Several types of sensory receptors provide information to the brain for the maintenance of equilibrium. The eyes and proprioceptors in joints, tendons, and muscles are important in informing the brain about equilibrium and the position and movement of body parts. However, unique receptors in the internal ear are crucial in monitoring two types of equilibrium. Static equilibrium involves the movement of the head with respect to gravitational force. Dynamic equilibrium involves linear acceleration in both horizontal and vertical directions, in addition to the rotational movement of the head.
The macula (mak’-u-lah; plural maculae), an organ of static equilibrium, is located within the utricle (u ‘-tri-kul) and the saccule (sak’-ul), enlarged portions of the membranous labyrinth within the vestibule. Each macula contains thousands of sensory receptors called vestibular hair cells that possess hairlike stereocilia embedded in a gelatinous material. Otoliths (o’-to-liths), crystals
- Changes in head position cause gravity to pull on the gelatinous mass, which bends the stereocilia. This change stimulates the vestibular hair cells to form nerve impulses that are carried by the vestibular branch of the vestibulocochlear nerve to the brain. No matter the position of the head, nerve impulses are formed that inform the brain of the head’s position.
- The cerebellum uses this information to maintain static equilibrium subconsciously.
- Our awareness of static equilibrium results when the nerve impulses are interpreted by the cerebrum.
The maculae in the utricle and saccule also sense linear acceleration in both the horizontal and vertical directions. The mechanism is similar to that used to detect changes in static equilibrium. When the head accelerates either vertically or horizontally, inertia of the gelatinous mass causes the stereocilia of the vestibular hair cells to bend. When motion ends, the gelatinous mass continues to move for a moment, which bends the stereocilia in the opposite direction. The changes in stereocilia movement alert the brain to changes in velocity.
The membranous labyrinths of the semicircular canals contain the sensory receptors that detect rotational motion of the head. Note that the semicircular canals are arranged at right angles to each other so that each occupies a different plane in space, roughly equal to the frontal, sagittal, and transverse planes.
Near the attachment of each membranous canal to the utricle is an enlarged region called the ampulla (am-p”ul ‘-lah). Each ampulla contains a sensory organ for dynamic equilibrium called the crista ampullaris (kris’-ta am-pul-lar’-is). Each crista ampullaris contains a number of vestibular hair cells, whose stereocilia extend into a dome-shaped gelatinous mass called the ampullary cupula. Axons of the vestibular branch of the vestibulocochlear nerve lead from the vestibular hair cells to the brain.
The mechanism for detecting rotational movement may be described as follows:
- When the head is turned, the endolymph pushes against the ampullary cupula, bending the stereocilia of vestibular hair cells, which stimulates the formation of nerve impulses. The nerve impulses are carried to the brain via the vestibular branch of the vestibulocochlear nerve.
Because each semicircular canal is oriented in
a different plane, the vestibular hair cells of the cristae are not stimulated equally with a given head movement. Thus, the brain receives a different pattern of nerve impulses for each type of head movement.
- The cerebellum uses the nerve impulses to make adjustments below the conscious level to maintain dynamic equilibrium.
Awareness of rotational movement, or lack of it, results from the cerebrum interpreting the pattern of nerve impulses it receives.