3.2.5. Equilibrium (Vestibulary sense)

Detects position and motion of the head

Consists of the semicircular canals and the otolith organs

Semicircular canals detect rotational or angular acceleration- change of movement. It consists of three circular canals arranged in planes that lie at right angles to each other, they are filled with endolymph

The receptive hair cells are located in the ampulla.  The hair cell holds 20-50 microvilli (stereocilia) and one cilium – the kinocilium which are embedded in cupula –a caplike gelatinous layer

When the head is set in motion the cupula moves through the fluid which does not initially move due to inertia, causing the cupula to sway or bend.

At constant speed, the fluid eventually catches up with the ampulla and the ampulla returns to resting position.

When motion of the head stops, the ampulla stops, but the fluid lags behind causing the ampulla to be bent in the opposite direction (similar to the motion of seaweed during the tide)

Each hair cell is oriented so that it depolarizes when the stereocilia are bent toward the kinocilium, bending in the opposite direction hyperpolarizes. The hair cells synapse on the vestibular nerve

Otolith organ – provides information on the position of the head relative to gravity and linear acceleration

The otolith organs, utricle and saccule are situated between the semicircular canals and the cochlea.

The hairs of the receptors in these organs also protrude into an overlying gelatinous sheet whose movement displaces the hairs and results in changes in hair cell potential

Tiny crystals of calcium carbonate (otoliths) are suspended in the layer making it heavier and giving it more inertia than the surrounding fluid

Utricle hairs are oriented vertically and the saccule hairs are lined up horizontally

Utricle hairs – when the head is tilted in any direction other than vertical, the hairs are bent in the direction of the tilt because of the gravitational force pulling on the top heavy gelatinous layer

The utricle hairs are also displaced by any change in linear motion because the top heavy layer labs behind the endolymph and hair cells causing the hairs to bend in the opposite direction of head movement.

The saccule functions similarly except that it responds to the tilting of the head away from a horizontal position (getting out of bed)

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