Located in the inner ear are several small organs responsible for balance. These organs belong to the vestibular system and give humans and many other mammals a sense of spatial awareness.
The vestibular apparatus lie within the vestibule, located in the inner ear labyrinth (along with the cochlea – involved in hearing). The major structures of the vestibular system are:
- The otolithic organs; the utricle and the saccule – Involved in detecting linear (forward and back or up and down) motions.
- The semicircular canals – Involved in detecting rotational movements
The Otolithic Organs
As mentioned above, the otolith organs comprise of the utricle and saccule, these are simply cavities which form part of the labyrinth of the inner ear. They contain hair cells and otoliths which send signals to the brain concerning the orientation of the head.
The hair cells of the utricle and saccule project into a gel like material. Within this gel lie many small calcareous structures – otoliths. Their function is to detect motion, which they do by movement. When the head is tilted the otoliths move and press onto the hairs protruding into the gel like substance. The hair cells are receptors which send sensory information to the brain, signalling the orientation of the head when stimulated by the otoliths.
The orientation of the utricle makes it sensitive to horizontal movement and acceleration (e.g. accelerating in a car) whilst the saccule is sensitive to vertical movement and acceleration (e.g. accelerating in an elevator).
The signals received from the sensory hairs can cause corrective motor movement of the posture or eyes, which helps to keep the animal balanced.
The Semicircular Canals
The semicircular canals of the inner ear are arranged over 3 spatial planes (x,y and z) to detect rotational movement across all 3 axis. At the base of all three canals lies a cluster of sensory hair cells (as found in the saccule and utricle). In a similar manner to the otolithic organs, the hair cells project into a gel, however the gel only forms a cap over the hairs – the rest of the canal is filled with fluid. The gelatinous cap is called a cupula.
Rotational movement causes the fluid in the semicircular canals to flow, this in turn pushes on the cupula which stimulates the hair cell receptors embedded within. The stimulation of the hair cells causes impulses to be sent to the brain by sensory neurones alerting the brain that the body is engaged in angular motion.
Sensory information from the vestibular system is detected by the vestibular nuclei via the vestibular nerve (one half of the vestibulocochlear nerve VIII).