The somatic sensations include temperature, touch, vibration, pressure, pain, and awareness of body movements and position. The five special senses (taste, smell, hearing, balance, and vision) originate from receptors that are restricted to particular areas of the body, such as the ears and eyes. The special senses deliver highly specialized information about the external world. Next, we look at the somatic sensations first and then discuss the five special senses in detail.
What is referred pain?
As much as we dislike pain, the ability to perceive pain is essential for survival. Pain warns us to avoid certain stimuli and informs us of injuries. Pain receptors are unencapsulated endings that respond to injury from excessive physical pressure, heat, light, or chemicals. Pain is described as either fast or slow, depending on its characteristics. Fast pain, also called sharp or acute pain, occurs as soon as a tenth of a second after the stimulus. Receptors for fast pain generally respond to physical pressure or heat and usually are located near the surface of the body. They inform us of stimuli to be avoided—for example, stepping on a nail or touching a hot burner on the stove. The reflex withdrawal response to fast pain is rapid and strong. In contrast, slow pain generally arises from muscles or internal organs. Slow pain, which may not appear until seconds or even minutes after injury, is due to activation of chemically sensitive pain receptors by chemicals released from damaged tissue. Slow pain from internal organs is often perceived as originating from an area of the body completely removed from the actual source. This phenomenon, called referred pain, happens because action potentials from internal pain receptors are transmitted to the brain by the same spinal neurons that transmit action potentials from pain receptors in the skin and skeletal muscles. The brain has no way of knowing the exact source of the pain, so it assigns the pain to another location. Referred pain is so common that physicians use it to diagnose certain disorders of internal organs. For example, a heart attack usually manifests itself as pain in the left shoulder and down the left arm. Figure 12.4 shows where pain from internal organs may be felt. The close association between areas of referred pain from the heart and esophagus is why it can be difficult, by sensations alone, to distinguish a heart attack from the less serious problem of esophageal spasm. Pain receptors generally do not adapt. From a survival standpoint, this is beneficial, because the continued presence of pain reminds us to avoid doing whatever causes it. However, it also means that people with chronic diseases or disabilities can end up experiencing constant discomfort.
Summarize how taste buds work. What are the five primary tastes qualities?
The tongue (Figure 12.5a) has a rough texture because its surface is composed of numerous small projections, called papillae, surrounded by deep folds (Figure 12.5b). About 10,000 taste buds are located at the surface of the papillae within the folds (Figure 12.5c). A taste bud is a cluster of about 25 taste cells and 25 supporting cells that separate the taste cells from each other. The exposed tip of a taste cell has hairs that project into the mouth (Figure 12.5d). The hairs contain chemoreceptors that are specific for certain chemicals (called tastants). When we eat or drink, tastants dissolve in our saliva and bind to the chemoreceptor. The binding of tastant to chemoreceptor causes the taste cell to release a neurotransmitter, which stimulates a nearby sensory neuron to produce action potentials. Note that the taste cell itself is not a sensory neuron. It takes two cells (the taste cell and a sensory neuron) to convert the chemical stimulus into a nerve impulse, but the ultimate effect is the same as if it were one cell. The five commonly accepted primary taste qualities are sweet, sour, salty, bitter, and umami. Taste receptors for umami, discovered by Japanese researchers, allow us to appreciate foods like parmesan cheese, mushrooms, beef, soy sauce, and Worcestershire sauce. Although an individual taste receptor and its associated sensory neuron seem to be “tuned” to respond most strongly to only one of the taste qualities, they do respond weakly to the other taste qualities as well. Most of the taste receptors are located around the edges and at the front and back of the tongue. Although all receptor types can be found in each of these regions, there is some evidence that their distribution is not entirely even. The tongue is slightly more sensitive to sweet tastes on the tip, sour on the sides, salty on the tip and sides, and bitter toward the back (the distribution of umami receptors is not yet well documented). Therefore, to gain the maximum taste sensation, we move food around in our mouth before swallowing. The combined sensitivities of our taste receptors allow us to distinguish hundreds of different flavors, far beyond the basic five qualities. On the other hand, if we don’t want to taste something, we pass it over the center of the tongue and swallow quickly. It is probably no accident that bitter taste receptors are heavily concentrated at the very back of the tongue. Bitterness is often a sign of an inedible or even poisonous substance, and the location of these receptors makes it difficult to bypass them.
What type of receptor cells detect odors? What is the relationship between taste and smell?
The sense of smell also relies on a chemoreceptor mechanism. There are two fundamental differences between taste and smell: (1) The receptors for smell are located on true sensory neurons and (2) there are receptors for over 1,000 different odorant chemicals, as opposed to just four tastant classes. With an abundance of different receptor types for smell and our sense of taste, we can distinguish as many as 10,000 taste and smell sensations. Odors are detected by olfactory receptor cells located in the upper part of the nasal passages. The receptor cells are sensory neurons, each with a modified dendritic ending that branches to become several olfactory hairs (Figure 12.6). The olfactory hairs extend into a layer of mucus covering the surface of the nasal passages. The mucus, produced by nearby olfactory glands, keeps these hairs from drying out. Gaseous and airborne odorants enter the nasal passages, become dissolved in the mucus, bind to chemoreceptors located on the olfactory hairs, and cause the olfactory receptor cell to generate impulses. Olfactory receptor cells synapse with olfactory neurons in a nearby area of the brain called the olfactory bulb, where information is partially integrated and then passed to higher brain centers. Our sense of smell complements our sense of taste because when food is chewed it releases chemicals that come into contact with olfactory receptors. The combined sensations create an effect that is interpreted at higher brain levels (Figure 12.7). When we have a cold, most foods taste less appetizing. In part this is because increased production of mucus prevents odorant molecules from reaching olfactory receptors, impairing our sense of smell.
What sound characteristic is measured by decibels?
Both intensity and perceived loudness are related to the amplitude of the sound waves, and both are measured in units called decibels (dB).
What determines the pitch (tone) of a sound?
The tone (or pitch) of a sound is determined by its frequency, the number of wave cycles that pass a given point per second (cycles/sec). Frequency is expressed in hertz (Hz). Higher tones are those of a higher frequency (Figure 12.8). The frequency range for normal human hearing is from 20 to 20,000 Hz.
What determines the loudness (intensity) of the sound?
Loudness (intensity) is determined by sound wave amplitude. Tone (pitch) is determined by sound wave frequency.
What is the function of the outer ear? What is the tympanic membrane?
The outer ear consists of the pinna, or visible portion of the ear, and the auditory canal. Sound waves arrive at the pinna and are directed into the auditory canal, which channels them to the tympanic membrane (eardrum). The tympanic membrane serves as a partition between the outer and middle ears.
What is the middle ear?
The middle ear consists of an air-filled chamber within the temporal bone of the skull, bridged by three small bones called the malleus (“hammer”), incus (“anvil”), and stapes (“stirrup”). When sound waves strike the tympanic membrane, it moves back and forth slightly (vibrates). Vibration of the tympanic membrane makes the malleus, incus, and stapes vibrate. The stapes touches a smaller membrane, called the oval window, causing it to vibrate, too.
What is the auditory (eustachian) tube?
The air-filled middle ear is kept at atmospheric pressure by the auditory tube (eustachian tube), a narrow tube that runs from the middle ear chamber to the throat. Although the tube is normally held closed by the muscles along its sides, it opens briefly when you swallow or yawn to equalize air pressure in the middle ear with that of the surrounding atmosphere. When you change altitude quickly, your ears “pop” as the auditory tube opens briefly and air pressures are suddenly equalized. When you have a cold or allergy, inflammation and swelling may prevent the auditory tube from equalizing pressures normally, which is why airplane trips can be painful when you have a cold or allergies. Children tend to get a lot of earaches (middle ear infections) via the auditory tube until the skull elongates, changing the angle of the tube to the ear.
What is the function of the inner ear’s cochlea?
The inner ear sorts sounds by tone and converts them into impulses. It consists of the cochlea, where sound is converted, and the vestibular apparatus, consisting of the vestibule and the three semicircular canals, which does not contribute to hearing at all. We will discuss the vestibular apparatus later in connection with the sense of balance. The cochlea is a coiled structure shaped a bit like a snail. Figure 12.10a shows what the cochlea would look like if it were uncoiled. It is a tapered tube containing two interconnected outer canals called the vestibular canal and the tympanic canal, surrounding a third, closed fluid-filled space called the cochlear duct
What are the hair cells on the basilar membrane for?
The base of the cochlear duct is formed by the basilar membrane (Figure 12.10c). The basilar membrane supports a population of about 15,000 hair cells, the mechanoreceptor cells of the ear. Hair cells have hairlike projections that are embedded in an overhanging structure called the tectorial membrane (the Latin word for “roof” is tectum), which is composed of a firm, gelatinous, noncellular material. Together, the hair cells and the tectorial membrane are called the organ of Corti, the organ that converts pressure waves to action potentials.
What is the function of the vestibular apparatus? What causes motion sickness?
Maintaining your balance against gravity requires integration of multiple sensory inputs. Those sensory inputs include signals from the joint receptors, muscle spindles, and tendon receptors described previously, as well as from special structures associated with the inner ear. Even visual input is involved. The inner ear structures, described next, provide information about rotational movement, position, and linear acceleration of the head. Looking again at the inner ear (Figure 12.12a), we see that next to the cochlea is the vestibular apparatus, a system of fluid-filled canals and chambers. The vestibular apparatus consists of three semicircular canals for sensing rotational movement of the head and body and an area called the vestibule, which senses static (nonmoving) position and linear acceleration and deceleration. The semicircular canals and the vestibule have mechanoreceptor-type hair cells embedded in a gel-like material (Figure 12.12b). When the head moves or changes position, the gel also moves, although a bit more slowly because of inertia. The movement of the gel bends the hairs, which causes the hair cells to release a neurotransmitter that ultimately generates impulses in sensory neurons of the vestibular nerve.
Summarize the functions and be able to identify the following: Cornea Iris Pupil Lens Retina Optic Nerve Macula Optic Disk
The structure and functional components of the eye are summarized in Table 12.4 and illustrated in Figure 12.14. A tough outer coat known as the sclera, or “white of the eye,” covers the outer surface except in the very front, where it is continuous with the clear cornea. Light passes through the cornea and a space filled with fluid called aqueous humor that nourishes and cushions the cornea and lens. Light then either strikes the iris, a colored, disk-shaped muscle that determines how much light enters the eye, or passes through the pupil, the adjustable opening in the center of the iris. Light that passes through the pupil strikes the lens, a transparent, flexible structure attached by connective tissue fibers to a ring of circularly arranged smooth muscle called the ciliary muscle. After passing through the main chamber of the eye filled with vitreous humor, light encounters the layers at the back and sides of the eye. This is the retina, comprising primarily photoreceptor cells, neurons, and a few blood vessels. Between the retina and the sclera at the back of the eye lies the choroid, consisting of pigmented cells and blood vessels. The pigmented cells absorb light not sensed by photoreceptors so that the image does not become distorted by reflected light, and the blood vessels nourish the retina. At the back of the eyeball is the optic nerve, which carries information to the thalamus, to be forwarded to the visual cortex for interpretation. Finally, skeletal muscles surround the eye and control its movements, so we can choose exactly where to look. Several sites on the retina deserve special mention. The macula is the central region of the retina, where photoreceptor density is the highest. When we want high visual acuity, we look directly at an object, focusing the image on the macula. At the very center of the macula is a small pit called the fovea centralis that is lined with highly packed photoreceptors. The optic disk is the area where the axons of the optic nerve and associated blood vessels exit the eye, so there are no photoreceptors there at all. The optic disk leaves us with a “blind spot” in each eye. To find the blind spots, try the exercise in Figure 12.15.
Differentiate between the following in terms of the type of vision problem and shape of eye.
Myopia
Hyperopia
Astigmatism
Eyeball shape affects focus Differences in the shape of the eyeball can affect the ability to focus properly. Glasses and contact lenses correct improper focus. Myopia is a common inherited condition in which the eyeball is slightly longer than normal (or more rarely, myopia occurs when the ciliary muscle contracts too strongly) (Figure 12.17a). Even when the lens is flattened maximally, distant objects focus in front of the retina. People with myopia can see nearby objects, but distant objects are out of focus, which is why it is called nearsightedness. Concave lenses, which bend incoming light so it focuses on the retina, can correct nearsightedness. Less common is the opposite condition, hyperopia (farsightedness), which occurs when the eyeball is too degree to which incoming light is bent, and therefore our ability to change focus between near and far objects, is accomplished solely by adjusting the curvature of the lens. This is done by the ciliary muscle.
Differentiate between the function of rods and cones.
Rods and cones respond to light Figure 12.19 takes a closer look at the photoreceptor cells, rods, and cones. One end of the cell consists of a series of flattened disks arranged to form either a rod- or cone-shaped structure, as the names imply. The flattened disks contain numerous molecules of a particular light-sensitive protein called a photopigment. A photopigment protein undergoes a change of shape when it is exposed to energy in the form of light. The change in photopigment molecule shape causes the photoreceptor(rod or cone) to close some of its sodium channels and reduce the amount of neurotransmitter it normally releases. Because the neurotransmitter released by rods or cones normally inhibits the bipolar cells, light ultimately increases the activity of the bipolar cells, which in turn activate the ganglion cells (see Figure 12.18). There are approximately 120 million rods and 6 million cones, but only 1 million ganglion cells with axons going to the brain. Clearly, a great deal of convergence and summation of signals occurs at each level of neuron transfer from the eye to the brain. Rods provide vision in dim light Rods all have the same photopigment, called rhodopsin. Rhodopsin is much more sensitive to light than the photopigments in cones, and therefore in dim light our vision is dependent primarily on rods. However, rods do not give us color vision, which is why objects appear less colorful in dim light. As we have seen, there are about 20 times more rods than cones. If we imagine that all 120 million rods converge on only half of the ganglion cells (one-half million), there would be 240 rods converging on a single ganglion cell. Thisconvergence increases our ability to see in very dim light, but at the expense of acuity (accuracy and detail). As a result, our vision in very dim light is not very colorful and not very detailed, but at least we can see. Rods and cones are not distributed evenly on the retina. Regions of the retina farthest away from the fovea have the highest ratio of rods to cones. If you want to see a dim star at night, don’t look directly at it—look off to the side just a little. Cones provide color vision and accurate images Not all animals perceive color, but humans do. We are able to see colors because we have three different kinds of cones: red, green, and blue. Each contains a photopigment that absorbs the energy of red, green, or blue light particularly well. Our ability to distinguish a variety of colors is due to the way the brain interprets the ratios of impulses coming from the ganglion cells connected to the three types of cones. When all three types are activated by all different wavelengths, we perceive white light. The perception of black is no light at allCones also are responsible for visual acuity. However, cones require stronger light to be activated because the cone photopigments are much less sensitive to light than the rhodopsin in rods. This is why your ability to distinguish between colors declines in dim light and it becomes difficult to tell whether a dark-colored car is green or red. In dim light, you are seeing primarily with your rods, which can detect only black and white.
