Neuroanatomy Lecture 24

Vestibular System

3 Functions of the Vestibular System

detects head position and head movements, helps maintain posture and balance, involved in some reflexive eye movements when head is moving

Vestibular Apparatus Location and Components

located in the inner ear near the cochlea, contains the semicircular canals (ducts) and otolith organs

Semicircular Canals (ducts)

detect rotational (angular) acceleration of the head, superior and posterior

Otolith Organs

detect static position (tipping back and forth) of head and linear acceleration, contains the utricle and saccule

Semicircular Canals (3)

horizontal which is parallel to the floor when the head is normal and upright, superior and anterior, posterior and inferior

Semicircular Canals (Info)

positioned at roughly right angles to one another, filled with fluid: endolymph which is high in K+

Ampulla and Cupula

one ampulla is located at the base of each semicircular canal (3 total), contain a gelatinous mass called the cupula that has water and ions, hair cells that are embedded in the cupula act as a barrier to be pushed on, the cupula forms the barrier to the endolymph in the scc

Inertia of the Endolymph

when you turn your head, the endolymph lags behind head motion due to inertia, endolymph and the cupula will move in the direction opposite to head rotation, the cupula is only displaced during a change in motion and then it goes back to being straight after endolymph

Hair Cells in the Vestibular System

stereocilia move towards the kinocilium because of input from cranial nerve VIII afferents, channels open so more NT and moves faster and there are many action potentials, kinocilium at rest have a certain firing rate, stereocilia move away from kinocilium and channels close so less NT and moving slower and so very few action potentials

Left Horizontal Semicircular Canal when Head turns Left

endolymph stays put and pushes the cupula and its hair cells to the right so stereocilia pushed towards kinocilium (on anterior end) and hair cells depolarize so more NT can be released

Right Horizontal Semicircular Canal when Head turns Left

endolymph stays put and pushes the cupula and its hair cells to the right towards the posterior side of the body so stereocilia are pushed away from kinocilium and hair cells hyperpolarize so less NT is released

Semicircular Canal Response to Left Ear Down

left superior/posterior scc is depolarized and right superior/posterior scc is hyperpolarized

Semicircular Canal Response to Right Ear Down

left superior/posterior scc is hyperpolarized and right superior/posterior scc is depolarized

Semicircular Canal Response to Nose Down

left and right superior scc are depolarized and left and right posterior scc are hyperpolarized

Semicircular Canal Response to Nose Up

left and right superior scc are hyperpolarized and left and right posterior scc are depolarized

Otolith Organs Detect…

static position of the head in space (with respect to gravity) at any angle and linear acceleration like in a straight line, car

Two Otolith Organs/Make-up

utricle and saccule, at different orientations to one another to detect in different dimensions, hair cell cilia embedded in the gelatinous layer and a thick otolithic membrane on top of this layer with otoconia embedded in it

Otoconia

calcium carbonate crystals, embedded in otolithic membrane, heavy and respond to gravity

Otolith Organ Membrane Shift

shifts relative to the hair cells when the head tilts (static) or in response to linear acceleration, pulls on the cilia of the hair cells, just like scc toward kinocilium is depolarization, hair cells are bent but stay in same position because gravity moving it

Otolith Organs Hair Cells

the hair cells don’t all face the same direction, striola divides utricle and saccule into two regions of opposite hair cell orientation

Utricle

oriented mainly horizontally, linear acceleration in the horizontal plane (car) and head tilt

Saccule

oriented mainly vertically, linear acceleration in the vertical plane (elevator) and head tilt

Vestibular Nuclei

nerves, ganglions, nuclei come together to form the entire thing; located in medulla and pons, receive projections from vestibular afferents (cell bodies in vestibular ganglion) and proprioceptive info from muscle spindles in the neck, knowing the difference between head tip because of falling or bending up/down (muscle stretch)

Angles of Cupulae and Semicircular Canals

all parts have a preferential angle for activity but usually multiple parts are active at once

Common Symptoms of Vestibular System Dysfunction

poor balance and feeling unsteady, nystagmus with uncontrolled eye movements, vertigo or illusion of movement, loss of perception of where “up” is, motion sickness

Common Causes/Disorders Related to Vestibular System Dysfunction

tumor on or other damage to CN VIII, benign paroxysmal positional vertigo (BPPV), Meniere’s disease (too much fluid/pressure in vestibular system), ototoxic drugs (antibiotics), alcohol, space travel

Benign Paroxysmal Positional Vertigo

happens periodically out of nowhere and we don’t know why, otoconia get dislodged through fluid against the cupula and messes with the hair cells because not actually moving but the illusion of feeling like it (vertigo), usually goes away in a couple of days and given positions to try and dislodge the otoconia

Vestibular Illusions in Flight: Sustained Angular Acceleration

perceiving rotation and then stopping again so no sensation of movement, semicircular canals detect the rotational acceleration of the head and so if rotational motion is constant then no rotation is perceived

Vestibular Illusions in Flight: Linear Acceleration

utricle doesn’t know the difference between linear acceleration and tipping the head back because all by gravity, pilot can activate this by tipping head back when preparing to take off so they don’t angle the plane downward

Light Buoyant Capula

can change because of alcohol, becomes less dense and can lift up and activate the hair cells, inhibit, because ethanol is less dense than water, the spins

Heavy Capula

can change because of alcohol, fluid becomes less dense but capula gets heavy since BAC going back down due to metabolism and so it will go down to excite