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Vestibular anatomy

refers to the structures in the inner ear that help maintain balance and spatial orientation, including the semicircular canals and otolith organs.

Each semi circular canal is about three-fourths of a toroid (donut) shape, measuring 15 mm long and 1.5 mm in diameter. These canals are filled with a fluid called perilymph.

A second, smaller toroid is found inside the larger toroid, measuring 0.3 mm in diameter. It is formed by a membrane filled with fluid called endolymph.

Crista contain the hair cells that project into the cupula- a jellylike material that, in response to a head movement, lags behind the movement of the endolymph. This lag displaces the hair cells.

Stereocilia (hair cells)

Each of the 5 vestibular organs (semi-circular canals and otolith) have stereocilia (hair cells). These are mechanoreceptors that, when deflected due to head motion, either increase or decrease neural firing.

A deflection towards the tallest hair cell increases firing. A deflection away from it decreases firing.

Hyperpolarization: a decrease in the neuron's firing rate due to inhibitory signals, resulting from the movement away from the tallest stereocilia. Potassium channels close for movements away from the tallest hair

Depolarization: an increase in the neuron's firing rate due to excitatory signals, resulting from the movement towards the tallest stereocilia. Potassium channels open for movements towards the tallest hair

Semi-circular canals: push/pull symmetry

Hair cells in opposite ers respond in a complementary manner to head movements, with one side increasing firing while the other decreases, maintaining balance. (one side hyperpolarizes while the other depolarizes)

Initial rotation: due to inertia, the fluid lags behind —> relative movement in the opposite direction —> hair cells activate —> sense of change in speed

Continued rotation: if speed is constant fluid moves with the same speed —> no relative movement —> cupula in neutral position —> no hair cell activation —> no sense of change in speed

Stop rotation: due to inertia, the fluid keeps moving —> relative movement in the same direction —> hair cells activate —> sense of change in speed

Sensitive to acceleration!!!

Otolith organs: linear acceleration and tilt

Each otolith organ contains a macula, which is sensitive to shearing forces.

Utricular macula: sensitive to horizontal linear acceleration

Saccular macula: sensitive to vertical linear acceleration

Also sensitive to tilt motion

Hair cells encased in gelatinous structure called otoconia. Larger accelerations result in greater otoconia displacement, which leads to greater deflection of the hair cell bundles (more neural firing)

Direction is determined based on the firing of hair cells and their arrangement. For example of utricular macula (horizontal motion) some hair cells are arranged so that their best sensitivity is to forward/backward motion and some are arranged so that their best sensitivity is to left/right motion. (ended at slide 53)