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CN VIII
CBN
32
Medical
Graduate
12/27/2010

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Term
what is the purpose of the vestibular system?
Definition
to provide information on the position of the head at any given moment. this allows regulation of posture and coordination of head/eye movement.
Term
what activates the vestibular system?
Definition
acceleration of fluid in the semicircular ducts/cristae (angular acceleration) and utricular/saccular membrane (linear acceleration) in the temporal bone.
Term
what do the vestibular receptors respond to?
Definition
change
Term
where are the receptors for the vestibular system?
Definition
in the bony labyrinth of the inner ear (semicircular canals, cochlea) which contains the the membranous labyrinth (semicircular ducts, cristae ampullaris) which contains *endolymph. the utricle and saccular membrane also play a slightly different part in this.
Term
what attaches to the oval window of the vestibule?
Definition
the stapes - which moves w/vibrations and stimulates receptors in the cochlea to create what we perceive as sound
Term
where are the bony labyrinths of the vestibular system located?
Definition
they are embedded in the petrous portion of the temporal bone
Term
how does the formation of the bony labyrinths allow awareness of movement in all 3 cartesian planes?
Definition
each bony labyrinth has three semicircular canals which are positioned in the 3 cartesian planes (orthogonal orientation).
Term
how are the 2 bony labyrinths arranged in relation to each other?
Definition
they are at right angles to each other (as also are their reciprocal semicircular canals)
Term
what do the semicircular canals respond to?
Definition
rotation
Term
where are the receptor cells located in the semicircular canals?
Definition
in the ampullae (enlargements) at the point where the canals meet centrally around the vestibule.
Term
what is the receptor in the semicircular ampulla called? how does it function?
Definition
the crista ampullaris (oriented at a right angle to the duct opening = maximum response to rotation) moves w/endolymphatic movement incurred during head movement.
Term
what is the amplification system inside the crista ampullaris?
Definition
inside the crista ampullaris there is a hollow cupula filled w/gelatinous material and a floor lined w/stereocilia and kinocilium (hair cells). the stereocilia (evagination of the plasma membrane at the apex of the cell) and kinocilium (actual cilium) are also oriented in the same perpendicular direction to maximize sensitivity to movement in that plane.
Term
what happens when a stereocilia/kinocilium moves?
Definition
the bottom of the stereocilia/kinocilium is surrounded by a nerve which experiences an influx of Na+ w/movement of the hair cells and thus an action potential is generated.
Term
what characterizes endolymphatic movement?
Definition
at the beginning of rotation, the temporal bone is moving, but the endolymph is not (and thus motion is briefly perceived as going in the opposite direction by the cupula), then the rotation of the endolymph and the temporal bone match speed (harmonic resonance: perceived as no motion by the cupula), then as the temporal bone ceases rotation the endolymph briefly continues moving in the direction it had been heading (perceived as a brief continuation of the stopped movement by the cupula).
Term
what do the utricle and saccule respond to? how are they arranged?
Definition
the utricle and saccule respond to linear acceleration and are arranged at right angles to each other (utricle on the floor of the vestibule and the saccule on the medial wall).
Term
what is the signal amplifier for the utricle and saccule?
Definition
the otolithic membrane, which contains otoconia (calcium carbonate crystals: heavier than endolymph = more momentum), which its connected to the vestibule by hair cells. these hair cells are arranged randomly (as opposed to those in the cupula) allowing response to linear acceleration in any direction to a degree.
Term
what is meniere's disease?
Definition
tinnitus and dizziness, which is idiopathic (potentially viral)
Term
what is the nerve path from the vestibule to the brain? what are the projections inside the brain?
Definition
the nerves running from the vestibule are considered "bipolar" (special sensory) and meet in the vestibular (scarpa's) ganglion in the internal auditory meatus. from the vesibular ganglion, CN VIII then projects into the CNS at the *pontomedullary junction. from this junction, the neurons bifurcate w/short ascending and long descending processes - some of which *decussate and project into the contralateral vestibular nuclei (of which there are 4), which project to the *vestibular ganglia, the *cerebellum, the **medial longitudinal fasiculus (group of neurons that hook up CN’s III, IV and VI and is responsible for conjugate gaze = allows coordination of head/eye movement), the *medial/lateral vestibulospinal pathways (allows local reflexes at all levels of the spinal cord to know where the head is), the *thalamus (and thus consciousness), and the RAS.
Term
what is (physiologic) nystagmus?
Definition
a mechanism by which the brain resets its conjugate gaze (look steadily in one direction) to the midline, through smooth movements toward one direction and saccadic movements back the to the opposite direction.
Term
what are the 3 kinds of nystagmus?
Definition
optokinetic, vestibular, and caloric
Term
what is optokinetic nystagmus?
Definition
the eyes are moving but the rest of the body is still (such as w/reading or watching telephone poles go by while on a train).
Term
what is vestibular nystagmus?
Definition
the body is moving, but the gaze is fixed on a target (medial longitudinal fasciculus is dealing with CN 3, 4 and 6 so that you remain looking at someone/something)
Term
what is caloric nystagmus? COWS?
Definition
an iatrogenic form of nystagmus involving placement of a cold then warm object into a pts ear to test the vestibular system. if a cold object is placed in the R ear, this sets up a convection current in the horizontal semicircular canal moving to the R, to replace the colder, sinking R-most endolymph. this movement of the fluid (which is usually stationary) in the temporal bone (which is usually moving) is perceived as movement to the L, which is the direction the eyes then try to reset their gaze to (CO of COWS = cold object causes nystagmus to the side opposite the inner ear being tested). if a warm object is placed in the R ear, this causes the most lateral endolymph to rise, causing the rest of the endolymph to move to the L. the L movement of the endolymph is perceived by the brain as the temporal bone moving to the R and the gaze resets to the R (WS of COWS = warm object causes nystagmus to the same side as the inner ear being tested).
Term
what determines the direction of optokinetic nystagmus?
Definition
the reset mechanism determines the direction of nystagmus - if the eyes keep reseting to the left, it is called a "L nystagmus"
Term
what determines the direction of vestibular nystagmus?
Definition
the reset mechanism determines the direction of nystagmus - if your body is spinning in a circle to the L, your midline is constantly going to the L, and therefore you are resetting to the L, and it is a "L nystagmus".
Term
what is the doll's eye reflex?
Definition
when checking an unconscious pt for brainstem damage (after checking the cervical spine), the head is gently turned laterally to each side and the eyes should move to remain facing midline = doll's eye reflex. if the eyes do not remain facing midline, and simply follow the direction the rest of the face aims, this is a negative doll's eye reflex and indicative that the brainstem is likely damaged.
Term
what pathways are involved in "standing still"?
Definition
the semicircular canals/maculae communicate w/the medial vestibulo-spinal tract (MVST) and lateral vestibule-spinal tract (LVST) to maintain posture.
Term
what pathways are involved in walking?
Definition
the maculae (acceleration) and CN 3,4,6 (eye position) communicate w/the lateral vestibule-spinal tract (LVST) to allow walking.
Term
how does the vestibular system project into the cerebellum?
Definition
through the inferior cerebellar peduncle
Term
what is the relationship between the vestibular system and the RAS?
Definition
the vestibular system projects nuclei into the RAS which projects to the cerebrum (awareness of balance or lack thereof) and the vagus (n/v if disoriented)
Term
what is the equilibrial triad?
Definition
the proprioceptive system (muscle spindles, golgi tendon organs, joint receptors, ruffinis, pacinian, hair receptors, dermatomes, etc), the visual system (CN II), and the vestibular system (CN VIII)
Term
how does EtOH affect the vestibular system?
Definition
EtOH alters the specific gravity of endolymph in the system - making it easier to move
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