Chapter 7 Outline
I. Audition
A. Sound waves are periodic compressions of air, water or other media. They vary according to amplitude and frequency.
1. Amplitude: Intensity of a sound wave. Loudness is the perception of intensity.
2. Frequency: Number of compressions per second, measured in hertz (Hz) of a sound.
3. Pitch: Perception closely related to frequency (the higher the frequency of a sound, the higher its pitch).
4. Most adult humans can hear vibrations from 15 or 20 Hz to just less than 20,000
Hz.
B. The anatomy of the ear is described in terms of three regions: the outer ear, the middle ear and the inner ear.
1. The outer ear includes the pinna (structure of flesh and cartilage attached to the side of the ear) and the auditory canal. The pinna helps us locate the source of a sound by altering reflections of sound waves.
2. The middle ear is comprised of the tympanic membrane (eardrum) which vibrates at the same frequency as sound waves which strike it. The tympanic membrane is attached to three tiny bones (hammer, anvil and stirrup).
3. The inner ear consists of the oval window, which receives vibrations from the tiny bones of the middle ear and the cochlea, which contains three fluid-filled tunnels: the scala vestibuli, scala media and scala tympani.
4. The stirrup causes the oval window to vibrate, setting in motion all the fluid in the cochlea.
5. The auditory receptors (hair cells) lie between the basilar membrane and the tectorial membrane in the cochlea.
6. When fluid in the cochlea vibrates, a shearing action occurs which stimulates hair
cells; these cells then stimulate the auditory nerve cells (eighth cranial nerve).
C. Pitch Perception.
1. Frequency Theory: We perceive certain pitches when the basilar membrane
vibrates in synchrony with a sound, causing the axons of the auditory nerve to
produce action potentials at the same frequency.
2. Place Theory: Each area along the basilar membrane is tuned to a specific frequency and vibrates whenever that frequency is present. Each frequency activates hair cells at only one place along the basilar membrane and the brain distinguishes frequencies by what neurons are activated.
3. The current prevalent theory combines modifications of both frequency and place
theories.
4. Volley Principle of pitch discrimination: Sound waves produce a volley of
impulses by auditory nerve fibers, which in turn signal high frequencies to the
brain.
5. High frequency vibrations strike the basilar membrane causing a traveling wave.
This causes displacement of hair cells near the base (where the stirrup meets the
cochlea). Low frequency sounds produce displacement near the apex of the
cochlea.
D. Pitch perception in the cerebral cortex:
1. Auditory information crosses over between the superior olive and inferior colliculus so that each hemisphere receives its major auditory input from the opposite ear.
2. Primary auditory cortex: Ultimate destination of auditory information located in the temporal lobe.
3. Each cell in primary auditory cortex responds best to one tone. Cells preferring a given tone in the auditory cortex cluster together providing a map of the sounds referred to as a tonotopic map. Thus, the cortical area with the greatest response indicates what sound or sounds are heard.
4. Damage to the primary auditory cortex leads to deficits in processing auditory
information as opposed to a loss of hearing.
E. Hearing Loss
1. Conductive deafness (middle-ear deafness): Failure of the bones of the middle
ear to transmit sound waves properly to the cochlea. This deafness can be
corrected by surgery or hearing aids.
2. Nerve deafness (inner-ear deafness): Damage to the cochlea, hair cells or auditory nerve causing a permanent impairment in hearing in one to all ranges of frequencies. Nerve Deafness can be inherited, due to prenatal problems or early childhood disorders.
3. Tinnitus: Frequent or constant ringing in the ear. Tinnitus is common in people
with nerve deafness and is due in some cases to a phenomenon like phantom limb.
F. Humans localize low frequency sounds by differences in phase. We localize high
frequencies by loudness differences. We can localize a sound of any frequency by its
time of onset if the onset is sudden enough.
II. The Mechanical Senses
1. The vestibular organ monitors head movements and directs compensatory movements of the eyes.
2. The vestibular organ is comprised of two otolith organs (the saccule and utricle) and three semicircular canals.
3. Calcium carbonate particles (otoliths) lie next to hair cells in the otolith organs and excite them when the head tilts in different directions.
4. The three semicircular canals are filled with a jelly like substance and lined with
hair cells. Acceleration of the head causes this substance to push against hair cells,
which in turn causes action potentials from the vestibular system to travel via part
of the eighth cranial nerve to the brainstem and cerebellum.
C. The somatosensory system is the sensation of the body and its movements.
1. Examples of touch receptors are pain receptors, Ruffin endings, Meissner's
corpuscles and Pacinian corpuscles.
2. Pacinian corpuscles detect sudden displacements or high-frequency vibrations on
the skin.
3. Somatosensory information from the head enters the CNS through the cranial
nerves. Information from touch receptors below the head enters the spinal cord
through the 31 spinal nerves and passes toward the brain.
4. Each spinal nerve has a sensory component and a motor component. Each sensory
spinal nerve innervates a limited area of the body called a dermatome.
5. Sensory information from the spinal cord is sent to the thalamus before traveling to the somatosensory cortex in the parietal lobe.
6. The somatosensory cortex receives information primarily from the contralateral
side of the body.
D. Pain
1. Substance P: A neuromodulator or cotransmitter with glutamate used by
unmyelinated or thinly myelinated axons to relay pain information to the spinal
cord.
2. Capsaicin: A chemical that causes neurons, which contain substance P to release it
suddenly. This chemical eventually leads to insensitivity to pain because neurons
release substance P faster than they can resynthesize it.
3. Gate Theory: Information not related to pain travels to the spinal cord and closes
the "gates" for each pain message.
4. Opioid Mechanisms: Systems that are responsive to opiate drugs and similar
chemicals.
5. Opiate receptors in the brain bind to endorphins (endogenous morphines) such as
met-encephalon and leu-enkephalin. Endorphin activity causes analgesia (pain
relief).
6. Certain painful stimuli activate neurons that release endorphins in the
periaqueductal gray area in the midbrain.
7. Endorphins block release of substance P in the spinal cord and brainstem.
Endorphins (and other opioids) block dull pain but not sharp pain.
8. Naloxone: A drug which blocks opiate receptors.
9. Transcutaneous electrical nerve stimulation (TENS): Prolonged, mild electrical
shock applied to the limbs to release endorphins and decrease pain.
10. Consequences of morphine use for analgesia are addiction, suppression of breathing (large doses) and a temporary weakening of the immune system. However, morphine taken under hospital conditions almost never becomes addictive.
11. The body also has mechanisms to increase pain after tissue has been damaged and inflamed. This pain sensitization is a result of the body releasing histamine, nerve growth factor, and other chemicals that are necessary to repair the body.
III. The Chemical Senses
A. Labeled-line principle: Receptors of a sensory system that respond to a limited range of stimuli and send a direct line to the brain.
B. Across-fiber pattern principle: Each receptor of a sensory system responds to a wide range of stimuli and contributes to the perception of each of them.
C. Taste: The stimulation of taste buds. Taste differs from flavor which is the combination of taste and smell.
1. Taste receptors are actually modified skin cells which last only about 10-14 days before being replaced.
2. Mammalian taste receptors are located in taste buds, located in papillae (structures on the surface of the tongue). In adult humans taste buds are located mainly on the outside edge of the tongue.
3.We have at least four types of taste receptors: sweet, sour, salty and bitter.
4. Adaptation: Decreased response to a stimulus as a result of recent exposure to it (e.g., After soaking the tongue in two sour solutions one after the other, the second solution will not taste as sour as the first).
5. Cross-adaptation: A reduced response to one taste because of exposure to another. There is little cross-adaptation in taste.
6. Urnami: A taste associated with glutamate. Chemicals that interfere with other tastes do not interfere with umami and responses of taste neurons to glutamate do not correlate strongly with response to other taste stimuli. Thus this taste may indicate we have more than four kinds of taste receptors.
7. Saltiness receptors work by allowing salt to cross its membrane. The higher the concentration of salt the greater the response of the receptors (i.e., the greater the action potential).
8. Sour taste receptors close potassium channels, preventing potassium from leaving the cell when acids bind to them.
9. Sweetness, bitterness, and umami receptors work by activating a G-protein that
releases a second messenger within the cell.
10. Phenythiocarbomide (PTC) is a chemical whose taste is controlled by a single dominant gene. Approximately one-forth of Americans hardly taste PTC, one-half taste it as bitter, and one-forth taste it as extremely bitter. People who taste it as extremely bitter are supertasters and have the largest number of fungiform papillae (the type near the tip of the tongue).
11.The perception of taste depends on a pattern of responses across taste fibers.
12. Taste information from the anterior two-thirds of the tongue travels to the brain via the chorda tympani, a branch of the seventh cranial nerve (facial nerve). Information from the posterior tongue and throat is carried to the brain along branches of the ninth and tenth cranial nerves. These three nerves project to the nucleus of the tractus solitarius (NTS) in the medulla. The NTS relays information to the pons, lateral hypothalamus, amygdala, thalamus and two areas of the cerebral cortex (one responsible for taste and the other for the sense of touch on the tongue).
D. Olfaction: the sense of smell
1. Olfactory cells: Neurons which line the olfactory epithelium and are responsible for smell. Each olfactory cell has cilia (threadlike dendrites) where receptor sites are located.
2. Continued stimulation of an olfactory receptor produces rapid adaptation.
3. Axons of olfactory cells carry information to the olfactory bulb. The olfactory bulb sends its axons to several other forebrain areas and especially activates the prefrontal cortex; strong connections are made between the olfaction system and brain areas responsible for feeding and reproduction.
4. Anosmia: A general lack of olfaction (a specific anosmia is the inability to smell a specific chemical).
5. Olfactory receptors are made up of a family of proteins which traverse the cell membrane seven times and respond to chemicals outside the cell by causing changes in a G-protein inside the cell. The G-protein provokes chemical activities that lead to an action potential.
6. Many estimate that humans have hundreds of different types of olfactory receptor proteins.
7. In the olfaction system the response of one receptor can identify the approximate
nature of the molecule and the response of a larger population of receptors enables
more precise recognition.
E. Vomeronasal Sensation and Pheromones
1. The vomeronasal organ (VNO): A set of receptors located near, but separate from the olfactory receptors. VNO receptors cross the membrane seven times like olfactory receptors, but there are relatively few VNO receptors. The VNO receptors also have a different amino acid sequence from those in the olfactory system and they do not show adaptation after continued exposure.
2. Pheromones: Chemicals released by an animal that affect the behavior of other members of the same species, especially sexually. Predominantly, the VNO controls the responses to pheromones.
3. Unlike most mammals, the VNO is small in adult humans. Moreover, no receptors have been found in the human VNO.
4. Although humans seem to lack a functional VNO, they still respond to pheromones
through an unknown mechanism.