Chapter 7
Audition, the Body Senses, and the
Chemical Senses
Audition
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The stimulus
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Sounds vary in their:
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Pitch – a perceptual dimension of sound; corresponds to their
fundamental frequency
Loudness – corresponds to intensity
Timbre – corresponds to complexity
Anatomy of the ear
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Sound is funneled via the pinna (external ear) through the ear canal
to the tympanic membrane (eardrum), which vibrates with the sound
The middle ear is located behind the tympanic membrane and
includes the middle ear bones, the ossicles (malleus, incus and
stapes)
The malleus connects with the tympanic membrane and transmits
vibrations via the incus and stapes to the cochlea, the sructure that
contains the receptors
Anatomy of the ear
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The cochlea is part of the inner ear; it is filled with fluid, therefore
sounds transferred through the air must be transferred into a
liquid medium; the ossicles aid in this transmission
The cochlea is divided into 3 sections: the scala vestibuli, scala
media, and scala tympani
The receptive organ, the organ of Corti, consists of the basilar
membrane, the hair cells, and the tectorial membrane
The auditory receptor cells are called hair cells, and they are
anchored, via Deiter’s cells, to the basilar membrane
Sound waves cause the basilar membrane to move relative to the
tectorial membrane, which bends the cilia of the hair cells; this
bending produces receptor potentials
Audition
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Hair cells
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Hair cells contain cilia (hair-like appendages involved in movement or
in transducing sensory info)
The hair cells form synapses with dendrites of bipolar neurons whose
axons bring auditory info to the brain
The Auditory pathway
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The organ of Corti sends auditory info to the brain by means of the
cochlear nerve, a branch of the vestibulocochlear nerve (8th cranial
nerve)
The pathway goes through the midbrain to the auditory cortex
located in the temporal lobe
Auditory info is represented tonotopically, i.e. topographically
organized mapping of different frquencies of sound that are
represented in a particular region of the brain
Perception of pitch
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Place coding
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Detecting moderate to high frequencies
The system by which info about different frequencies is coded (i.e.
neural representation of info) by different locations on the basilar
membrane
Good evidence is seen for place coding with cochlear implants (an
electronic device surgically implanted in the inner ear that can enable
a deaf person to hear) because most speech sounds are of higher
frequencies, and cannot be represented by rate coding
Rate coding
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Detecting low frequencies
The system by which info about different frequencies is coded by the
rate of firing of neurons in the auditory system
Auditory perception
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Perception of loudness
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The axons of the cochlear nerve inform the brain of the loudness of a
stimulus by altering their rate of firing (Louder the sound, higher rate
of firing)
Perception of timbre
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When the basilar membrane is stimulated by a complex sound (such
as a musical instrument), different portions respond to each of the
overtones (the frequency of complex tones that occurs at multiples of
the fundamental frequency)
Perception of spatial location
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Neurons in our auditory system respond selectively to different
arrival times of the sound waves at the left and right ears in order
to perceive the spatial location of a sound (phase difference)
By analyzing the timbre of a sound, we can perceive if a sound is
in front or behind us
Vestibular system
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Consists of the vestibular sacs (respond to the force of gravity
and inform the brain about the head’s orientation) and the
semicircular canals (respond to angular acceleration, i.e. changes
in head rotation, but not to steady acceleration)
The functions include balance, maintenance of the head in an
upright position, and adjustment of eye movement to compensate
for head movements
Anatomy
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Vestibular sacs:
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Utricle & saccule
In the canals, there is an enlargement called the ampulla, which is
where the sensory receptors reside
The sensory receptors are hair-like and their cilia are embedded in
the cupula
Vestibular system
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Pathway
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Vestibular nerve is part of the 8th cranial nerve
Project to cerebellum, spinal cord, medulla, and pons
Also connects to 3rd, 4th and 6th CN (control eye muscles) in order to
adjust eyes during any head movements
Somatosenses
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Provide info about what is happening on the surface of our body
and inside it
Cutaneous sense – sensitivity to stimuli that involve the skin;
touch
Kinesthesia – perception of the body’s own movement
Organic sense – a sense modality that arises from receptors
located within the inner organs of the body
Cutaneous senses respond to pressure, vibration, heating,
cooling, and events caused by tissue damage
Kinesthesia is provided by stretch receptors in skeletal muscles
and tendons that report changes in muscle length to the CNS
Somatosenses
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Anatomy of the skin and its receptive organs
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Humans have both hairy and glabrous (hairless) skin
Hairy skin:
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Free nerve endings – detect painful stimuli and changes in temp
Ruffini corpuscles – respond to indentation of skin
Pacinian corpuscles – respond to rapid vibrations
Glabrous skin
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Free nerve endings, Ruffini and Pacinian corpuscles
Meissner’s corpuscles – touch-sensitive end organs
Merkel’s disk – the touch-sensitive end organs found adjacent to sweat
ducts
Somatosenses
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Touch perception
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When the Pacinian corpuscle is bent relative to the axon, the
membrane becomes depolarized
Most info about tactile stimulation is precisely localized
Adaptation
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Temperature
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A moderate, constant stimulus applied to the skin fails to produce any
sensation after it has been present for a while
Due to the physical
2 types of thermal receptors: one responds to warmth, the other to
coolness
Pain
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Accomplished through free nerve endings on skin
At least 3 types of nociceptors
Gustation
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Stimuli
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5 qualities: bitter, sour, sweet, salty, umami
Flavor is the combination of gustation (taste) and olfaction (smell)
Anatomy of taste bud and gustatory cells
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Tongue, palate, pharynx and larynx contain ~10,000 taste buds
Most of these receptive organs are arranged around papillae:
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Fungiform papillae (anterior 2/3 of tongue)
Foliate papillae (edges of back of tongue)
Circumvallate papillae (posterior third of tongue)
Taste buds consist of groups of ~20-50 receptor cells, with cilia
located at the end of each cell that project through the opening of the
taste bud (pore) into the saliva
Taste receptor cells form synapses with bipolar neurons whose
axons convey gustatory info through the 7th, 9th and 10th CN
Gustation
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Perception of gustatory info
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The tasted molecule binds with the receptor and produces changes in the
membrane potential
Different substances bind with different types of receptors producing different
taste sensations
Salty
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Sour
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Typical stimulus is plant alkaloid such as quinine
Perhaps family of bitter receptors
GPCR called gusducin
Sweet
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Respond to hydrogen ions in acidic solutions
Bitter
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Simple sodium channel, blocked by the drug amiloride
Respond to sugar molecules (e.g. glucose, fructose)
Also coupled to gusducin
Umami
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Taste of MSG
Specialized metabotropic glutamate receptor may be responsible
Gustation
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Gustatory pathway
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Info from anterior tongue travels through chorda tympani, a branch of
the facial (7th) nerve; info from posterior tongue travels through
lingual branch of 9th CN; 10th CN carries info from palate and
epiglottis
First “relay station” is the nucleus of the solitary tract (NTS), in the
medulla, which then projects to the thalamus, then to the primary
gustatory cortex, located at base of frontal cortex and in the insular
cortex
Olfaction
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Stimulus
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Odorants
Anatomy
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6 million olfactory receptors located on olfactory epithelium, located
at the top of the nasal cavity
Receptor cells are bipolar neurons whose cell bodies lie in the
cribiform plate
Olfactory bulbs lie at the base of the brain on the ends of the
olfactory tracts
Each olfactory cell sends an axon onto the olfactory bulb, where it
synapses with dendrites of mitral cells (in the olfactory glomeruli),
and the projects thorough the olfactory tracts to the amygdala,
pyriform cortex, and entorhinal cortex
Olfaction
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Transduction
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A G protein called Golf activates an enzyme that opens sodium
channels of the olfactory cell
In humans there are ~500-1000 different olfactory receptors
Perception of specific odors
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How can a (relatively) small amount of receptors lead to such a vast
array of smells?
A particular odorant binds to more than one receptor, thus different
odorants produce different patterns of activity in different glomeruli
The spatial pattern of olfactory info is maintained in the olfactory
cortex