vestibular system objectives - michael

Question 1

1) Describe the structure of the vestibular apparatus of the inner ear, the receptive organs and hair cells.

Answer 1

a) Vestibular apparatus: memorize these two slides:
b) Receptive organs:
i) Christae: located in the dilated portion of the semicircular duct called the ampulla
(1) Hair cells in christae are all oriented in one direction
ii) Maculae: reside within the saccule and utricle
(1) Hair cells are polarized around the striola, the central line of the macula
(2) Hair cells in maculae come in small patches with an L-shaped appearance, oriented in multiple directions
c) Hair cells: see discussion in the Audition lecture.
i) (Some additional points)
ii) In vestibular hair cells, a motor protein is driven toward low intracellular calcium. If a stereocilium remains slack for too long, the motor protein will tighten the stereocilium, resulting in partial K and Ca channel opening and some neurotransmitter release. This ensures slow but steady neurotransmitter release under resting conditions in the hair cell.

Question 2

2) Explain how movement is transduced to an electrical signal and the particular sensitivities of the maculae and christae.

Answer 2

a) Mechanoelectrical transduction by hair cells: see discussion in the Audition lecture.
b) Maculae:
i) The hair cells of the maculae protrude into a gelatinous material studded with calcium carbonate crystals (otoliths)
ii) When the head accelerates, the crystals are left behind, bending the hair cells; this signals linear acceleration; also detects gravity when the head is positioned vertically.
c) Christae:
i) The hair cells of the christae protrude into a gelatinous mass called a cupula (no otoliths)
ii) Mainly and simply movement of fluid that deflects the christae. As the head turns in the plane of the semicircular duct, the fluid moves in the opposite direction.
iii) Because there are three christae (one for each semicircular duct), and each semicircular duct represents one plane of motion (x, y, z), by combining the amount of activation or inhibition in each christa you can detect how the head is moving (angular acceleration). Angular acceleration includes flexion, extension, lateral bending or rotation of the head.

Question 3

3) Describe the central projections of vestibular afferents and the particular functional roles of central vestibular structures.

Answer 3

a) Hair cells of the maculae and christae synapse onto sensory axons located in the vestibular ganglion (Scarpa’s ganglion). These neurons give rise to CNVIII (vestibulocochlear) that runs through the internal acoustic meatus to the medullary-pontine junction
b) Most of the axons in the vestibular part of the vestibulocochlear nerve terminate in the vestibular complex (vestibular trigone) of nuclei in the region of the rostral medulla.
c) Some collateral branches of CNVIII directly enter the cerebellum and terminate in the flocculonodular lobe. This is called the vestibulocerebellum.
d) Other afferents:
i) Cerebellum: mostly flocculus and fastigial nucleus (important for adaption of vestibular responses)
ii) Spinal cord: incorporates proprioception from the rest of the body (especially the upper neck)

Question 4

4) Discuss the importance of the vestibule-ocular reflex, the central connections involved in the reflex and the regulation of this reflex by the vestibulocerebellum.

Answer 4

a) The vestibular complex passes through the medial longitudinal fasciculus to connect to the nuclei controlling CNIII, CNIV, and CNVI (eye movements) – stabilizes the eyes during jarring movements. This is the vestibule-ocular reflex.
b) The vestibulocerebellum changes the vestibule-ocular reflex in periods of adapatation, such as after inner ear damage.

Question 5

5) Explain the role of connections of the vestibular system with other areas of the brain, including the spinal cord, brain stem and thalamus.

Answer 5

a) Spinal cord: the vestibular complex sends projections that descend through the medulla toward the spinal cord. These include the medial vestibulospinal tract (medial VST) and lateral vestibulospinal tract (lateral VST). These tracts help regulate muscle tone in response to vestibular stimuli (e.g. prevent you from tipping over)
b) Brain stem: projections to the reticular formation can produce autonomic responses such as nausea, vomiting, flushing, and sweating in response to vestibular stimulation (e.g. motion sickness)
c) Thalamus: relays to the sensory cortex, allows for perception of movement. Produces vertigo when there are abnormalities in the vestibular system.