Balance Flashcards
what is the vestibular sense
sense of movement, tilt, and rotation
what are the vestibular stimuli
1) angular acceleration (head turning)
2) linear acceleration (head tilt with respect to gravity)
how does vestibular sense affect behavior
reflexes that maintain balance and posture, visual fixation, orientation in space
kinetic labyrinth
*part of the vestibular labyrinth
*contains semicircular canals
*detects angular acceleration and deceleration
static labyrinth
*path of the vestibular labyrinth
*utricle and saccule
*detects linear acceleration and gravity
endolymphatic sac
a drainage pathway for endolymph into subarachnoid space
bony labyrinth of inner ear
a very dense, bony shell containing perilymph, which resembles extracellular fluid in general
vestibular labyrinth of inner ear
*comprises the otolith organs (utricle & saccule) and 3 semicircular canals (superior, posterior, and lateral/horizontal)
anterior (superior) semicircular canal
located in the sagittal plane
lateral (horizontal) semicircular canal
located in the transverse plane
posterior (inferior) semicircular canal
located in the frontal plane
how are semicircular canals stimulated by angular acceleration
1) all hair cells are located on a crest in the ampulla of each semicircular canal; the hair bundles of cells are oriented or polarized in the same direction
2) the bundles extend into a gelatinous structure called the cupula
3) when the head turns, inertia of fluid resists movement and deforms the hair bundle, displacing the cupula in the OPPOSITE direction
4) turning the head in one direction causes depolarization of the hair cell and hyperpolarization in the other, thereby increasing or decreasing the firing rate of afferent nerve fibers that synapse on the cell
5) the change in firing rate of the afferent nerve fibers is proportional to the angle and degree of acceleration and deceleration
ampulla
duct at base of semicircular canal, filled with endolymph
cupula
gelatinous mass inside ampulla
vestibular hair cells and kinocilium
*rows of stereocilia with a single kinocilia
*pushing kinocilium in one direction OPENS CATION CHANNELS (depolarization)
*neurotransmitter is released and activates primary vestibular afferent fibers
flow of endolymph toward kinocilium is ?
excitatory
maculae
*within the saccule and utricle, the maculae are the sensory organs of the static labyrinth that detect head position and linear acceleration
vestibulocerebellar pathway
*bidirectional (between cerebellum and vestibular nuclei)
*helps with motor planning, movement, balance, etc
lateral vestibulospinal tract
*descending
*originates in the lateral vestibular nucleus; axons descend IPSILATERALLY down spinal cord to motor neurons of limbs and trunk to maintain balance
lateral vestibulospinal tract - functions
*senses falling/tripping
*contracts leg muscles for postural support
*descending = motor; lateral = limbs
medial vestibulospinal tract
*descending
*originates in the medial ventricular nucleus; axons descend bilaterally to the cervical and thoracic spinal card
medial vestibulospinal tract - functions
*head and shoulder reflexes
*acts on neck musculature to stabilize the head if the body moves
vestibular projections to thalamus and cortex
*ascending
*projects to the ventroposterior (VP) nucleus of the thalamus
*then projects to the cortex
*provides awareness of motion and body position
vestibulo-ocular reflexes
*ascending
*originate in vestibular nucleus and travel up the medial longitudinal fasciculus to the nuclei responsible for eye movements (CN III, IV, and VI)
vestibulo-ocular reflexes - functions
*eye reflexes
*stabilizes visual image during head movement
*causes eyes to move simultaneously in the opposite direction and in equal magnitude to head movement
vestibulo-ocular reflex: step by step
*if you rotate head to left, eyes move to right
*medial longitudinal fasciculus (MLF) connects abducens nucleus on one side to the oculomotor nucleus on the contralateral side
*if the right eye abducts via CN VI, connections through MLF to left oculomotor nucleus causes left medial rectus to ADDUCT the left eye
oculocephalic reflex (doll’s head reflex)
*used in unconscious patients to test brainstem function
*NORMAL/positive (reflex present) = head rotated to R, eyes move to L
*ABNORMAL/negative (reflex absent) = head rotated to R, eyes follow to R
*absence of reflex bilaterally can indicate damage to lower brainstem
vertigo
*an illusion of body motion, like spinning or turning, when no real motion occurring
*typically uncontrollable
*usually accompanied by nausea, vomiting, sweating
*onset is usually sudden; could persist for hours to days
peripheral vertigo
*commonly associated with a problem in the inner ear
*most common type of vertigo
*many causes: excessive alcohol intake, meningitis, etc
*may also include hearing loss, nystagmus, and gait abnormalities
central vertigo
caused by vertebrobasilar stroke or cerebellar stroke
nystagmus
*abnormal deviations or beating movements of the eyes
*slow drift of eyes in one direction, followed by fast beat in opposite direction
*note - named for the direction of the FAST beat (ex. right nystagmus = eye slowly drifts to the left, then quickly beats back to the right)
how is the direction of nystagmus defined
direction of FAST BEAT = direction of nystagmus
caloric testing for nystagmus
*irrigation of the external ear canal with either cold or warm water to determine nystagmus
*NORMAL (COWS): cold water causes nystagmus to the OPPOSITE side; warm water causes nystagmus to the SAME side [COWS: cold, opposite; warm, same]
*no response can help you determine where the issue is
benign paroxysmal positional vertigo (BPPV)
*most common cause of vertigo
*etiology = canalithiasis (displaced otoconia end up in semicircular canals)
*usually lasts seconds to minutes
Meniere’s disease
*excess endolymph
*4 major symptoms:
1) periodic episodes of rotary vertigo
2) fluctuating, progressive hearing loss (sensorineural)
3) tinnitus
4) a sensation of “fullness” or pressure in the ear
*episodes last for hours
labyrinthitis
*infection of the inner ear and labyrinth, usually caused by viral infection
*can lead to nystagmus, tinnitus, hearing loss
*vertigo and dizziness persist even when head remains still
*lasts for several days
Dix-Hallpike test
*testing for BPPV
1) patient’s head is first turned 45 degrees
2) patient is laid supine with neck extended below the table
3) after a few seconds, observe vertical and rotatory nystagmus and assess for vertigo
romberg test
inability to keep balance with eyes closed indicates either vestibular dysfunction or loss of proprioception (sensory ataxia)
