The human body is composed of a series of linkages, block on top of block. These blocks can be arranged in a multitude of patterns called postures. In order to produce and control these postures, the human brain utilizes a great number of continuous inputs telling the brain the instantaneous condition of the body posture.
Overall, we refer to this process as the general body sense. The internal ear provides one of the input systems for the general body sense. The internal ear responds to gravitational forces, of which there are two kinds--static and kinetic (in motion). Of the kinetic stimuli, the motion may be in a straight line (linear) or angular (curvilinear).
The membranous labyrinth of the internal ear has two sac-like parts—the sacculus and the utriculus. On the wall of each of these sacs is a collection of hair cells known as the macula. The hairs of these hair cells move in response to gravitational forces, both static and linear kinetic.
The maculae are particularly sensitive to small changes in the orientation of the head from an upright position. Thus, the maculae are very important in maintaining a standing or upright position.
The Semicircular Ducts:
In addition, three tubular structures are associated with the utriculus. The circle of each of these semicircular ducts is completed by the cavity of the utriculus. At one end of each semicircular duct is a crista, a ridge of hair cells across the axis of the duct.
When a jet takes off, a passenger tends to remain in place at first and can feel the resulting pressure of the seat against his back. Also, when the jet is no longer accelerating, the passenger can feel that the pressure of the seat against his back has returned to normal.
Likewise, in the appropriate semicircular duct, the endolymph ("passenger") tends to remain in place early during acceleration. Because the duct ("seat") itself is moving with the body ("jet"), the hairs of the crista are affected by the change in movement. Later, when acceleration stops, the effect upon the hairs of the crista is also registered.
However, the cristae of the semicircular ducts detect rotation of the head (angular acceleration and angular velocity). Linear acceleration, as with our example of the passenger and the jet, is detected primarily by the maculae, discussed above.
Resulting inputs for the Special Sense of Equilibrium:
The combined inputs from the maculae of the sacs and the cristae of the semicircular ducts provide continuous, instantaneous information about the specific location and posture of the head in relationship to the center of gravity of the earth.
These inputs are transmitted by the vestibular neurons to the hindbrainstem.