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sound and the ear

- audition depends on ability to detect sound waves

- waves are periodic compressions or air, water, other media



refers to the intensity of the sound wave



refers to the perception of the sound wave

- Amplitude is one factor



refers to the number of compressions per second and is measured in hertz (Hz)

-Related to the pitch (high to low)


Hearing and age...

Children hear higher frequencies than adults; the ability to recognize high frequencies diminishes with age and exposure to loud noises


Anatomists distinguish between:

- The outer ear
- The middle ear
- The inner ear


outer ear

- pinna, structure of flesh and cartilage attached to each side of the head


outer ear responsible for

- Altering the reflection of sound waves into the middle ear from the outer ear

- Helps to locate the source of a sound


middle ear

- tympanic membrane, which vibrates at the same rate when struck by sound waves
(aka the ear drum)

- Connects to three tiny bones (malleus, incus, & stapes) that transform waves into stronger waves to the oval window


inner ear

oval window membrane

-Transmits waves through the viscous fluid of the inner ear

contains a snail shaped structure called the cochlea

- Contains three fluid-filled tunnels (scala vestibuli, scala media, & the scala tympani)


hair cells

are auditory receptors that lie between the basilar membrane and the tectorial membrane in the cochlea

- When displaced by vibrations in the fluid of the cochlea, they excite the cells of the auditory nerve


pitch perception theories:

- place theory
- frequency theory


Place theory

each area along the basilar membrane has hair cells sensitive to only one specific frequency of sound wave


Frequency theory

the basilar membrane vibrates in synchrony with the sound and causes auditory nerve axons to produce action potentials at the same frequency


The current pitch theory combines modified versions of both the place theory and frequency theory:

- Low frequency sounds best explained by the frequency theory

- High frequency sounds best explained by place theory


Volley principle

auditory nerve as a whole produces volleys of impulses (for sounds up to about 4,000 per second)

- No individual axon solely approaches that frequency

-Requires auditory cells to precisely time their responses

Hearing of higher frequencies not well understood



the impaired detection of frequency changes (tone deafness)


People vary in their sensitivity to pitch

Associated with thicker than average auditory cortex in the right hemisphere but fewer connections from auditory cortex to frontal cortex


Absolute pitch (“perfect pitch”) is the ability to hear a note and identify it

- Genetic predisposition may contribute to it

- Main determinant is early and extensive musical training

- More common among people who speak tonal languages


The primary auditory cortex (area A1) is the destination for most information from the auditory system

Located in the superior temporal cortex

Each hemisphere receives most of its information from the opposite ear


Organization of the auditory cortex parallels that of the visual cortex

Superior temporal cortex contains area MT
- Allows detection of the motion of sound

Area A1 is important for auditory imagery

Requires experience to develop properly
- Axons leading from the auditory cortex develop less in people deaf since birth


The cortex is necessary for the advanced processing of hearing

Damage to A1 does not necessarily cause deafness unless damage extends to the subcortical areas


The auditory cortex provides a tonotopic map in which cells in the primary auditory cortex are more responsive to preferred tones

Some cells respond better to complex sounds than pure tones


Areas around the primary auditory cortex exist in which cells respond more to changes in sound than to prolonged sounds

Cells outside A1 respond to auditory “objects” (animal cries, machinery noise, music, etc.)

- Because initial response is slow, most likely responsible for interpreting the meaning of sounds


Two categories of hearing impairment include:

- Conductive/middle ear deafness

- Nerve or inner-ear deafness


Conductive/middle ear deafness

- Occurs if bones of the middle ear fail to transmit sound waves properly to the cochlea

- Caused by disease, infections, or tumerous bone growth

- Can be corrected by surgery or hearing aids that amplify the stimulus

- Normal cochlea and normal auditory nerve allows people to hear their own voice clearly


Nerve or inner-ear deafness

- Results from damage to the cochlea, the hair cells, or the auditory nerve

- Can vary in degree

- Can be confined to one part of the cochlea
(People can hear only certain frequencies)

- Can be inherited or caused by prenatal problems or early childhood disorders



is a frequent or constant ringing in the ears

- Experienced by many people with nerve deafness

Sometimes occurs after damage to the cochlea

- Axons representing other part of the body innervate parts of the brain previously responsive to sound
- Similar to the mechanisms of phantom limb


Sound localization depends upon comparing the responses of the two ears

Three cues:
- Sound shadow
- Time of arrival
- Phase difference

Humans localize low frequency sound by phase difference and high frequency sound by loudness differences


Sound localization - 3 mechanisms

- High-frequency sounds (2000 to 3000Hz) create a “sound shadow”

- Difference in time of arrival at the two ears most useful for localizing sounds with sudden onset

- Phase difference between the ears provides cues to sound localization with frequencies up to 1500 Hz

- Readjusting sound localization is slow process


The mechanical senses include:

- The vestibular sensation
- Touch
- Pain
- Other body sensations

The mechanical senses respond to pressure, bending, or other distortions of a receptor


Vestibular Sensation

sense refers to the system that detects the position and the movement of the head

- Directs compensatory movements of the eye and helps to maintain balance

- The vestibular organ is in the ear and is adjacent to the cochlea


vestibular organ

consists of two otolith organs (the saccule and utricle) and three semicircular canals

- Otoliths are calcium carbonate particles that push against different hair cells and excite them when the head tilts

The three semicircular canals are filled with a jellylike substance and hair cells that are activated when the head moves

- Action potentials travel to the brain stem and cerebellum


somatosensory system

refers to the sensation of the body and its movements

- Includes discriminative touch, deep pressure, cold, warmth, pain, itch, tickle and the position and movement of the joints


Touch receptors may be:

- Simple bare neuron ending
- A modified dendrite (Merkel disks)
- An elaborated neuron ending
- A bare ending surrounded by non-neural cells that modify its function

Stimulation opens sodium channels to trigger an action potential


Pacinian corpuscle


a type of touch receptor that detects sudden displacement or high-frequency vibrations on the skin

- Onion-like outer structure resists gradual or constant pressure

- Sudden or vibrating stimulus bends the membrane and increases the flow of sodium ions to triggers an action potential


Chemicals can stimulate receptors for:

heat and cold: e.g., capsaicin and menthol


Merkel disks

receptors that respond to light touch (i.e., gentle stroking of the skin)

Men and women generally have the same number of Merkel disks, but women tend to have smaller fingers

- Results in Merkel disks compacted into a smaller area

- More sensitive to feeling the distances between grooves


Information from touch receptors in the head enters the CNS through the cranial nerves

Information from receptors below the head enters the spinal cord and travel through the 31 spinal nerves to the brain

Each spinal nerve has a sensory component and a motor component and connects to a limited area of the body


A dermatome refers to

the skin area connected to or innervated by a single sensory spinal nerve

Sensory information entering the spinal cord travel in well-defined and distinct pathways

Example: touch pathway is distinct from pain pathway


Various aspects of body sensations remain separate all the way to the cortex

Various areas of the somatosensory thalamus send impulses to different areas of the somatosensory cortex located in the parietal lobe

Different sub areas of the somatosensory cortex respond to different areas of the body

Damage to the somatosensory cortex can result in the impairment of body perceptions



is the experience evoked by a harmful stimulus, directs our attention, and holds it


Pain sensation begins with the least specialized of all receptors (bare nerve endings)

Some pain receptors also respond to acids, heat, or cold

Capsaicin: a chemical found in hot pepper; stimulates these receptors


Axons carrying pain info have little or no myelin:

impulses travel slowly

However, brain processes pain information rapidly and motor responses are fast


Mild pain triggers the release of glutamate in the spinal cord and stronger pain triggers the release of glutamate and substance P

Substance P results in the increased intensity of pain


Pain pathways cross to a tract ascending the contralateral side of the spinal cord

Pain-sensitive cells in the spinal cord relay information to several areas of the brain

- Somatosensory cortex responds to painful stimuli, memories of pain, and signals that warn of impending pain

- Central nuclei of the thalamus, amygdala, hippocampus, prefrontal cortex and cingulate cortex are associated with emotional associations


Opioid mechanisms

systems that are sensitive to opioid drugs and similar chemicals

Activating opiate receptors blocks the release of substance P in the spinal cord and in the periaqueductal gray area of the midbrain



group of chemicals that attach to the same brain receptors as morphine

Different types of endorphins for different types of pain


Gate theory

suggests that the spinal cord areas that receive messages from pain receptors also receive input from touch receptors and from axons descending from the brain

- These other areas that provide input can close the “gates” and decrease pain perception

- Non-pain stimuli around it can modify the intensity of the pain



a drug or other procedure with no pharmacological effect

- Decreases pain perception by decreasing the brain’s emotional response to pain perception, not the sensation itself

- Decreases response in cingulate cortex but not in the somatosensory cortex



are chemicals related to marijuana that also block certain kinds of pain

- Act mainly in the periphery of the body


Mechanisms of the body to increase sensitivity to pain include:

Damaged or inflamed tissue releases histamine, nerve growth factor, and other chemicals that increase the responses of nearby pain receptors


Certain receptors become potentiated after an intense barrage of painful stimuli

Leads to increased sensitivity or chronic pain later


Emotional pain resembles physical pain in many regards:

Increased activity in the cingulate cortex when someone felt left out of an activity

People taking acetaminophen (Tylenol) reported less incidences of hurt feelings and social pain



The release of histamines by the skin produce itching sensations

- Activates a distinct pathway in the spinal cord to the brain

- Impulses travel slowly along this pathway (half a meter per second)


Pain and itch have an inhibitory relationship

Opiates increase itch while antihistamines decrease itch


chemical coding

Coding in the sensory system could theoretically follow:

- The labeled-line principle: each receptor responds to a limited range of stimuli and sends a direct line to the brain

- Across-fiber pattern: each receptor responds to a wider range of stimuli and contributes to the perception of each of them


Vertebrate sensory systems probably have no pure label-lined codes

The brain gets better information from a combination of responses

- Example: auditory perception and color perception both rely on label-lined codes

Taste and smell stimuli activate several neurons: meaning of the response of a single neuron depends on the context of responses by other neurons



Taste refers to the stimulation of the taste buds, which are receptors on the tongue

Our perception of flavor is the combination of both taste and smell

- Taste and smell axons converge in the endopiriform cortex

Receptors for taste are modified skin cells

Taste receptors have excitable membranes that release neurotransmitters to excite neighboring neurons

Taste receptors are replaced every 10 to 14 days



are structures on the surface of the tongue that contain the taste buds

Each papillae may contain up to ten or more taste buds

Each taste bud contains approximately 50 receptors

Most taste buds are located along the outside edge of the tongue in humans


Procedures that alter one receptor but not others can be used to identify taste receptors

Adaptation refers to reduced perception of a stimuli due to the fatigue of receptors

Cross-adaptation refers to reduced response to one stimuli after exposure to another


Western societies have traditionally described

sweet, sour, salty and bitter tastes as the “primary” tastes and four types of receptors

- Evidence suggests a fifth type of glutamate receptor (umami)

- Different chemicals also result in different temporal patterns of action potentials and activity in the brain

- Taste is a function of both the type of cell activity and the rhythm of cell activity


The saltiness receptor

permits sodium ions to cross the membrane

(Results in an action potential)


Sourness receptors

close potassium channels when acid binds to receptors
(Results in depolarization of the membrane)


Sweetness, bitterness, and umami receptors

activate a G protein that releases a second messenger in the cell when a molecule binds to a receptor


Bitter receptors

sensitive to a wide range of chemicals with varying degrees of toxicity

About 25 types of bitter receptors exist


Taste add. info

We are sensitive to a wide range of harmful substances, but not highly sensitive to any single one

Different nerves carry taste information to the brain from the anterior two-thirds of the tongue than from the posterior tongue and throat

Taste nerves project to a structure in the medulla known as the nucleus of the tractus solitarius (NTS)

- Projects information to various parts of the brain


Various areas of the brain are responsible for processing different taste information

- The somatosensory cortex responds to the touch aspect of taste

-The insula is the primary taste cortex


Each hemisphere of the cortex is also responsive to the ________ of the tongue

ipsilateral side


Genetic factors and hormones can account for some differences in taste sensitivity

Variations in taste sensitivity are related to the number of fungiform papillae near the tip of the tongue

Supertasters have higher sensitivity to all tastes and mouth sensations in general



the sense of smell and refers to the detection and recognition of chemicals that contact the membranes inside the nose

Critical in most mammals for finding food and mates, and avoiding danger

Even humans can follow a scent trail to some extent, and we get better with practice

Important for our food selection; linked to taste


Humans tend to prefer the smell of potential romantic partners who smell different from themselves and their family members

Decreases the risk of inbreeding

Increases the probability that children will have a wide range of immunities


Olfactory cells

line the olfactory epithelium in the rear of the nasal passage and are the neurons responsible for smell

Olfactory receptors are located on cilia which extend from the cell body into the mucous surface of the nasal passage


Vertebrates have hundreds of olfactory receptors which are highly responsive to some related chemicals and unresponsive to others

- Proteins in olfactory receptors respond to chemicals outside the cells and trigger changes in G protein inside the cell

- G protein then triggers chemical activities that lead to action potentials


Olfaction movement

Axons from olfactory receptors carry information to the olfactory bulb

Chemicals that smell similar excite neighboring areas; chemicals that smell different excite more separated areas

Coding in the brain is determined by which part of the olfactory bulb is excited

The olfactory bulb sends axons to the cerebral cortex where messages are coded by location


Olfaction over time

Olfactory receptors are replaced approx every month, but are subject to permanent impairment from massive damage

Receptors regenerate w/in a month

Individual diff in olfaction exist

Women detect odor more readily than men; brain responses are stronger

Ability to detect a faint odor and become more sensitive to it is characteristic of young adult women; seems to be influenced by hormones

Mice w/ a gene that controls a channel through which most K+ travels to reach the olfactory bulb developed a sense of super smell


vomeronasal organ (VNO)

is a set of receptors located near the olfactory receptors that are sensitive to pheromones



are chemicals released by an animal to affect the behavior of others of the same species


The VNO and pheromones are important for most mammals, but less so for humans

The VNO is tiny in human adults and has no receptors


Humans unconsciously respond to some pheromones through receptors in the olfactory mucosa

Example: synchronization of menstrual cycles in women



is the experience of one sense in response to stimulation of a different sense

Estimates suggest 10% of 6- and 7- year olds, though it develops gradually

Some axons from one cortical area may branch into another cortical area