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
|
|
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
|
|
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 |
|
|
