Vestibular System

Question 1

Anatomy

Answer 1

3 semicircular canals: filled w/ endolymph, connects w/ utricle, base is ampulla
Ampulla: angular acceleration, crista extends into lumen, capulbi bridges width of ampulla
Utricle & Saccule: linear displacement, contain macula, utricle is horizontal, saccular is certical
Macula: gelatinous layer overlies stereocilia, above this is otolithic membrane embedded in calcium carbonate crystals provide weight

Question 2

Function

Answer 2

Detect angular acceleration and linear displacement
bend towards tallest stereocilium -> depolarisation
bend towards shortest -> hyperpolarisation
Depolaristion -> increase in APs -> work in pairs left/right

Question 3

Main targets for signals from CNVIII

Answer 3

Brainstem (vestibuloocular reflex)
Cerebellum (vestibuloocular adaption)
Spinal Cord: MVST (vestibulocervical reflex), LVST (vestibulospinal reflex)

Question 4

Mediation of vestibuloocular reflex

Answer 4

coordinates eye movements with head movements
++ connections to contralateral CNVI nucleus & ipsilateral CNIII -> – connections to ipsi. CNVI nucleus & contra. CNVIII nucleus -> eyes move to the right when head moves to the left and vice versa

Question 5

Physiological Nystagmus

Answer 5

Rotary: semicircular canals sense rotation in one direction -> eyes rotate slowly in opp direction followed by rapid rotational ‘reset’ in rotational direction, fast-phase control -> pontine gaze centre, slow-phase control -> vestibular nuclei
Postrotatory: if animal is spun -> andolymph movement will reach rotation speed and if suddenly stopped, flow continues -> nystagmus but in opp direction

Question 6

Vestibulocervical Reflex

Answer 6

spinal cord: MVST arises from medial, rostral and caudal vestibular nuclei -> passes caudally in MLF -> both sides mainly ipsilateral -> innervates cervical and cranial thoracic segments

Question 7

Vestibulospinal Reflex

Answer 7

spinal cord: LVST axons descend in LVST to entire ipsilateral spinal cord -> innervates axial and proximal limb m. -> coordinates limb, trunk, and head movements in resp. to vestibular sig.

Question 8

Unilateral vestibular disease

Answer 8

Abnormal nystagmus: fast phase AWAY from abnormal side
Abnormal Posture and Ataxia: imbalance, head tilt down towards affected side, trunk tipping, rolling or falling to affected side, blind folding will increase ataxia -> excessive extensor tonus
Abnormal postural reactions and strabismus: righting resp. is abnormal, exaggerated resp. towards abnormal side, eyeball on affected side displays strabismus