Biological Bases of Behavior
7: Audition, the Body Senses,
and the Chemical Senses
Sound Waves
7.2
Divisions of the Ear

Outer ear:


Middle ear:


Channel to
tympanic
membrane
Ossicles
Inner ear:

Cochlea
7.3
The Cochlea

The cochlea is formed from three chambers:




Scala vestibuli and scala media are separated by a membrane
Scala tympani and scala media are separated by the basilar
membrane
Hair cells within the organ of Corti transduce sound
waves into nerve impulses
The organ of Corti consists of



Basilar membrane (the base)
Tectorial membrane (the roof)
Hair cells in between
7.4
Auditory Hair Cells

Two types of hair cells are located within the human organ
of Corti

Inner hair cells (approximately 3500) form a single line of cells
along the basilar membrane
 Destruction

Outer hair cells (approximately 12,000) are arranged in three rows
along the basilar membrane
 Outer

of inner hair cells eliminates hearing
hair cells serve a structural function
Cilia project from the top of each hair cell
 The
tectorial membrane is attached to the outer hair cell cilia
 When sound waves move the basilar and tectorial membranes, the cilia
bend in one direction or the other
 Shear of the cilia generates a receptor potential that releases a
neurotransmitter
7.5
Auditory Transduction



Cilia tips are joined by a
fiber link
Cilia movement produces
tension of the link which
opens an ion channel in
the adjacent tip
Calcium and potassium
ions flow into the cilia
and produce a
depolarization
7.6
Auditory Pathways

Afferent pathways:
Spiral (cochlear nerve)
ganglion
 Through cochlear nuclei

 To
superior olivary complex
 To inferior colliculus
 To medial geniculate
 To auditory cortex

Efferent pathway:

Olivocochlear bundl
From
superior olivary complex
7.7
Place Coding of Pitch

Different frequencies produce maximal distortion of
basilar membrane

Sound vibration produces a traveling wave
High frequency: near base of basilar membrane
 Moderate frequency: near apex of basilar membrane


Different regions of the basilar membrane project to
different areas of auditory cortex



base to medial and posterior, apex to lateral and anterior
Throughout the auditory system there is a tonotopic
representation in which adjacent neurons receive signals
from adjacent areas of the basilar membrane
Place coding can account for medium to high sound
frequencies, low frequency sounds are coded by rate
of firing
7.8
Support for Place Theory

Observations of traveling waves
by von Bekesy


Antibiotics


Different frequencies produce
maximal displacement at different
points along the basilar membrane
Induce hair cell loss first at base of
basilar membrane, which produces a
loss of hearing for high frequency
sounds
Cochlear implants restore speech
perception by stimulating
different regions of the basilar
membrane
7.9
Perception of Spatial Location

Arrival time difference for high frequency sound
7.10
Perception of Spatial Location

Horizontal locations
Arrival time difference for high frequency sound
 Arrival phase difference for low frequency sound
 Intensity difference for high frequency sound


Vertical location
All above methods won’t work for vertical
middle-plane
 Timbre difference pinna (external ear)


Same for front versus back
7.11
Analysis of the Auditory System

The various components of the auditory system detect
sounds, determine sound location, and recognize sound
identity



Ventral (what) system: auditory cortex, inferior frontal gyrus
Dorsal (where) system: superior parietal cortex, superior
frontal gyrus
Lesions placed at different levels of the auditory
system:



Auditory association cortex: auditory agnosia
Bilateral auditory cortex: animal can detect pitch, intensity
diff, but not “tunes”
Lateral lemniscus: animal is deaf
7.12
Anatomy of the Vestibular Apparatus

Vestibular sac:


Semicircular canal:


One of the three ring-like structures of the vestibular apparatus that detect
changes in head rotation, related to dizziness and nystagmus (rhythmic eye
movements)
Ampulla:


One of a set of two receptor organs (utricle and saccule) in each inner ear
that detects changes in the tilt of the head, related to nausea
An enlargement in a semicircular canal; contains the cupula.
Cupula:

A gelantinous mass found in the ampulla of the semicircular canals; moves
in response to the flow of the fluid in the canals.
7.13
The Vestibular Pathway

The receptor cells:


Similar to the hair cells found in the cochlea; method
of transduction is also similar to hair cells of the
cochlea.
Vestibular ganglion:

A nodule on the vestibular nerve that contains the cell
bodies of the bipolar neurons that convey vestibular
information to the brain.
7.14
7.15
Somatosenses

The somatosenses provide information relating to events on
the skin and to events occurring within the body

Cutaneous senses receive various signals from the skin that form
the sense of touch
 Pressure
 Vibration
 Heating/cooling
 Stimuli

that damage tissue (and produce pain)
Kinesthesia provides information about the body position and
movement
 Kinesthetic
signals arise from receptors located within the joints, tendons,
and muscles

Organic senses arise from receptors in and around internal organs
7.16
Morphology of Skin
Epidermis
Dermis
7.17
Cutaneous Senses

Three different sensations are reported to the brain by
receptors localized within skin

Touch involves perception of pressure and vibration of an object
on the skin
 Pacinian
corpuscles detect rapid vibrations, with large receptive fields
 Meissner’s corpuscle (glabrous skin only) detect slow vibrations
 Ruffini corpuscles (plus Merkel’s disks on glabrous skin) respond to
indentation, with slow adaptation
 Free nerve endings around hairs detect movements of hairs

Temperature is detected by warmth and cold receptors
 Receptor
activation is relative to the baseline temperature
 The receptors (free nerve endings) lie at different levels of the skin (cold
are close to the surface of the skin)

Pain is associated with skin tissue damage
 Detected
by free nerve endings (nociceptors)
7.18
Somatosensory Pathways



The dorsal columns carry
information related to touch
(precisely localized)
The spinothalamic tract
carries pain and temperature
signals (poorly localized)
Mostly contra lateral
7.19
Somatosensory Pathways

Somatosensory cortex is
organized into columns

There may be 5-10 cortical
maps of the body surface
7.20
Pain

Pain serves a functional role for survival


Pain stimuli induce species-typical escape and
withdrawal responses


Persons lacking pain receptors are at great risk
Pain is a motivational force that can activate behavior
Pain involves tissue destruction induced by
Thermal stimuli
 Mechanical force



Pain reception is poorly localized (as is temperature)
Pain involves an emotional component (that can be used
to modify the magnitude of pain perception)
7.21
Pain Receptors

Receptors for pain (nociceptors)




Free nerve endings networks within the skin that respond to
intense pressure
Free nerve endings that respond to heat, acids, and capsaicin
(the active ingredient in chili peppers)
Receptors that are sensitive to ATP
Pain receptors are found in:
Skin
 Sheath around muscles, internal organs
 Cornea of the eye
 Pulp of the teeth


Pain receptors are activated by mechanical, chemical
stimulation
7.22
Analgesia


Analgesia refers to the reduction of the perception of pain
Analgesia can be induced by external and internal stimuli







Hypnosis
Massage
Acupuncture
Opiates
Placebo
Attention shifts
Pain stimuli activate primary somatosensory cortex
(sensory) and anterior cingulate cortex (emotional)


anterior cingulate cortex is involved in the aversiveness of pain
hypnosis and PET scanner study: intensity/unpleasantness
7.23
Opiates and Pain



Exogenous opiates (drugs such as opium, morphine,
and heroin) induce analgesia
Environmental stimuli which can activate endogenous
opioids also induce analgesia
Certain analgesia could be blocked by Naloxon


Naloxone reversibility is taken as an indication of opiate
involvement (such as acupuncture induced analgesia)
Focal brain stimulation can reduce pain


At periaqueductal gray matter in particular is effective
Brain stimulation activates a descending pathway that
modulates pain (Basbaum and Fields model)
7.24
Opiate-Induced Analgesia Circuit

Why have pain sensors and analgesia circuit?
(experiments on rats, Maier etal 1982)
7.25
Gustation

Gustation is related to eating foods and drinking
liquids
Molecules within the food dissolve in saliva and
activate one of four receptor types
 Each receptor type provides information about a food

Sweet:
safe foods
Salty: source of sodium ions
Bitter: poisonous foods
Sour: spoiled foods

Flavor involves a mixture of taste and olfaction
7.26
Transduction of Taste

Taste molecules bind with a receptor, alter membrane
potential, and induce receptor potentials

Saltiness: best stimulus is sodium chloride
 Receptor


for saltiness may be a simple sodium channel
Sourness receptors respond to hydrogen ions present in acid
solutions
Bitterness: typical stimulus is an alkaloid (e.g. quinine)
 Receptors

involve a hydrophobic residue
Sweetness: typical stimulus is a sugar
 Receptors
have a hydrogen ion site
7.27
Transduction of Taste
7.28
7.29
Gustatory Processing

Gustatory information is transmitted through cranial
nerves 7 (anterior tongue), 9 (posterior tongue), and 10
(palate and epiglottis)



First relay station for taste information is the nucleus of the
solitary tract (medulla)
Taste information is then transmitted to primary gustatory
cortex, to the amygdala, and to the hypothalamus
Recordings from chorda tympani (7th cranial nerve)
indicate that taste fibers respond to more than one taste
quality and to temperature

In cortex, the major groups of taste-sensitive neurons were salty
and sweet
7.30
7.31
Olfaction

Olfaction is the second chemical sense
The stimulus for odor (known as ordorants) consists
of volatile substances having a molecular weight in
the range of approximately 15 to 300
 Almost all odorous compounds are lipid soluble and
of organic origin
 For humans, olfaction is the most enigmatic of the
modalities


Flavor involves a mixture of taste and olfaction
7.32
Anatomy of the Olfactory Apparatus

Olfactory epithelium:


Olfactory bulb:


The protrusion at the end of the olfactory tract; receives information from
the olfactory receptors.
Mitral cell:


The epithelial tissue of the nasal sinus that covers the cribiform plate;
contains the cilia of the olfactory receptors.
A neuron located in the olfactory bulb that receives information from
olfactory receptors; axons of mitral cells bring information to the rest of
the brain.
Olfactory glomerculus:

A bundle of dendrites of mitral cells and associated terminal buttons of
the axons of olfactory receptors.
7.33
7.34
7.35