Vestibule or otolith organ is formed by saccule and utricle. Often, the saccule and utricle are together termed as otoliths. Utricle and saccule communicate with each other through the utriculosaccular duct. Saccule and cochlear duct communicate through ductus reuniens (canalis reuniens of Hensen). Another duct known as the endolymphatic duct arises from the utriculosaccular duct and it terminates in a bag-like structure on the cranial surface of the petrous bone (temporal bone) known as the endolymphatic sac.
Receptor Organs
The receptor organ in the otolith is called the macula. The macula is also formed by neuroepithelium and supporting cells. The neuroepithelium of the macula also has two types of hair cells – Type I hair cells and type II hair cells. The macula is also covered by a flat gelatinous membrane called the otolith membrane. The otolith membrane contains some crystals, which are called ear dust or otoconia. Otoconia are mainly constituted by calcium carbonate.
Functions of Otolith Organ
The otolith organ is concerned with linear acceleration and detects acceleration in both vertical and horizontal planes. The utricle responds during horizontal acceleration and the saccule responds during vertical acceleration.
- The position of the macula’s hair cells aids the utricle’s ability to react to horizontal acceleration. In the utricle, the macula is located in the horizontal plane and the hair cells are located in the vertical plane. Due to inertia, otoconia pull the cilia of hair cells as they travel horizontally in the opposite direction and stimulate the hair cells.
- The cilia of hair cells are located in the horizontal plane, while the macula of the saccule is in the vertical plane. As with a utricle, a saccule’s otoconia move vertically, but they do so in the opposite direction, pulling the cilia, which stimulate the hair cells.
Role of Otoliths in Resting Conditions
Hair cells are continuously stimulated while at rest (when the head is not moving) because of the gravitational force tugging on the otoconia. Hair cell stimulation causes reflex movements of the head and limbs to maintain posture with respect to gravity. Thus, the otolith organ’s receptors are referred to as gravity receptors because of their function.
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