Chapter 7 The Other Sensory

Systems

Audition

• Our senses have evolved to allow us to detect and interpret biologically useful information from our environment .

• However, we do not detect all sensory information in the world.

• Some sensory information lies beyond our ability to detect it.

• We also tend to focus on information that is important or relevant.

Audition

• Audition refers to our sense of hearing.• Audition depends upon our ability to detect

sound waves.• Sound waves are periodic compressions of

air, water or other media.• Sound waves are “transduced” into action

potentials sent to the brain.

Audition

• The amplitude refers to the height and subsequent intensity of the sound wave.

• Loudness refers to the perception of the sound wave.– Amplitude is one factor.

• Frequency refers to the number of compressions per second and is measured in hertz.– Related to the pitch (high to low) of a

sound.

Fig. 7-1, p. 196

Audition

• Anatomist distinguish between:– The outer ear– The middle ear– The inner ear

Audition

• The outer ear includes the pinna and is responsible for:– Altering the reflection of sound waves into

the middle ear from the outer ear.– Helping to locate the source of a sound.

Audition

• The middle ear contains the tympanic membrane which vibrates at the same rate when struck by sound waves.

• Three tiny bones (malleus, incus, & stapes) transmit information to the oval window or a membrane in the middle ear.

Fig. 7-2, p. 197

Audition

• The inner ear contains a snail shaped structure called the cochlea which contains three fluid-filled tunnels (scala vestibuli, scala media, and the scala tympani).

• Hair cells are auditory receptors that excite the cells of the auditory nerve when displaced by vibrations in the fluid of the cochlea.– Lie between the basilar membrane and the

tectorial membrane in the cochlea.

Fig. 7-3, p. 198

Audition

• Pitch perception can be explained by the following theories:

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

• Place theory - each area along the basilar membrane is tuned to a specific frequency of sound wave.

Fig. 7-4, p. 199

Audition

• 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.

Audition

• Volley principle states that the auditory nerve can have volleys of impulses (up to 4000 per second) even though no individual axon approaches that frequency by itself. – provides justification for the place theory

and

Audition

• The primary auditory cortex is the ultimate destination of information from the auditory system.– Located in the superior temporal cortex.

• Each hemisphere receives most of its information from the opposite ear.

Audition

• The superior temporal cortex contains area MT which allows for the detection of the location of sound.

• Area A1 of the brain is important for auditory imagery.

• The auditory cortex requires experience to develop properly.– Auditory axons develop less in those who

are deaf from birth.

Fig. 7-5, p. 200

Audition

• 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.

Fig. 7-6, p. 201

Audition

• Areas around the primary auditory cortex exist in which cells respond more to changes in sound.

• 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.

Audition

• About 99% of hearing impaired people have at least some response to loud noises.

• Two categories of hearing impairment include:

1. Conductive or middle ear deafness.

2. Nerve deafness.

Audition

• Conductive or 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 near the middle ear.

• 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.

Audition

• Nerve or inner-ear deafness results from damage to the cochlea, the hair cells or the auditory nerve.

• 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 (rubella, syphilis, inadequate oxygen to the brain during birth, repeated exposure to loud noises, etc).

Audition

• Tinnitus 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

invade parts of the brain previously responsive to sound.

– Similar to the mechanisms of phantom limb.

Audition

• Sound localization depends upon comparing the responses of the two ears.

• Humans localize low frequency sound by phase difference and high frequency sound by loudness difference.

Audition

• Three mechanisms:1. High-frequency sounds (2000 to 3000Hz)

create a “sound shadow”, making the sound louder for the closer ear.

2. The difference in the time of arrival at the two ears is most useful for localizing sounds with sudden onset.

3. Phase difference between the ears provides cues to sound location for localizing sounds with frequencies up to 1500 Hz.

Fig. 7-7, p. 202

Fig. 7-8, p. 203

Fig. 7-9, p. 203

The Mechanical Senses

• 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.

The Mechanical Senses

• The vestibular 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.

The Mechanical Senses

• The vestibular organ consists of two otolith organs (the saccule and untricle) and three semicircular canals.

• The otolith organs have calcium carbonate particles (otoliths) that activate hair cells when the head tilts.

• The 3 semicircular canals are oriented in three different planes and filled with a jellylike substance that activates hair cells when the head moves.

Fig. 7-10, p. 206

The Mechanical Senses

• The vestibular sense is integrated with other sensations by the angular gyrus.– Angular gyrus is an area at the border

between the parietal and temporal cortex.

The Mechanical Senses

• The 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 neurons, an elaborated neuron ending, or a bare ending surrounded by non-neural cells that modify its function.

Fig. 7-11, p. 207

The Mechanical Senses

• The pacinian corpuscle is a type of touch receptor that detects sudden displacement or high-frequency vibrations on the skin.

• Mechanical pressure bend the membrane.– increases the flow of sodium ions and

triggers an action potential.

Fig. 7-12, p. 207

The Mechanical Senses

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

• Information from receptors below the head enter the spinal cord and travel through the 31 spinal nerves to the brain.

Fig. 7-13, p. 208

The Mechanical Senses

• 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 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.

Fig. 7-14, p. 208

The Mechanical Senses

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

• Various areas of the 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.

The Mechanical Senses

• Pain depends on several types of axons, several neurotransmitters, and several brain areas.

• Mild pain triggers the release of glutamate while stronger pain triggers the release of glutamate and substance P.– Substance P results in the increased

intensity of pain. – Morphine and opiates block pain by

blocking these neurotransmitters.

Fig. 7-15, p. 210

The Mechanical Senses

• Opioid mechanisms are systems that are sensitive to opioid drugs and similar chemicals.

• Activating opiate receptors blocks the release of substance P in the spinal chord and in the periaqueductal grey of the midbrain.

• Enkephalins refer to opiate-type chemical in the brain.

• Endorphins- group of chemicals that attach to the same brain receptors as morphine.

Fig. 7-16, p. 211

The Mechanical Senses

• Discrepancy in pain perception can partially be explained by genetic differences in receptors.

• Gate theory suggests that the spinal cord areas that receive messages from pain receptors also receive input from other skin receptors and from axons descending from the brain.– These other areas that provide input can

close the “gates” and decrease pain perception.

The Mechanical Senses

• Special heat receptors account for the pain associated with a burn.

• Heat receptors can also be activated by acids.

• Capsaicin is a chemical found in hot peppers that directly stimulates these receptors and also triggers an increase in the release of substance P.

The Mechanical Senses

• Pain activates the hypothalamus, amygdala, and cingulate cortex.– results in an emotional component to pain.

• A placebo is a drug or other procedure with no pharmacalogical effect.

• Placebo’s decrease pain perception by decreasing the brains emotional response to pain perception and not the sensation itself.

Fig. 7-17, p. 211

The Mechanical Senses

• 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 number of sodium gates in nearby pain receptors. • Pain responses are thus magnified.

– Certain receptors become potentiated after an intense barrage of painful stimuli.• leads to increased sensitivity or chronic

pain later.

The Mechanical Senses

• Pain is best controlled by preventing the brain from being bombarded with pain messages.

• Bombarding the brain with pain messages results in the increased sensitivity of the pain nerves and their receptors.

The Mechanical Senses

• Morphine and other opiates are the primary drugs for controlling serious pain.

• Morphine inhibits slow, dull pain carried by thin unmyelinated axons.– Sharp pain is conveyed by thicker

myelinated axons.– Not influenced by morphine and

endorphins.• Ibuprofen, an anti-inflammatory drug, controls

pain by reducing the release of chemicals from damaged tissues.

The Mechanical Senses

• The release of histamines by the skin produce itching sensations.

• The release of histamine by the skin 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.

The Chemical Senses

• Coding in the sensory system could theoretically follow:

1. The labeled-line principle in which each receptor responds to a limited range of stimuli and sends a direct line to the brain.

2. Across-fiber pattern in which each receptor responds to a wider range of stimuli and contributes to the perception of each of them.

The Chemical Senses

• 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 and the meaning of the response of a single neuron depends on the context of responses by other neurons.

The Chemical Senses

• Taste refers to the stimulation of the taste buds.

• Our perception of flavor is the combination of both taste and smell. – Taste and smell axons converge in the

endopiriform cortex.

The Chemical Senses

• 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.

The Chemical Senses

• Papillae are structures on the surface of the tongue that contain the taste buds.

• Each papillae may contain zero to ten taste buds.

• Each taste bud contains approximately 50 receptors.

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

Fig. 7-18, p. 217

The Chemical Senses

• 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.

The Chemical Senses

• 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.

The Chemical Senses

• 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.

The Chemical Senses

• 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, as well as the rhythm of cell activity.

The Chemical Senses

• Bitter receptors are sensitive to a wide range of chemicals with varying degrees of toxicity.– Over 40 types of bitter receptors exist.

• Most taste cells contain only a small number of these receptors.

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

The Chemical Senses

• Taste coding in the brain depends upon a pattern of responses across fibers in the brain.

• The brain determines taste by comparing the responses of several types of taste neurons.

• Receptors converge their input onto the next cells in the taste system.

• Cells thus respond best to a particular taste but others as well.

The Chemical Senses

• 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 other parts of the

brain.

Fig. 7-19, p. 219

The Chemical Senses

• 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 ipsilateral side of the tongue.

The Chemical Senses

• 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.

Fig. 7-20, p. 220

The Chemical Senses

• Olfaction is the sense of smell and refers to the detection and recognition of chemicals that contact the membranes inside the nose.

• Olfaction is more subject to adaptation than our other senses.

• Olfactory cells line the olfactory epithelium in the rear of the nasal passage and are the neurons responsible for smell.

Fig. 7-21, p. 221

The Chemical Senses

• 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.

• Olfaction processes a wide variety of airborne chemicals, hence the need for many different types of receptors.

The Chemical Senses

• 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.

Fig. 7-22, p. 222

The Chemical Senses

• Axons from olfactory receptors carry information to the olfactory bulb in the brain.

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

• Chemicals excite limited parts of the olfactory bulb with similar chemicals exciting similar parts.

The Chemical Senses

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

• The cortex connects to other areas that control feeding and reproduction.– Both behaviors are highly influenced by

smell.

Olfaction

• Olfactory receptors are replaced approximately every month, but are subject to permanent impairment from massive damage.

• Anosmia refers to a general lack of olfaction.• Specific anosmia refers to the inability to

smell a single type of chemical.

The Chemical Senses

• Individual differences in olfaction exist regarding olfaction.

• Women detect odor more readily than men and brain responses are stronger.

• The ability to attend to a faint odor and become more sensitive to it is characteristic of young adult women and thus seems to be influenced by hormones.

The Chemical Senses

• The vomeronasal organ (VNO) is a set of receptors located near the olfactory receptors that are sensitive to pheromones.

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

The Chemical Senses

• 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

cycle’s in women.

The Chemical Senses

• Synesthesia is the experience of one sense in response to stimulation of a different sense.– Estimates suggest 1 in every 500 people

(Day, 2005).• fMRI case studies show activity in both the

auditory and visual cortex responsive to color when exposed to spoken language.– Suggests some axons from one area have

branches to other cortical regions.