vestibular system

Question 1

define Otoliths

Answer 1

• found in utricle and saccule
• Calcium carbonate crystals that are denser than any other tissue and can bend cilia of hair cells RESULTING in organs sensitive to LINEAR ACCELERATION and position of head with respect to gravity
• nerves have relatively steady high firing rate when head is upright and changes in response to head tilt

Question 2

define macula

Answer 2

• found mainly on the floor of the utricle
• when head is uprightm macula of the utricle is oriented almost PARALLEL TO THE GROUND
• when head is upright, macula of the SACCULE is VERTICAL TO THE GROUND

Question 3

define semicircular cannals

Answer 3

• ROTATIONAL ACCELERATION to head
• anterior (superior), posterior, and horizontal canals are arranged at RIGHT ANGLES to each other
• if the head is bent forward 30 degrees, horizontal cells become parallel to the ground
• BRAIN COMPARES firing rate in nerves coming from right and left ears
• -> anterior canal output on one side is compared to output from the posterior canal from the other ear
• -> respond to angular acceleration by CHANGING FIRING RATE from TONIC FIRING LEVEL (PHASIC) = endolymph stops moving)

Question 4

vestibular hair cells

Answer 4

• receptors of the estibular system deteriorate w age

- 40% decrease by 70 years old

Question 5

describe the vestibule-ocular reflexes in response to head tilt or head turns

Answer 5

• reflex developed to move eyes in response to small head turns
• eye move opposite to head turn to facilitate fixation on visual target (even if eyes are closed)
• LOSS of the reflex would make fixation difficult or impossible while the head was turning
• VOR sensory input
• -> reflex is driven by vestibular input
• -> reflex occurs even when pts have eyes closed or are in the dark
• -> visual input keeps this reflex accurate. Amount of eye mov’t can be experimentally manipulated

Question 6

post-rotatory nystagmus

Answer 6

• vestibular nystagmus that occurs AFTER a person stops turning
• Endolymph (increase K+) has inertia so it continues to move after head stops turning
• signals from semicircular canals make person feel they are turning in OPPOSITE DIRECTION from preceding actual head turn
• Eye movements correspond to ILLUSIONARY turning

Question 7

Define slow phase nystagmus

Answer 7

• if turn head to the RIGHT slow phase of nystagmus is to the LEFT

Question 8

Define fast phase nystagmus

Answer 8

• moves eye back to the midline, so if turning patient to RIGHT, fast phase is to the RIGHT
• DEPENDENT ON DIFFERENT NEURAL CIRCUIT and DOES NOT OCCUR in COMATOSE PATIENTS

Question 9

What type of nystagmus occurs during turning

Answer 9

• SLOW phase in opposite direction of the turn

Question 10

what type of nystagmus occurs after turning

Answer 10

• FAST phase back to midline, in the SAME DIRECTION of the turn

Question 11

Describe positional vertigo

Answer 11

• BPPV is one of the MOST common types of vertigo
• Benign term used because vertigo not caused by intracranial neoplasm but by free floating particles within the posterior semicircular canal
• BPPV can be severe disabling problem
• ADDITIONALLY patients experiences nystagmus, usually with both torsional and vertical components

Question 12

What is the cause of Meniere syndrome

Answer 12

• caused by an imbalance between the production and reabsorption of endolymph
• endolymphatic hydrops leading to rupture of membraneous labyrinth
• contamination of endolymph with sodium ions depolarizes afferent and hair cells can be killed

Question 13

describe the symptoms of Meniere syndrome

Answer 13

• include transient attacks (lasting minutes to hours of:
• -> vertigo, Nausea, vomiting, abnormal eye movements, sensorineural hearing loss
• disease has variable course
• some patients have spontaneous recovery
• many patients have permanent damage and become deaf in the affected ear

Question 14

what is the main input into vestibular nuclei

Answer 14

• primary vestibular afferents, neck proprioceptors

Question 15

what are the main outputs of vestibular nuclei

Answer 15

• main outputs via:

- -> thalamus to cerebral cortex (provide conscious awareness of body orientation and motion)

Question 16

major roles of vestibular system

Answer 16

• subjective awareness of body position and movement of body in space
• postural tone and equilibrium
• stabilization of the eyes in space during head movements
• vestibular system receives help from visual and proprioceptive systems to fulfill the roles*

Question 17

describe doll’s eye maneuver

Answer 17

• Use of VOR to evaluate brainstem injury in UNCONSCIOUS PATIENT
• very important to determine that there is NO INJURY to cervical spine BEFORE PERFORMING this test
• when patient is lying face upwards, the horizontal semicircular canals lie nearly in vertical plane
• rotating the head left or right stimulate the hair cells in these canals and elicits the VOR
• VOR normally moves eyes conjugately in direction opposite to the movement of the head
• if eyes do not track together and move opposite to the direct of head turn the patient probably has brainstem lesion

Question 18

describe caloric testing of vestibular system

Answer 18

• throwing cold water on the vestibular system
• COWS = cold opposite, warm same
• each side will respond accordingly

Question 19

describe vertigo

Answer 19

• sense that the world is spinning around or that one’s head or body is whirling. It is the illusion of motion
• Patients often use the imprecise term dizzy or dizziness when they feel vertigo. they also use the term dizzy when they mean light-headed, faint, etc