Chapter 7Chapter 7Hearing, Balance, and the Cutaneous and Chemical Senses

The Auditory System:The Auditory System:

Sound - Vibrations in a material medium, such as air, water, or metal.

Sound waves vary along three dimensions:• Frequency refers to the number of vibrations

per second and is measured in hertz (Hz). We perceive the frequency of a sound as pitch.

• Amplitude refers to the loudness of a sound wave and is measured in decibels (dB).

• Timbre refers to the combination of multiple frequencies that make up complex sounds and give them their characteristic qualities.

Wave Forms for the Wave Forms for the Three Dimensions of a Three Dimensions of a

SoundSound

The The HumanHumanEarEar

The Transduction of Sound The Transduction of Sound Waves into Neural ImpulsesWaves into Neural ImpulsesThe inner hair cells have a

resting potential of -60 mV.When cilia bend in the direction

of the longest cilium the membrane depolarizes.

This leads to a rapid influx of Ca2+ ions into the hair cells, which results in the release of glutamate.

Auditory Auditory PathwaysPathways

Cochlea Cross Cochlea Cross SectionSection

Pitch Perception: Early Pitch Perception: Early ResearchResearch

Place Theory of Pitch Perception• The view that different sounds activate

nerve fibers at different locations on the basilar membrane.

• High-pitched sounds activate the nerve fibers at the base of the membrane near the oval window

• Low-pitched sounds stimulate nerve fibers at the opposite end of the basilar membrane.

Pitch Perception: Early Pitch Perception: Early ResearchResearchFrequency Theory of Pitch Perception

• The view that the firing rate in the auditory nerve matches the frequency of the sound. That is, the basilar membrane vibrates in synchrony with the sound wave.

• We now know that the firing rate matches the frequency because of the volley principle:• while one group of neurons in the auditory nerve

is firing, another group is recovering from its previous activity

• the end result being that the combined firing of all the groups matches the frequency of the sound.

Pitch Perception: Current Pitch Perception: Current TheoryTheoryThe current theory of pitch perception uses a

combination of the previous theories:• From 20 Hz to 400 Hz, frequency theory accounts for

pitch perception (the firing rate of individual neurons in the auditory nerve directly matches the frequency of the sound).

• From 400 Hz to 4 kHz, volley principle takes over.• Beyond 4 kHz, place theory comes into play (the place

of maximal vibration on the basilar membrane determines the pitch that we perceive).

• Additionally, both place theory and the volley principle work for sounds from about 1 kHz to 4 kHz (may explain our greater sensitivity to pitches within this range).

Detection of LoudnessDetection of LoudnessThe nervous system has two

mechanisms for determining the intensity of a stimulus:• The rate of firing of individual neurons• The number of neurons firing

The higher the firing rate, or the greater the number of neurons firing, the more intense the stimulus.

Detection of Sound Detection of Sound ComplexityComplexityPure tones are sounds of only one

frequency; complex sounds have two or more frequencies.

Combination of frequencies produces what we perceive as the timbre of a particular sound.

According to the place theory, because each sound frequency activates a specific part of the basilar membrane, a complex sound produces a unique pattern of neural activity.

Sound LocalizationSound LocalizationFor both low-pitched sounds and high-

pitched sounds, the cues to sound localization are based on differential time of arrival at the two ears.

As long as the sound does not come from the median plane, the sound will arrive at one ear slightly before it gets to the other ear, which allows us to locate the direction from which a sound comes from.

The Role of the Auditory The Role of the Auditory Cortex in Sound RecognitionCortex in Sound RecognitionAuditory receptors encode sound:

• Frequency• Intensity• Timbre

Receptors send this information to the primary auditory cortex.

In auditory cortex, some neurons respond selectively to specific aspects of sounds; others react to more complex aspects of the sound stimulus.

The Role of the Auditory The Role of the Auditory Cortex in Sound RecognitionCortex in Sound Recognition

Sound is identified as the neural information moves from the primary auditory cortex to the anterior part of the lateral surface of the superior temporal gyrus

Sound is localized as it moves to the posterior part of the superior temporal gyrus and then to the parietal cortex

Cochlea ImplantsCochlea Implants

The Vestibular SenseThe Vestibular SenseThe sense responsible for

maintaining balance.• Enables us to walk on two feet, keep

our head upright, and adjust our eye movements to compensate for our head movements.

Phillippe Petit

Components of the Components of the Vestibular SystemVestibular SystemVestibular sacs - Provide

information about the position of the head relative to the body.• Utricle and saccule -The two

vestibular sacs containing the vestibular receptor cells, or hair cells.

Semicircular canals - Fluid-filled canals that provide information related to head movements or rotations.• Ampulla, crista, cupula

Vestibular PathwaysVestibular PathwaysVestibular hair cells

• convert information about passive head movement and active head rotation into an increase or decrease in neurotransmitter release

• synapse with bipolar neurons

Cell bodies of bipolar neurons form:• vestibular ganglia (receive input from

vestibular hair cells)

• axons of the vestibular ganglia become the vestibular nerve (combine with cochlear nerve fibers to form the auditory nerve)

Motion SicknessMotion SicknessFeelings of dizziness and nausea; occur

when the body is moved passively without motor activity and corresponding feedback to the brain.

Two types of motion sickness:• Detects movements but motor actions that

could have produced the movement have not occurred

• Detects movement inconsistent with the information about movement sensed by the eyes

The SomatosensesThe SomatosensesThe skin sensations of touch, pain,

temperature, and proprioception.Proprioception -The somatosense

that monitors body position and movement, acts to maintain body position, and ensures the accuracy of intended movements• located in the muscles, tendons, and

joints• essential to the control of movement.

Skin ReceptorsSkin Receptors

• The functions of the skin include:• protecting the internal organs from

injury• helping regulate body temperature by

producing sweat, which cools the body when it becomes too hot

• providing a first line of defense against invading microorganisms.

Receptive Fields and Receptive Fields and Adaptation Rates of Touch Adaptation Rates of Touch

ReceptorsReceptors

Skin ReceptorsSkin Receptors

Skin Receptors:Glabrous Skin Receptors:Glabrous SkinSkin

Somatosensory PathwaysSomatosensory Pathways• The dorsal column-medial lemniscal

system • begins in the spinal cord and transmits

information about touch and proprioception to the primary somatosensory cortex.

• The anterolateral system • begins in the spinal cord and transmits

information about temperature and pain to the brain stem, reticular formation, and the primary and secondary somatosensory cortices.

• The spinocerebellar system • begins in the spinal cord and transmits

proprioceptive information to the cerebellum.

The The SomatosensorSomatosensory Cortexy Cortex

The Experience and Control The Experience and Control of Painof PainPain has both negative and

positive functions:• Chronic pain can be the bane of a

person’s existence.• However, under ordinary

circumstances, pain is extremely useful, warning us of potential injury and inducing us to seek appropriate treatment.

Gate-Control Theory of Gate-Control Theory of PainPainMelzack & Wall (1965)Gate-control theory of pain - Input from

pain receptors will produce the perception of pain only if the message first passes through a “gate” in the spinal cord and lower brain stem structures.

Theory emphasizes that messages from the brain can open or close the spinal cord gate, helping us to understand the psychological nature of pain - why our sensation of pain can be affected by our thoughts and feelings.

Gate-Control Theory of Gate-Control Theory of PainPain

Neuromatrix Theory of Neuromatrix Theory of PainPainMelzack (1999)Neuromatrix theory of pain - A

theory that accounts for types of pain unexplained by the gate-control theory of pain.• Severe, chronic pain existing in the

absence of injury or disease.

The Chemical SensesThe Chemical Senses

Chemical senses include the gustatory and olfactory systems.

Both are intermingled in our eating experiences, in that much of what we report as the taste of food actually comes from its odor.

Taste and SmellTaste and Smell

GustationGustationGustatory sense -The sense of

taste.Tastes can be classified according

to four primary sensations:• Sweet (stimulate sugar receptors)• Sour (stimulate H+ receptors)• Bitter (stimulate alkaloid compound

receptors)• Salty (stimulate NaCl receptors)

Taste ReceptorsTaste Receptors

Papilla - A small, visible bump on the tongue that contains taste bumps.

Taste bud - A cluster of taste receptors that lie either near or within a papilla.

Three kinds of papillae contain taste buds:

• Foliate• Circumvallate• Fungiform

Types of Types of Papillae Papillae and and DistributioDistribution of Taste n of Taste ReceptorsReceptors

Genetics of TasteGenetics of TastePeople differ in their sensitivity to

bitter and some sweet tastes.These individual differences appear to

be partly related to the number of taste buds on the tongue:• Supertasters (25% of people) have the

most taste buds - about 425 per square cm on the tongue tip.

• Medium tasters (50% of people) have about 184 taste buds per square cm.

• Non-tasters (25% of people) have about 96 per square cm.

Mechanisms of Taste Mechanisms of Taste ReceptionReceptionMechanism differs for each of the

four basic tastes:• Salty food activates a taste receptor by

causing Na+ ions to move through Na+ ion channels in the cell membrane.

• H+ ions in sour foods and sugar molecules in sweet foods close the K+ ion channels in receptor membranes, preventing K+ ions from leaving the cell.

• In bitter foods, alkaloid compounds trigger the movement of Ca2+ ions into the cytoplasm from storage sites in the taste receptor, increasing the release of neurotransmitters.

GustatorGustatory y PathwayPathwayss

OlfactionOlfaction

The sense of smell.Habituation - can occur quickly with

smells. Whether pleasant or unpleasant, we rapidly “get used to” smells.

This sensory adaptation is caused by decreased responding by receptors when they are exposed to the same stimulus for a continuous period of time.

Olfactory ReceptorsOlfactory ReceptorsOlfactory epithelium - The mucous

membrane in the top rear of the nasal passage; lined by olfactory receptors.• Humans have approximately 50

million olfactory receptors that detect smell

• other species, such as dogs, may have up to 20 times as many, with each cell having more than 10 times as many cilia.

Olfactory Olfactory Receptors Receptors and and PathwaysPathways