NEUROSCIENCE: VESTIBULAR SYSTEM -- Semicircular Canals

SEMICIRCULAR CANALS

A. Afferent Endings: hair cells embedded in gelatinous cupula in ampula

B. Stimulation of Receptors: sequence of events

1. movement of fluid or development of pressure gradient in canal (endolymph inertia)

2. cupula and embedded hairs move (flex, bend)

3. primary afferent axons stimulated/inhibited, depending upon direction of movement

C. Adequate Stimulus: angular acceleration

Note response to prolonged rotation below ("velocity" refers to angular velocity of rotation, so sequence is begin to rotate, continue to rotate at constant angular velocity, then stop rotating)

Question: What would be the response of the contralateral semicircular canal?

Note: If duration of rotation is brief, only the initial response is seen

D. Orientation

The 3 semicircular canals are oriented approximately at right angles to each other. Thus, angular acceleration in all 3 dimensions can be detected

E. Role

1. Somatic reflex response to rotation (maintain equilibrium by posture adjustment)

2. Reorientation of eyes to fix the visual field: vestibulo-ocular reflex

F. Nystagmus related to semicircular canal stimulation

1 Define Nystagmus: pattern of eye movement with slow eye movement in one direction and rapid movement in the opposite direction

Note: nystagmus can be a normal response to an appropriate stimulus or it can be an indication of abnormal function, e.g. spontaneous nystagmus

2. Causes: several, one of which is angular acceleration, particularly when the original object of visual fixation passes out of the visual field

a. direction of nystagmus is defined as the direction of the fast component
b. for brief rotation, nystagmus is in the direction of rotation
c. for rotation of long duration

1) upon starting, nystagmus is in the direction of rotation
2) after continuing rotation, no nystagmus
3) on stopping, nystagmus is opposite to previous rotation

3. Nystagmus can be elicited by thermal gradients in the ear (due to endolymph convection currents); used as clinical test