Week 6 Flashcards
Auditory System
• 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
Sound
amplitude- loudness
frequency- pitch
complexity- timbre
Ear anatomy
inner ear- cochlear, semi-circular canals, round window, oval window
middle ear- ossicles
outer ear- external auditory canal, auricle or pinna
Transmit Sound – Outer Ear
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
Transmit Sound – Middle Ear
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
Transmit Sound – Inner Ear
• Action of the stapes at the oval window produces pressure changes that propagate through cochlear • Pressure causes basilar membrane to vibrate
Transduction
At auditory threshold, the hair cell
displacement is 100 picometers
Equivalent to 10mm at the top of the
Eiffel Tower
Pitch Perception
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
Pitch Perception
Auditory processing is tonotopic
Auditory Pathway
• 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)
Subcortical - Sound Localisation
• 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
Interaural Time Difference
• As sound source moves left or right of centre, time to each ear differs • Medial SO generates a map of time differences • Coincidence detectors
Interaural Intensity Difference
• Head acts to block sound reaching one ear • Lateral SO - intensity comparison • Cross Inhibition
Subcortical - Sound Localisation
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
Auditory Cortex
Tonotopic Columnar organisation (like V1) but based on frequency (rather than orientation) Both ears contribute to processing from early
Auditory Dysfunction – Hearing Loss
3 broad classes of hearing loss: 1. Conduction deafness 2. Sensorineural deafness 3. Central deafness
Conduction Deafness
• 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
Sensorineural Deafness
• 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
Central Deafness
• 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
Vestibular System
• 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
Labyrinth of the Inner Ear
Labyrinth of the Inner Ear Ampulae Utricle Saccule Vestibular part of Cranial Nerve VII Facial Nerve Auditory Nerve Cochlea
Semicircular Canals
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
Otolith Organs
• 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
Vestibular System
• 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
Vestibulo-Occular Reflex
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
Somatosensory System
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.
Exteroception
• 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
Exteroceptive Receptors
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
Proprioception
• 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
Interoception
• 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
Somatosensory Pathways
• 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)
Dermatomes
• 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
Pathways
Dorsal Column-Medial
Lemniscus
• 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
Pathways
The Anterolateral System
• 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
Cortex
• 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
Pain Perception
• 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
Pain
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
Pain Dysfunction
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
Chemical Senses
• 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
Chemical Senses - Olfaction
• 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
Olfactory Receptors
• 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)
Olfaction - Pathway
• 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)
Chemical Senses - Gustatory
• 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
Taste Receptors
• 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
Gustatory
• 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)
Super-Tasters
• 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)
Chemical Senses - Dysfunction
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
