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objectives
Review the function of the auditory, visual and vestibular sy Examine the functional connections of the 3 sensory system
Highlight relevant clinical correlates
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introduction
Sight is dependent not only on a substantial portion of the ce
cortex, but also upon six cranial nerves (IIVII).
Perception is the function of the retina, optic nerve, tract, raand cortex.
The oculomotor, trochlear and abducens nerves move the ey
Eyeball sensations such as pain, touch and pressure are medthe ophthalmic nerve, and the facial nerve innervates orbicumuscle.
70% of all sensory receptors are in the eyes
40% of the cerebral cortex is involved in processing visual inf
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visual system
The eye is the organ of vision; modified to suit its function Neural impulses arise from the eye to the brain via the optic
tract(CNII)
The nerve is in fact a tract of the CNS: Rods and cones Bipolar cells
Ganglion cells
Rods, cones and intrinsically photosensitive retinal ganglion[IPRGCs] are the photosentive cells
The eye also has non-image forming pathways to parts of th
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internal structure of the eye
Fig 16.7
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anterior structures of the eye
Fig 16.8
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retina
a) Light passes outward
through the entirethickness of the retinabefore exciting thephostoreceptor cells; thelectrical signals flowinward from neuron toneuronFig 16.9
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optic chiasma
is a flattened bundle of nerve fibres situated at the junction of thwall and floor of the third ventricle
The superior surface is attached to the lamina terminalis, and infrelated to the hypophysis cerebri, from which it is separated by tdiaphragma sellae.
The anterolateral corners of the chiasma are continuous with the
nerves, and the posterolateral corners are continuous with the oA small recess, the optic recess of the third ventricle, lies on its ssurface.
fibres originating from the nasal half of each retina cross the meat the chiasma to enter the optic tract of the opposite side.
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pathway of transmission of visualimpulses
From the retina the impulses reach the LGN of the thalamus The LGN is a 6 layered structure connected to the superior c
via the superior brachium
Layers 1&2 have giant cells [magnocellular]
Layers 3-6 have small sized cells [parvocellular]
Fibres that would have crossed in the optic chiasma relay in&6
Fibres from the temporal hemi-retina relay in layers 2,3 &5.
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the retinofugal projection
The Optic Nerve, Optic Chiasm, and Optic Tract
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the lateral geniculate nucleus (LGN
Inputs Segregated by Eye and Ganglion Cell Type
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the lateral geniculate nucleus (LGN Receptive Fields
Receptive fields of LGN neurons: Identical to the gangliocells that feed them
Magnocellular LGN neurons: Large, monocular receptivefields with transient responsegross detail: low acuity,movement, peripheral
Parvocellular LGN cells: Small, monocular receptive fieldwith sustained response fine detail: color, high acuity,foveal
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the lateral geniculate nucleus (LGN
Nonretinal Inputs to the LGN
Primary visual cortex provides 80% of the synapticinput to the LGN
Brain stem neurons provide modulatory influence onneuronal activity
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geniculocalcarine tract
From LGN the fibres of the optic radiation take 2 courses to occipital lobe via the geniculocalcarine tract
Traverses the sublentiform and retrolentiform parts of intercapsule curving posteriorly towards the calcarine sulcus
Some of the fibres travel or loop over the temporal horn of
ventricle as Meyers Loop These fibres carry fibres from the upper quadrants of the ey
the contralateral eye
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optic radiations
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the primary visual cortex
Brodmanns area V1 17, V2:association areas 18,19.
Also called the striate cortex
Found on the occipital lope(posterior hemispheric pole)
There is a point to point projection of the retina onto the cacortex
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anatomy of the striate cortex Retinotopy
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anatomy of the striate cortex
Inputs to the Striate Cortex Magnocellular LGN neurons: Project to layer IVC
Parvocellular LGN neurons: Project to layer IVC
Koniocellular LGN axons: Bypasses layer IV to makesynapses in layers II and III
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stripe/band of Gennari in area 17
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summary of point-point projection
Axons from Rt halves of both retinae terminate in the Rt LGvisual info is relayed to visual cortex of Rt Hemisphere.
Axons from upper quadrants peripheral to the macula end ipart of LGN----- ant 2/3 of visual cortex above calcarine sul
Lower quadrants peripheral to macula project to lateral porLGN----ant 2/3 below the calcarine sulcus.
Macula projects to the large posterior region of the LGN----pof the visual cortex in the region of the occipital pole.
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visual field representation on calcarcortex Left visual fieldrep on Rt LGN and visual cortex of right
Upper half of visual field rep on lat portion of LGN and in thbelow the calcarine cortex
Lower half of visual field projected on the medial portion ofon cortex above calcarine cortex
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non-image forming visual
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non-image forming visualpathways
Other visual pathways include optic tracts tothe:
Superior colliculi to control extrinsic eyemuscles,
Pretectal nuclei in the midbrain to mediatepupillary light reflexes,
Suprachiasmatic nucleusin the hypothalamus toregulate daily biorhythms.
These are referred to as the non-imageforming pathways of the eye.
i f h i l
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connections of the visual system anreflexes LGN connected to sup colliculus via sup brachium---pretecta
EW---CNIII---Sphincter pupillae
Pupillary light reflex/Consensual light reflex Light shone into one eye
retina sends fibres from both eyes to the optic tract
Impulses stimulate olivary pretectal nucleus
Pretectal nucleus sends impulses to both ipsilateral and contralat
EW sends pregang fibres to ciliary ganglion
Destination: sphincter pupillae
Constriction of BOTH PUPILS
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direct and consensual light reflexes
Afferent nervous impulses travel from the retina through the opt
optic chiasma, and optic tract A small number of fibres leave the optic tract and synapse on ne
the pretectal nucleus (close to the superior colliculus).
Impulses are passed by axons of the pretectal nerve cells to theparasympathetic nuclei (Edinger-Westphal nuclei) of CNIII on bot
Here, the fibres synapse, and the parasympathetic nerves travel
CNIII to the ciliary ganglion in the orbit. Postganglionic parasympathetic fibres pass through the short cilto the eyeball and to the constrictor pupillae muscle of the iris. Bconstrict in the consensual light reflex because the pretectal nucfibres to the parasympathetic nuclei on both sides of the midbra
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accommodation reflex
When eyes are directed from a distant to a near object, con
the medial recti brings about convergence of the ocular axe
Lens thicken to increase refractive power by contraction of tmuscle,
Pupils constrict to restrict the light waves to the thickest cenof the lens.
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argyll-robertson pupil
Characterised by a small pupil, of fixed size that does not re
Contracts with accommodation
Usually caused by a neurosyphilitic lesion to fibres that run pretectal nucleus to the parasympathetic nuclei (Edinger-Wnuclei) of CNIII on both sides
The fact that the pupil constricts with accommodation implthe connections between the parasympathetic nuclei and thconstrictor muscle of the iris are intact
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horners syndrome
consists of (1) constriction of the pupil (miosis),
(2) slight drooping of the eyelid (ptosis),
(3) enophthalmos,
(4) vasodilation of skin arterioles,
(5) loss of sweating (anhydrosis).
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horners syndrome
symptoms result from an interruption of the sympathetic ne
supply to the head and neck.
Pathologic causes include lesions in the brainstem or cervicathe spinal cord that interrupt the reticulospinal tracts descefrom the hypothalamus to the sympathetic outflow in the lacolumn of the first thoracic segment of the spinal cord.
Such lesions include multiple sclerosis and syringomyelia. Trthe stellate ganglion due to a cervical rib or involvement of tganglion in a metastatic lesion may interrupt the peripheral the sympathetic pathway
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lesions of visual pathway
Optic nerve- Blindness
Chiasmabitemporal hemianopia
Tract- homonymous hemianopia
Geniculocalcarine tract- homonymous hemianopia with macsparing.
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auditory systemwhen he had said these things, he cried, He that hath ears to hear, let [Luke 8:8 -The Holy Bible; King James Version]
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auditory system
The ear is an organ of hearing and equilibrium adapted to it
Nuclei associated with these functions are found in the brai
The nuclei also project to other areas so as to facilitate refle
Hearing is second in importance among the special senses oyielding first place only to sight.
The auditory system consists of the external ear, middle ear,of the internal ear, cochlear nerve, and pathways in the cennervous system (CNS).
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Outer ear
Middle ear
Inner ear
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four major divisions of auditory syste
1. The outer ear
- pinna- ear canal
- eardrum
2. The middle ear
- three ossicle bones;
(malleus, incus, stapes)
- two major muscles
(stapedial muscle, tensor
tympani)- Eustachian tube
3. The inner ear
- cochlea (hearing)
- vestibular system (balance)
4. The central auditory system
four major divisions of auditory syste
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four major divisions of auditory syste
t
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Three parts of outer ear
1) Pinna
2) Ear canal
3) Ear drum
Major function of outer ear
1) protection
2) amplification
3) sound localization
outer ear
outer ear: pinna (binaural cue to sound
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t Left
t Right
* Different distances from source to each ear
=> different arrival times (Interaural time-difference)
and different sound level (interaural level-difference)
Right
ear
Left
ear
Sound
outer ear: pinna (binaural cue to soundsource location)
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t l d iddl
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external and middle ear
The external ear consists of the auricle or pinna and the ext
acoustic meatus, with the latter being separated from the mby the tympanic membrane.
The function of the external ear is to collect sound waves, wcause vibration of the tympanic membrane.
t l d iddl
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external and middle ear
The vibration is transmitted across the middle ear by a chain
ossicles (little bones): the malleus, incus, and stapes. The malleus is attached to the tympanic membrane and art
with the incus, which articulates in turn with the stirrup-shastapes.
The footplate of the stapes occupies the fenestra vestibuli (o
window) in the wall between the middle and internal ears; tthe foot plate is attached to the margin of the fenestra vestithe annular ligament, composed of elastic connective tissue
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external and middle ear
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external and middle ear
Protection against the effect of sudden, excessive noise is pr
reflex contraction of the tensor tympani and stapedius muscwhich are inserted on the malleus and stapes, respectively.
The tensor tympani is innervated by the trigeminal nerve, astapedius is innervated by the facial nerve
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inner ear cochlea
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perilymph
perilymph
endolymph
inner ear- cochlea
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inner earresonance of basilar
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Fig
membrane
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inner earinner hair cells (IHC) & outer hacells (OHC)
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Inner hair cells: produce sensation of hearing
Outer hair cells: modify BM response and act as amplification system
cells (OHC)
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bony and membranous labyrinth
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bony and membranous labyrinth
The bony labyrinth is located in the petrous part of the temp
bone, which forms a prominent oblique ridge between the mand posterior cranial fossae.
The labyrinth is a system of tunnels within the bone.
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bony labyrinth
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bony labyrinth
Three semicircular canals extend posterolaterally from the v
and the cochlea constitutes the anteromedial part of the bolabyrinth. The cochlea has the shape of a snail shell; its baseagainst the deep end of the internal acoustic meatus, whichinto the posterior cranial fossa.
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membranous labyrinth
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membranous labyrinth
The delicate membranous labyrinth conforms, for the most
the contours of the bony labyrinth. There are two dilations, the utricle and the saccule, in the ve
the bony labyrinth. Three semicircular ducts arise from the
A patch of sensory epithelium is present on the inner surfacutricle, the saccule, and each semicircular duct.
The saccule is continuous with the cochlear duct through a channel known as the ductus reuniens. The cochlear duct coalong its entire length, the organ of Corti.
membranous labyrinth
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membranous labyrinth
Whereas the lumen of the membranous labyrinth is filled w
endolymph, the interval between the membranous and bonlabyrinths is filled with perilymph.
The vestibular part of the membranous labyrinth is suspendthe bony labyrinth by trabeculae of connective tissue.
The cochlear duct is firmly attached along two sides to the b
of the cochlear canal.
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cochlea
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The thin unspecialized wall of the cochlear duct apposing the scais called the vestibular or Reissner's membrane, and the thicker w
apposing the scala tympani constitutes the specialized basilar mon which the organ of Corti rests.
The basilar membrane is of special importance in the physiologybecause it responds to vibration of the stapes in the following m
vibration of the foot plate of the stapes produces correspondingthe perilymph, beginning with that of the vestibule. Sound wavepropagate through the scala vestibuli, Reissner's membrane, theendolymph in the cochlear duct, and the basilar membrane to thtympani. These same waves create a vibration of the membranefenestra cochleae at the base of the scala tympani; this is essenteliminate the damping of pressure waves that would otherwise obone-encased fluid.
pathway of audition
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p y
Organ of corti
Cochlea nerve Dorsal and ventral cochlea nuclei
3 acoustic striae
Commisural fibres
Superior olivary body
Lateral lemnsicus
Inferior colliculi
MGN
Auditory cortex
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Tonotopy!
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pathway of audition
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p y
The last link in the auditory pathway consists of the auditory
in the sublentiform part of the internal capsule, through whmedial geniculate body projects to the primary auditory cortemporal lobe.
pathway of audition
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y
This primary auditory area, corresponding to Brodmann's ar
and 42, is located in the floor of the lateral sulcus, extendingslightly onto the lateral surface of the hemisphere. A landmprovided by the anterior transverse temporal gyri (Heschl'sconvolutions) on the dorsal surface of the superior tempora
pathway of audition
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The area receives afferent fibres from the tonotopically orga
ventral part of the medial geniculate body. The tonotopic pattern in the auditory area is such that wher
for low-frequency sounds end in the anterolateral part of thfibres for high-frequency sounds go to its posteromedial par
pathway of audition
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Projection fibres to the auditory area arise principally in the
geniculate body and form the auditory radiation of the intercapsule. The anterior part of the primary auditory area is cowith the reception of sounds of low frequency, and the postof the area is concerned with the sounds of high frequency.unilateral lesion of the auditory area produces partial deafnboth ears, the greater loss being in the contralateral ear. Thi
explained on the basis that the medial geniculate body recemainly from the organ of Corti of the opposite side as well afibres from the same side.
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projections
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Superior temporal gyrus
Projection fibres to brainstem From inferior colliculus to spinal cord[tectospina tract]
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auditory reflexes
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A few acoustic fibres from the inferior colliculus pass forwar
superior colliculus, which influences motor neurons of the cregion of the spinal cord through the tectospinal tract. The scolliculus also influences neurons of the oculomotor, trochleabducens nuclei through indirect connections in the brain stThese pathways provide for reflex turning of the head and etoward the source of a sudden loud sound.
The tectospinal tract is concerned with reflex postural moveresponse to visual stimuli
auditory reflexes
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Some axons from the superior olivary nucleus terminate in t
nuclei of the trigeminal and facial nerves for reflex contractitensor tympani and stapedius muscles, respectively. Contrathese muscles in response to loud sounds reduces the vibratympanic membrane and the stapes, thereby protecting thestructures in the cochlea from mechanical damage.
high-tone deafness
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Persistent exposure to loud sounds causes degenerative cha
the organ of Corti at the base of the cochlea, causing high-todeafness. This is prone to occur in workers exposed to the scompression engines or jet engines and in those working fohours on farm tractors. High-tone deafness was formerly enmost frequently among workmen in boiler factories and is ssometimes called boilermakers' disease.
disorders of hearing: conductiondeafness
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deafness Conduction deafness
conductive deafness - blockage of sound transmission throuand/or middle ear without damage to cochlea
Sound vibrations cannot be conducted to the inner ear
e.g. in ruptured tympanic membrane, otitis media, otosclero
Conductive deafness may resolve or may be treatable in som
disorders of hearing: conductiondeafness
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deafness
normal tympanic membrane ruptured tympanic membrane otitis me
disorders of hearing: sensorineuraldeafness
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Results from damage to any part of the auditory pathway
loss of auditory function because of loss of cochlear hair celcochlear nerve neurons they connect to
Sensorineural deafness can result from direct damage to thecells, or indirectly from damage to the blood supply.
Sensorineural deafness is not reversible in mammals.
important facts 1
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The ossicles of the middle ear transfer vibrations from the aperilymph. Movement of the ossicles is restrained by the te
tympani and stapedius muscles, innervated by CNV and VII,respectively.
In the cochlea, the oscillations of the basilar membrane are by the inner and outer hair cells of the organ of Corti. The ocells respond with movement, which is transmitted to the temembrane and thence to the inner hair cells, increasing thesensitivity of the latter to sound. The inner hair cells responreleasing their excitatory transmitter and stimulating the seterminals of the cochlear division of CNVIII.
important facts 2
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The primary sensory neurons have their somata in the spiral ganthe cochlea. Their axons end in the dorsal and ventral cochlear n
Axons from the dorsal cochlear nucleus cross the midline, travel the lateral lemniscus, and end in the inferior colliculus.
Axons from the ventral cochlear nucleus end in the superior olivof both sides. The convergence of signals from the left and right allows neurons in the superior olivary nucleus to respond to the
times of arrival of sound in the two ears, thus providing the abilidetermine the direction of the source. The neurons in each supenucleus have axons that travel in the lateral lemniscus and end ininferior colliculus
important facts 3
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The inferior colliculus projects (through the inferior brachiumedial geniculate body, which projects to the primary auditof the cerebral cortex.
The primary auditory cortex is located on the superior surfatemporal lobe. It is connected with auditory association corsuperior temporal gyrus and nearby parts of the parietal lobleft cerebral hemisphere (of most people), these regions are
coextensive with the receptive language area.
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vestibular systemhe who thinks that he is standing must be afraid of falling.
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The Otolith Organs: Detect changes in head angle, linear acceleration
The Vestibular System
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Found in utricule and saccule
Macular hair cells responding to tilt
The Vestibular System
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The Semicircular Canal
Structure
The Vestibular System
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Push-Pull Activation of SemicircularCanals
Three semicircular canals on oneside
Helps sense all possiblehead-rotation angles
Each paired with another onopposite side of head
Push-pull arrangement ofvestibular axons:
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Ascending Pathways
b l
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Vestibular nerve
Vestibular nuclei
Cerebellum
Oculomotor complex CN 3, 4, and 6
Along with vestibulospinal reflexes coordinate head andmovements
Relay Centers
Thalamus
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Connection with vestibular cortex and reticularformation arousal and conscious awareness of
body; discrimination between self movement vs. thatof the environment
Vestibular Cortex Junction of parietal and insular lobe
Target for afferents along with the cerebellumBoth process vestibular information with somatosensory
and visual input
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The Vestibulo-Ocular Reflex(VOR)
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Vestibular-Ocular Reflex(VOR)
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Causes eyes to move in the opposite directionto head movement
Speed of the eye movement equals that of thehead movement
Allows objects to remain in focus during headmovements
Compensatory Eye Movements
VOR
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VOR
Optokinetic reflex
Smooth pursuit reflex, saccades, vergence
Neck reflexes combine to stabilize object on the same area of the
retina=visual stability
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Purves 2001.
Vestibular ProcessingGain
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Keeps eye still in space while head is moving
Ratio of eye movement to head movement (equal1)
VOR Dysfunction
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Direction of gaze will shift with the head movement
Cause degradation of the visual imageIn severe cases, visual world will move with each head mov
Menieres disease
Vertigo
Nausea
Nystagmus(oscillatory mvmnt of the eyes consisting of fastcomponents)
Concluding Remarks
Hearing and Balance
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Nearly identical sensory receptors (hair cells)
Movement detectors: Periodic waves, rotational,
and linear force
Auditory system: Senses external environment
Vestibular system: Senses movements of itself