The inner ear contains several sensory organs two of which provide information about the direction of the force of gravity and the angular rotation of the hear. The Saccule and Utricle sense the direction of gravity, and are responsible for the reflex (automatic) responses to tilting.
One way of dealing with moving the centre of gravity outside the base is to broaden the base - by separating the feet or by side-stepping. Upright Posture and Balance are maintained by automatic adjustments of muscle tone; such movements are involuntary (they are not willed) and determine one’s posture.
This reflex adjustment is mediated by descending pathways from the vestibular nuclei (vestibulo-spinal tracts), and adjust the position and tone of the muscles of the limbs and the axial muscles so as to keep the centre of gravity within the base. If the tilt is sideways, the reflex produces some side-stepping in order to a wide base beneath the centre of gravity.
The Semicircular Canals constitute a second organ within the vestibular apparatus. The semicircular canals sense rotation of the head in three planes - vertical, horizontal and lateral. Rotation in each plane is sensed by a semicirular canal on each side of the head.
The detection of sudden movements (accelerations) to the head is performed by the three Semi-Circular Canals. This information is sent to the brainstem, to and induces sudden changes in posture that compensate for the acceleration.
Vestibulospinal tracts. The vestibular nuclei give rise to the medial and lateral vestibulospinal tracts. The medial vestibulospinal tract descends in the anterior funiculus and innervates the upper half of the cord (above T6). It innervates neck muscles and stabilises the head on with respect to the trunk during changes of position of the body. It works in partnership with pathways that control the position of the head and eye movements (see above).
The lateral vestibulospinal tract projects ipsilaterally through the anterior funiculus of the spinal cord to all segments of the spinal cord. It increases the tone of antigravity muscles and is concerned with compensatory adjustments in posture to accommodate tilts and movements of the body.
Tilting and Eye Movements When the head is tilted, the eyes also rotate in the opposite direction, so as to maintain a horizonal horizon in the visual fields This reflex response involves a short chain of neurones, starting in the vestibular apparatus and vestibular nuclei.
On each side of the midline of the brainstem, a band of axons called the medial longitudinal bundle and communicates information concerned with the position the head to the nuclei of the oculomotor, trochlear and abducent nerves, and to the paramedian pontine reticular formation (PPRF). These pathways are responsible for the vestibulo-ocular reflexes, and the PPRF is concerned with coordinating eye movements in response to vestibular and other inputs.
Vestibular Apparatus and the Control of Muscle Tone Compensatory changes in muscle tone during changes in posture are transmitted from the vestibular nuclei to spinal motoneurones using the vestibulospinal tracts. The neurones involved have cell bodies in the vestibular nuclei in the medulla, and long axons that project to the interneurones and motoneurones in the spinal cord.
The Frontal Eye Fields are an area (Brodmann's area 8) of each frontal lobe, which when stimulated electrically result in coordinated movements of both eyes in a horizontal direction.
This area receives input from the visual asssociation cortex and connects with the superior colliculi and the paramedian pontine reticular formation (PPRF), a region of the midbrain that has vestibular inputs and is involved in coordinating eye movements. The superior colliculi are involved in initiating changes in the diretion of gaze when objects suddenly move into the periphery of the visual fields.
Vestibular Connections with the Cerebellum. The cerebellum is concerned with the control of movement, integrating signals from different parts of the nervous system and generating error signals that allow adjustments to be made to any muscles involved so as to achieve the desired objective of the movement.
The vestibular apparatus and the vestibular nuclei have close links with the flocculonodular lobe, which can modulate vestibulo-ocular reflexs and stabilize the position of the eyes during rotation and tilting.
Neurons in this part of the cerebellum extract from the vestibular signals about the speed and acceleration of movement and command the eyes to follow a moving object smoothly. These movements that follow an object while the head is moving are known as pursuit movements.
Vestibulo-cerebellar circuits through the flocculonodular lobe allow the cerebellum to compensate for damage to the inner ear. Damage to this lobe will result in vertigo and/or nystagmus.
Nystagmus - 'Dancing Eyes'- is an involuntary eye movement consisting of a rapid movement of the eyes in one direction, followed by a slow movement back to the starting position.
Physiological Nystagmus. When the head is rotated, distant visual images would move beyond the visual field very rapidly, and reflexes initiated by the semicircular canals attempt to fixate the gaze on the moving object. This involves a slow movement of the eyes in the direction opposite to the rotation, during which the retina fixates and follows the distant object, followed by a rapid movement of the eyes in the direction of rotation after whch the eye fixates on a new distant object, and follows it.
The semicircular canals sense angular momentum, and send information to the vestibular nuclei. The medial longitudinal fasiculus (or bundle) runs up the brainstem near the midline below the fourth ventricle and makes contact with the cranial nerve nuclei that cause the extraocular muscles peroform the tasks mentioned above. The direction of ocular movement depends on which semicircular canals are activated by the rotation.
Pathological Nystagmus. Pathological nystagmus may be caused by congenital disorders, acquired or central nervous system disorders and drugs such as alcohol. The CNS disorders include disorders of the vestibular system, brainstem or cerebellum (particularly the flocculo-nodular lobe). Nystagmus is sometimes associated with vertigo, a sense of dizziness accompanied by a sensation of rotatory movement.
The vestibulo-ocular reflexes can be tested using caloric testing, in which warm or cold water is passed into the external auditory canal. The change in temperature initiates convection currents in an adjacent semicircular canal, causes movements of the crista ampullaris, and initiates activity in the medial longitudinal bundle, producing nystagmus.
The control of the intrinsic muscles of the eye is not concerned with Balance or Eye movements, but is of importance in determining the amount of light entering the eye and focusing that light on the fovea.
The autonomic (parasympathetic) fibres of the III nerve originate in the Edinger-Westphal nucleus and travel to the ciliary ganglia inthe orbit. There they synapse with post-ganglionic parasympathetic neurones that innervate the iris and the ciliary muscles of the anterior chamber of the eye. The intrinsic muscles of the eye also have a sympathetic nerve supply consisting of the postganglionic neurones of the superior cervical ganglion.