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CN VIII: Vestibulocochlear, Pathways - Coggle Diagram
CN VIII: Vestibulocochlear
Anatomy of the vestibulocochlear nerve
2 types of special sensation:
Vestibular
(balance)
Auditory
(hearing)
Sensory R =
Hair cells
located on the inner walls of
membranous labyrinth
Membranous labyrinth filled with fluid within the petrous temporal bone called "bony labyrinth"
Each membranous labyrinth is composed of:
Spiral cochlear
Vestibular division (saccule and utricule)
3 semicircular canals
Cell bodies of the primary sensory neurons are located in:
Vestibular (Scarpa’s) ganglion
Spiral ganglion
Form CN VIII and travel through:
Internal auditory meatus together with CN VII
Enters posterior fossa
Nerve joins the brainstem at the pontomedullary junction
Terminate within vestibular and cochlear nuclei
Hair cells
Auditory system and anatomy
1) Central auditory pathways
Transduces
sound waves
Sound waves are collected and amplified
by physical structure in
external
and
middle ear
-->
transfer
to neural elements in
inner ear
Biomechanical properties of inner ear decompose
complex sound waves
into
sinusoidal components
(component frequencies, their amplitude, and their phase can be encoded in the firing of the receptor cells)
Sound comes from =
spherical pressure waves
generated by vibrating air molecules that can be characterized by:
Amplitude
(loudness)
Frequency
(pitch)
Phase
(temporal displacement)
20Hz -> 20kHz
Auditory system anatomy
Middle ear
Tympanic membrane
ossicles
Transmit acoustic energy from air to the inner ear
Amplifies the pressure of acoustic energy some 200-fold
Inner ear
Cochlea
Auditory nerve
Biomechanical: decompose complex acoustic energy (frequency) into component
sinusoidal waveforms
Neural (sensory transduction): transduce this mechanical energy into neural signals that are then communicated to the brain (thanks to the auditory nerve)
External ear
Pinna
Concha
Auditory meatus
Concentrate sound energy and focuses it on the tympanic membrane (=ear drum)
Filters and collect different sound frequencies to provide cues about sound localization
Sound characteristics:
The
cochlea
encodes for
sound intensity
(volume) and
frequency
(pitch)
Detecting the location of a sound source = important for survival (location is determined by CNS)
Sound waves -> external acoustic meatus -> strike the tympanic membrane -> vibration
Vibration carried through middle ear cavity by malleus, incus and stapes to oval window of the cochlea
Action of the muscles
Tensor tympani (CN V)
Dampen noise produced by chewing
When tensed =
muscle pulls the malleus medially
, tensing tympanic membrane and
damping vibration in the ear ossicles
and thereby reducing the perceived amplitude of sounds
Stapedius muscles (CN VII)
Dampens
the
vibrations of the stapes
( by pulling on the neck of that bone)
It prevents:
Excess movement of stapes
Helping control amplitude of sound waves from the general external environment to the inner ear
The stapedius muscle dampens the ability of the stapes vibration and protects the inner ear from high noise levels, primarily the volume of your own voice
Anatomy of the cochlea
House cochlear duct and turns around a bony center called the “
modiolus
”
The cochlea communicates with the middle ear cavity via two openings in the bone:
Oval window
(fenestra vestibuli)
Round window
(fenestra cochlea)
The cochlear duct divides the cochlear space into three compartments:
Scala vestibuli
Scala tympani
Scala media (Cochlear duct itself) = Contains the
organ of Corti
The cochlea duct:
Full of endolymph rich in K+
Deformation of the stereocilia:
toward the longest stereocilia =
depolarization
-> more NT release
away from the longest stereocilia =
hyperpolarization
-> less NT release
Organ of Corti
Basilar membrane: supports hair cells
Tectorial membrane
Cochlear hair cells
Inner
Outer
Inner:
single row in the cochlea
straight rows
shortest of the cilia facing the center of the cochlea
synapse with more than 90% of the cochlear sensory neurons
Primarily responsible for hearing
Outer:
smaller
more numerous than inner hair cells
three parallel rows
“U” or “W” shape
less than 10% of the auditory sensory traffic to the brain
Vestibular system and anatomy
1) Central vestibular pathways and reflex
Anatomy
Set of interconnected canals that arise from the same embryological precursor = Otic placode
Canals are filled with endolymph and surrounded by perilymph
2 otolith organs:
Utricule
Saccule
3 semi-circular canals
Vestibular labyrinth
= designed to sens the motions that arise from head movements and the inertial effect due to gravity
Vestibular signal are relayed to integrative centers in
BRAINSTEM
and
CEREBELLUM
to adjust
postural reflexes
and
eye movement
Vestibular hair cells:
Kinocilium = longest stereocilium
Depolarization: Shortest to longest
Hyperpolarization: Longest to shortest
Otolithic organs anatomy =
Detect movement of the head and the position of the head relative to gravotanional pull
Uricular macula orientation
In utricule:
Kinocilia are orientated toward the striola
Utricular maculae oriented horizontally =
horizontal plane
(x axis)
Sensitive to linear acceleration on horizontal plane
Saccular macula orientation
Clinical test and conditions
Auditory sensory transduction
1) Pressure
2) ossicles of the middle ear
3) oval window to bulge inward
4) pressure is conducted throughout the fluid of the scala vestibuli (called perilymph)
5) scala tympani
6) round window
7) vibrations in the basilar membrane
Movement of basilar mb = initiates sensory transduction (displace sensory hair cells)
Shearing motion between basilar and tectorial membrane
That deform ends of inner hair cells = transduction of mechanical energy into neural impulses
Pathways
Neuron from DORSAL cochlear nucleus
Cross the midline via = Dorsal acoustic stria
Rostrally in the lateral lemniscus to synapse in the contralateral inferior colliculus
Neuron from VENTRAL cochlear nucleus
Project bilaterally to superior olivary nuclear complex
Superior olivary nuclear complex neurons project via lateral lemnisci to inferior colliculi
Inferior colliculus -> Medial geniculate body of thalamus
Audotory cortex in transverse temporal gyrus