Week 6

Question 1

Auditory System

Answer 1

• Transmit sound to the sensory organ
• Transduce sound energy into a neural signal
• Transmit the neural signal to the brain
• Processing of the neural signal to provide
meaningful (and useful) auditory information

Question 2

Sound

Answer 2

amplitude- loudness
frequency- pitch
complexity- timbre

Question 3

Ear anatomy

Answer 3

inner ear- cochlear, semi-circular canals, round window, oval window
middle ear- ossicles
outer ear- external auditory canal, auricle or pinna

Question 4

Transmit Sound – Outer Ear

Answer 4

Auricle (Pinna)
• Collect sound waves and channel them into the
auditory canal
• Important role in localising sounds - folds
selectively reflect sounds of various frequencies
around the ear and into the auditory canal
• As a sound source changes its location relative to
the head, the frequency profile of these reflections
changes - offering a cue to the location of the
source
Auditory Canal
• Channel sound energy to the tympanic membrane
Tympanic Membrane (ear drum)
• Vibrates in response to air pressure changes of the
sound waves
• Middle ear ossicles are attached to the TM

Question 5

Transmit Sound – Middle Ear

Answer 5

Ossicles
• Middle ear is for impedance matching – sounds in air but sensory
in fluid
• If TM transmitted directly – air to fluid – almost all sound energy
would be lost (reflected back)
• Concentrate the vibrations of the tympanic membrane on a very
small area on the oval window
• Think of how pressure is increased by concentrating a given mass
on a small area - like when someone stands on your foot with a
stiletto heel compared to a wide boot
• In the case of the middle ear, this is a 17 fold increase
• The lever action of the ossicles amplify the vibrations by
approximately 1.3 times
• Combined, this accounts for a 22 fold increase in the strength of
vibrations hitting the tympanic membrane

Question 6

Transmit Sound – Inner Ear

Answer 6

• Action of the stapes at
the oval window
produces pressure
changes that propagate
through cochlear
• Pressure causes basilar
membrane to vibrate

Question 7

Transduction

Answer 7

At auditory threshold, the hair cell
displacement is 100 picometers
Equivalent to 10mm at the top of the
Eiffel Tower

Question 8

Pitch Perception

Answer 8

Auditory processing is tonotopic
• Basilar membrane is a mechanical analyser of
sound frequency
• Structure of the membrane changes continuously
along its length
• Much wider at the apex than the base
• Each point along the membrane responds
preferentially to a different frequency – high at the
base, low at the apex
• Preserved throughout early processing

Question 9

Pitch Perception

Answer 9

Auditory processing is tonotopic

Question 10

Auditory Pathway

Answer 10

• No major pathway (cf
retina-geniculate-striate
of vision) – complex
network
• First ipsilateral cochlear
nuclei
• Ultimately medial
geniculate nucleus of
thalamus (MGN) then
primary auditory cortex
(A1)

Question 11

Subcortical - Sound Localisation

Answer 11

• Localisation of sound sources mediated
subcortically at superior olives (SO)
• 2 ears - sound impinging on each ear slightly
different depending on where the sound is coming
from
• Differs in 2 detectable ways:
• Interaural time difference
• Interaural intensity difference
Each is used to localise sound source

Question 12

Interaural Time Difference

Answer 12

• As sound source
moves left or
right of centre,
time to each ear
differs
• Medial SO
generates a map
of time
differences
• Coincidence
detectors

Question 13

Interaural Intensity Difference

Answer 13

• Head acts to block sound
reaching one ear
• Lateral SO - intensity
comparison
• Cross Inhibition

Question 14

Subcortical - Sound Localisation

Answer 14

Teng et al. (2012):
• Human expert echolocators can discriminate target
offsets of as little as 1.2 degrees (similar to bats)
• Acuity is similar to visual acuity in the far periphery

Question 15

Auditory Cortex

Answer 15

Tonotopic
Columnar
organisation (like
V1) but based on
frequency (rather
than orientation)
Both ears
contribute to
processing from
early

Question 16

Auditory Dysfunction – Hearing Loss

Answer 16

3 broad classes of hearing
loss:
1. Conduction deafness
2. Sensorineural deafness
3. Central deafness

Question 17

Conduction Deafness

Answer 17

• Damage to the tympanic
membrane and ossicles
• E.g. ossicles become fused
and no longer transmit
sound vibrations from the
outer ear to the cochlea
• Does not involve the
nervous system
• Treatment – hearing aid or
bone conduction implants

Question 18

Sensorineural Deafness

Answer 18

• Auditory nerve fibres are
not stimulated properly
• Deafness is permanent
• Infection, trauma,
exposure to toxic
substances
• Loud sounds (e.g. noise
pollution, personal
headsets)
• Streptomycin (antibiotic)
has ototoxic properties
• Tuberculosis patients
treated with streptomycin
had cochlear damage
• In some cases, all the hair
cells in the cochlea were
destroyed - leading to
total deafness
Cochlear Implant
• Bypass hair cells and stimulate
auditory nerve fibres directly
• External processor converts
sound into digital code
• Internal electrode array (in the
cochlear) stimulate the nerve
accordingly
• Uses tonotopic principle
• Time and training to learn to
interpret the signals

Question 19

Central Deafness

Answer 19

• Caused by brain lesions in the
temporal cortex (e.g. stroke) (also
brainstem)
• Results in loss of specific faculties -
like language processing (left lobe)
or discrimination of non-language
sounds (right lobe)
• Unilateral lesions may result in
unilateral hearing loss; bilateral
lesions for bilateral loss
• Remapping may improve hearing
with time and rehab

Question 20

Vestibular System

Answer 20

• Proprioception – information about the movement and
position of body parts
• Especially important – movement and position of the
head – position of whole body; balance; control of
vision
• 5 receptor organs that sense accelerations of the head
• 3 semicircular canals (sense head rotations)
• 2 otolith organs (utricle and saccule) sense linear
acceleration – horizontal movement and tilt
• NOTE – measure accelerations (i.e. changes in speed)
and not constant motion
• Each has a cluster of hair cells that transduce head
motion/position into vestibular signals

Question 21

Labyrinth of the Inner Ear

Answer 21

Labyrinth of the Inner Ear
Ampulae
Utricle
Saccule
Vestibular part of Cranial Nerve VII
Facial Nerve
Auditory Nerve
Cochlea

Question 22

Semicircular Canals

Answer 22

3 perpendicular canals (horizontal, anterior vertical, posterior
vertical) to sense rotations around the three principle axes
• Ampulla contains
diaphragm – cupula –
hair bundles insert into
cupula
• Inertia of fluid exerts
force on hair cells
• Start rotation, fluid lags
so cupula distorts
• Stop rotation, fluid keeps
going so cupula distorts
• In between, no change
Hair cells deform one way for depolarisation (excitation),
other for hyperpolarisation (inhibition)
• Excitation (or inhibition)
as motion initiated
• Baseline through most of
the motion
• Inhibition (or excitation)
as motion stopped
• Left and right act
together as functional
pairs

Question 23

Otolith Organs

Answer 23

• Hair cells into flat membrane
covered in tiny ‘stones’
• Linear acceleration exerts
force that moves the
membrane, distorting the
hair cells
• Translational motion or
gravity
• Otolith system cannot distinguish between tilt and linear
acceleration
• Use tilt to simulate G-force in VR 
• Gravity is a constant
linear acceleration
• So head tilts illicit
continuous activity above
or below baseline firing
rates

Question 24

Vestibular System

Answer 24

• Most movements illicit complex patterns of vestibular
stimulation
• Individual organ signals may be ambiguous due to
combined (complex) movement plus tilts and gravity
• Integrate 3 canals + 2 otoliths + visual and
somatosensory to interpret head and body movement
and positions

Question 25

Vestibulo-Occular Reflex

Answer 25

Head movements illicit compensatory eye movements
to maintain fixation – minimise motion on the retina
• Loss of VOR – oscillopsia (“bouncing vision”)
• Bilateral loss of VOR leaves the patient with the
sensation that the world is moving whenever the
head moves
• Information about head movements signalled by the
vestibular organs is unavailable
• Compensatory eye movements are not made

Question 26

Somatosensory System

Answer 26

Is comprised of three systems:
• Exteroceptive system: senses external stimuli applied to
the surface of the skin.
• Proprioceptive system: senses the position of limbs via
joint angles; body posture; vestibular senses.
• Interoceptive system: senses the general conditions
within the body such as temperature, blood pressure.

Question 27

Exteroception

Answer 27

• 5 senses – touch
• Different aspects – physical contact, temperature, pain
• Many types receptors:
• Mechanoreceptors (different for stroke, pressure, vibration, stretch,
light stroke, erotic touch)
• Temperature receptors (cool, warm, cold, hot)
• Nociceptors (sharp, burn, freeze, slow burn)
• Lots fibres – vary in diameter and myelination
• Conduction speed can vary from 100m/s (large myelinated – most
non-stroking mechano) down to 0.5 m/s (unmyelinated – erotic
touch and slow burn nociceptors)
• Typically touch and proprioception fast, thermal and nociception
slower

Question 28

Exteroceptive Receptors

Answer 28

Meissner corpuscles
• Surface
• Adapt quickly – no sustained
response
• Best response to lateral motion
• Medium sensitivity
Pacinian corpuscles
• Deep
• Respond rapidly, adapt quickly
• Sudden displacement of the
skin
• Respond best to vibration
• High sensitivity
Merkel’s disks
• Surface
• Sustained response, slow adaptation
• Gradual skin indentation
• Best response to edges and points
• Medium sensitivity
Ruffini Endings
• Deep
• Sustained response, slow adaptation
• Best response to skin stretch
• Low sensitivity
Free nerve endings
• No specialised structure
• Temperature and pain

Question 29

Proprioception

Answer 29

• Sensory and motor systems rep info about state of
muscles and limbs
• Muscle length and speed, muscle stretch, muscle
contraction, joint angle, excess stretch or force
• A variety of receptors embedded in muscles, tendons,
and joint capsules
• Some involved in conscious sensation of muscle
activity, some in unconscious monitoring of body for
posture, some in reflexes
• Patellar reflex – stretches muscle spindle in quadricep
– spinal reflex to contract quad and relax hamstring

Question 30

Interoception

Answer 30

• Visceral sensations rep status of internal body organs
• Drive behaviour for survival: respiration, hunger,
thirst, nausea (food aversion), arousal
• Loss of air – feeling of suffocation becomes all
consuming goal – hold breath when needed knowing
that signal will tell you when to stop
• O2 and CO2 sensors in carotid bodies and in
respiratory centres of medulla and hypothalamus
• Ondine’s Curse – damage to medullary centres and
loss of ‘air hunger’ – death and failure of auto
breathing in sleep

Question 31

Somatosensory Pathways

Answer 31

• Somatosensory information ascends from each side of
the body to the cortex via two major pathways
• Dorsal Column-Medial Lemniscus carries information
about touch and proprioception
• Anterolateral System carries information about pain
and temperature
• Both fed by dorsal roots of spinal nerves (or
trigeminal sensory nerves in the head)

Question 32

Dermatomes

Answer 32

• Afferent nerve fibres
over a specific area of
the body converge on
specific dorsal roots in
the spinal cord
• Considerable overlap so
damage or loss of one
does not result in a large
deficit in sensation of the
dermatome
• Quadruped setup

Question 33

Pathways
Dorsal Column-Medial
Lemniscus

Answer 33

• Touch and proprioception
(fast – large myelinated)
• Ascend ipsilaterally in
dorsal columns and
synapse on dorsal column
nuclei in medulla
• Decussate then ascend via
medial lemniscus to
contralateral VPN

Question 34

Pathways

The Anterolateral System

Answer 34

• Pain and temperature (slow –
small myelinated and
unmyelinated)
• Synapse on cord entry
• Decussate and ascend by
contralateral anterior lateral
spinal cord (some ascend
ipsilaterally
• Multiple tracts
• Spinothalamic to thalamus
(several nuclei) – main noxious,
thermal and visceral
• Others to various brainstem
structures

Question 35

Cortex

Answer 35

• Primary (S1) and secondary (S2)
somatosensory cortex (anterior
parietal)
• Sensory Homunculus –
somatotopic organisation (map of
the body) in S1 (contralateral)
and S2 (bilateral)
• S1 and S2 output to association
cortex – posterior parietal
• Damage to S1 is not marked by
major deficits in sensation. This is
probably due to the numerous
parallel pathways in the two
systems

Question 36

Pain Perception

Answer 36

• Pain is adaptive because it stops us from doing damage to
ourselves
• Encourages us to seek treatment or to treat ourselves
• The cortical representation of pain is diffuse - no single
structure is responsible
• S1 and S2 respond to painful stimuli but are not necessary
- removal does not reduce sensitivity to pain.
• Full removal of an entire hemisphere has little effect

Question 37

Pain

Answer 37

Anterior Cingulate Cortex
• Implicated in mediating the
perception of pain
• PET studies showed increase
in activity when participants
touched very hot or very cold
objects
• Likely involved in the
emotional response to pain
• Prefrontal lobotomy results in
reduced emotional response
to pain but no change in pain
threshold
Descending control
Periaqueductal grey (PAG)
has analgesic effects
• Electric stimulation
reduces pain
• Receptors for opiate
based pain drugs
• Endorphins modulate
PAG activity

Question 38

Pain Dysfunction

Answer 38

No Pain – Congenital Insensitivity to Pain (CIP)
• Miss C. - the woman who felt no pain
• Normal university student had never felt pain
• Bit the tip of her tongue off while chewing; burnt her legs on a
heater; felt no electric shock
• No sneezing or coughing; no corneal reflex
• Not even autonomic reaction to pain (e.g. no increased heart
rate)
• Had multiple health problems - bad joints (lack of protection)
• Died age 29 of massive infection
• Genetic – mutation in gene for subunit of a sodium channel
found in nociceptors – extremely rare

Question 39

Chemical Senses

Answer 39

• Chemicals in the environment are cues
• General state of the local environment – toxins, pH, ionic,
etc
• Food (constituent and emitted); predator/prey scent;
mate scent; kin identification
• Volatile chemicals – odours
• Non-volatile chemicals – tastes
• Single cells react and respond to local chemical
environment
• Chemical senses evolved very early
• Multiple responses – identification; affective; initiate
physiological changes

Question 40

Chemical Senses - Olfaction

Answer 40

• Receptors in upper
nasal passage
embedded in
olfactory mucosa
• Olfactory receptor
neuron (ORN)
• Dendrites in nasal
passage
• Axons pass through
cribriform plate
into olfactory bulbs
• Synapse then
project via olfactory
tracts to brain

Question 41

Olfactory Receptors

Answer 41

• Olfactory receptors (OR) – G-protein coupled receptors
located on cilia on the ORN dendrite
• ~400 OR types (~1000 genes but most broken)
• Only 1 OR type per ORN
• Each OR responds in varying degrees to many odours –
component processing – odours identified by pattern of
activity across many receptor types
• Intensity changes the perceived smell as lower affinity
receptors come online
• Rapid turn over of ORNs (1-2 months) – continually
replaced from basal stem cells
• Human 10M ORNs (dog 200M)

Question 42

Olfaction - Pathway

Answer 42

• Different receptor types
scattered throughout
olfactory mucosa
• Same receptor types
project to same general
spot in olfactory bulb
• Some type of
topographic layout but
not based on similarity
of smell
• Large convergence at
olfactory bulb – 100
times
ORNs-> olfactory bulb (Olfactory tract - bulb to cortex directly (not via thalamus) and mostly ipsilateral): (Each olfactory tract projects to several structures in medial temporal lobe)
- 2 main pathways from amygdala/piriform
->amygdala->hypothalamus and other limbic structures (Limbic: motivational responses, autonomic, emotional)
->piriform cortex (primary olfactory)->thalamus->orbital frontal cortex (Thalamic-orbitofrontal: conscious perception of odours, memory, attention)

Question 43

Chemical Senses - Gustatory

Answer 43

• 5 primary tastes: salty, sweet, sour, bitter, umami
• More complex taste from higher level cortical
processing of combined input
• Information provided
• Umami – protein
• Sweet – carbs/high caloric
• Salty – ion/water balance
• Bitter and sour – warning system (detect bitter 1000 times
better than salty)
• Maybe 6th taste of fat – usually thought of as
texture/feel – some evidence for tastebud response –
fat is important
• Taste receptors – tongue
and oral cavity – clusters
of 50-100 in taste buds
• Taste buds located on
small protuberances –
papillae

Question 44

Taste Receptors

Answer 44

• Few taste buds on centre tongue
• Receptors not neural but synapse like connection to
neuron
• Multiple receptors feed each neuron
• Non-taste papillae – secretory and somatosensory -
mouthfeel, temperature and nociception (irritants
like capsaicin, CO2, acetic acid)
• Receptor for each of the 5 primary tastes – 1
receptor protein in each receptor cell
• High receptor turnover (10-30 days) drops with age
especially after 70
• 33 gustatory
receptor proteins –
1 umami, 2 sweet,
30 bitter
• Sour and salty act
directly on ion
channels

Question 45

Gustatory

Answer 45

• 3 gustatory afferents
• Solitary nucleus in
medulla then project
to ventral posterior
medial (VPM) nucleus
of thalamus
• VPM projects to
primary gustatory
cortex (superior lip of
lateral fissure near
face area of
somatosensory
homunculus)

Question 46

Super-Tasters

Answer 46

• 25% super (and 25% nontasters)
• Taste buds vary individually – 120 to 670 per cm2 –
women more than men
• Super-tasters experience much more intense taste
(especially bitter) and more pain to irritants
• Young children very sensitive to bitter – protection
from accidental poisoning (put everything in their
mouths!)
• More papillae seems to be the underlying cause
(more densely packed)

Question 47

Chemical Senses - Dysfunction

Answer 47

Anosmia - the inability to smell
• Common cause: blow to the head such that the
brain shifts in the skull and the axons from the
olfactory receptors are sheared off where they
enter the skull (cribriform plate)
Ageusia - the inability to taste
• Very rare
• Probably due to the diffuse afferent tracts from the
taste receptors