General concepts
The vestibular system (vestibulum - L = entrance) is the neural system that sets body equilibrium or balance. Equilibrium is defined as the condition in which all influences acting on a structure are cancelled out by others, resulting in a stable, balanced system.
For example, if the head moves to the left, there has to be increased extensor muscle activity on the left side of the body to accommodate the shift in weight. Head movement can be primary (animal turning to look at something) or secondary due to movement of the body. The latter can be generated by the animal (e.g. sitting, locomotion) or by external forces, such as being pushed by another animal. Head movement is detected by the vestibular system, the input from which results in modification of somatic muscle activity to accommodate the new set of forces acting on the body.Cephalisation is the concentration of the exteroceptors and neural processing centres at the leading end of the animal. Use of exteroceptors such as vision, audition, olfaction and gustation, as well as tactile receptors, (e.g. whiskers) necessitates precise control of head position and movement. Controlling head position and movement requires detailed proprioceptive input that modifies motor activity in the neck muscles. Changes in head and neck position require postural changes in axial and appendicular muscle activity to provide the correct postural support. This is reflected in increased extensor muscle activity on the side to which the head is turned. Therefore head position dictates tone in muscles through the neck, body and limbs. To maintain appropriate visual input requires that the position and movement of the eyes be linked to head position and movement.
The vestibular apparatus, located in the inner ear, houses the receptor organs for detecting head position and movement. Stimulation of the vestibular apparatus provides the major input to the vestibular nuclei in the brainstem.
However, the vestibular nuclei also integrate input from the gravity and movement sensors in the inner ear with proprioceptive information from muscles and joints.The vestibular apparatus on each side consists of five distinct organs. There are receptors in three semicircular ducts, which are sensitive to angular acceleration/head rotation, and in two otolith organs (utriclus and sacculus), which are sensitive to linear accelerations and the influence of gravity in static head positions. The receptors are housed in the inner ear, in a series of membrane-lined, fluid-filled ducts and spaces located in the bony vestibule of the petrous temporal bone.
The receptors that detect head position and movement are hair cells. These are specialised epithelial cells with fine, hair-like processes on top of them. The base of the hair cell synapses with sensory nerves that convey action potentials via the vestibular portion of cranial nerve VIII (vestibulocochlear nerve) to the vestibular nuclei. The cell bodies of these bipolar sensory nerves are spread along the fibres forming the vestibular ganglion in the petrous temporal bone. There are four pairs of vestibular nuclei, located in the brainstem, just ventral to the cerebellum. The vestibular nuclei have five main efferent connections. They give rise to UMN spinal cord tracts that reflexively stimulate ipsilateral extensor muscle activity in the limbs, trunk and neck, inhibit antagonist muscles and contralateral extensor muscles. Rostrally directed output from the vestibular nuclei affects function of the cranial nerve nuclei, III, IV, VI, causing extraocular muscle contraction for eyeball position and movement. The vestibular nuclei send information to the cerebellum and cerebrum about head proprioception for subconscious modulation of postural muscle activity, and for conscious awareness, respectively. The vestibular nuclei also connect to the vomiting centre of the brainstem reticular formation; this is the basis of motion sickness.
The basic mechanism that stimulates the hair cells is similar for the detection of either static, or changing, head position. On each hair cell are 40-80 stereocilia (long microvilli) and a single kinocilium. These processes project into an overlying gelatinous layer, which, when it moves, causes deviation of the cellular processes. This deviation induces changes in membrane potential of the hair cell and may trigger an action potential in the sensory nerve fibre that synapses with the base of the hair cell. Gravity constantly stimulates hair cells that detect static head position. However, hair cells that detect head movement will only do so during acceleration or deceleration of the head, not when the head has reached constant velocity.