The second functional unit of the ear is the vestibular apparatus which is concerned with posture and balance. This is the sense of equilibrium. The structures that provide for this sense are located in the inner ear and they include the utricle and saccule, and the semi circular canals.
As in hearing the receptors for equilibrium are hair cells. These receptors are proprioceptors and they detect rotational and linear accelerations. The vestibular apparatus is innervated by the vestibular nerves which together with the cochlear nerve from the auditory apparatus make up the vestibulo cochlear (8th cranial) nerve.
The vestibular apparatus and the snail like cochlea involved in hearing form the inner ear within the temporal bones of the skull. The vestibular apparatus consists of two parts the otolith organs which are the utricle and saccule, and the semi circular canals.
The sensory structures of the vestibular apparatus and cochlea are located with in a tubular structure called the membranous labyrinth which is filled with a fluid that is like intracellular fluid in composition; called endolymph. The membranous labyrinth is also located within a tubular bony cavity of the same shape the bony labyrinth.
Within the bony cavity, between the membranous labyrinth and the bone is a fluid called perilymph, which is similar in composition to cerebrospinal fluids.
The Semi Circular Canals
The three Semi circular ducts and canals are perpendicular to each allowing each other one to be oriented in one of the three planes of space. The ducts are lined by the membranous labyrinth which is suspended in perilymph within the bony labyrinth while the canals are lined by bone.
The semi circular ducts contain endolymph. On the basis of their locations the semi circular canals and ducts are called superior (anterior) lateral and posterior. Each duct has an expanded end called an ampulla which contains a receptor structure called the crista ampullaris. Each crista consists of hair cells and sustentacular cells surmounted by a gelatinous material that closes off the ampulla.
The Utricle and Saccule
Within the vestibule which is the central chamber of the labyrinth are two endolymph filled sacs called the utricle and saccule housed within their bony chambers between the semi circular canals and the cochlea. Each sac also contains a sensory patch called macula. The maculas contain sustentacular and hair cells also surmounted by an otolithic membrane in which is an embedded crystal of calcium carbonate the otoliths.
Sensory hair cells of the vestibular apparatus. The receptors for next line equilibrium are modified epithelial cells known as hair cells because they contain twenty to fifty hair like extensions stereocilia i.e. processes that contain filaments of protein surrounded by part of the cell membrane.
One of the extensions is larger and has the structure of a true cilium with a clubbed end. That one is known as kinocilium. Each hair cell is oriented so that it depolarizes when its stereocilia are bent toward the kinocilium. Bending in the opposite direction hyperpolarizes the cell.
Functions of the Vestibular Apparatus
The vestibular apparatus provides information essential for the sense of equilibrium and for coordinating head movements with eye and postural movements. It detects changes in position and motion of the head. All the components of the vestibular apparatus contain endolymph and are surrounded by perilymph.
The main components of the vestibular apparatus are the utricle, saccule and the fluid filled semi circular ducts of the membranous labyrinth. The receptors of the utricle and saccule regulate static equilibrium and the receptors in the ampullae of the semi circular ducts respond to movements of the head.
The equilibrium system also receives input from the eyes and from some propioceptors in the body especially the joints. The movement of hairs of a hair cell, in the axis of sensitivity makes the hair cell to generate a receptor potential in the nerve ending, synapsing with the hair cells.
Functions of the Utricle and Saccule Static Equilibrium
The receptor region of the utricle and saccule called the macula contains hair cells embedded in a jelly like otolith membrane. Loosely attached to the membrane, and piled on top of it are hundreds of thousands of calcium carbonate crystals called otoliths (ear stones).
These stones make the membrane heavier resulting in a higher inertia than the surrounding fluid (resistance to change in position). Hair cells in the utricle respond to motion changes that occur during the straight-line acceleration and deceleration of the head (back and forth).
The hair cells also monitor the position of the head in space, controlling posture. It actually tells you automatically the position of your head in relation to gravity. The utricles are also responsible for initiating the righting reflex which is seen when a cat is dropped upside down and it lands on its feet.
This is because the maculas discharge tonically in the absence of head movement because of the pull of gravity on otoliths. The saccule generally responds more to vertical acceleration.
The mechanism of static equilibrium response depends on the difference in density between the otoliths (otoconia) and the endolymph inside each utricle and saccule. Otoliths have greater density than endolymph.
As a result when the head moves in a change of posture, the otoliths resist the external force and lag behind the motion of the endolymph. The otolith and membrane remain relatively still and bend the hairs of the hair cell. Each macula hair cell responds to the gravitational force exerted upon the hairs by the dense otoliths.
When the head is held horizontally, the gravitational force is directed downward upon the hair bundle. When the head is tilted to the side, the hair bundle of each hair cell is displaced towards the axis of sensitivity, and can excite the afferent nerve fibre. Atilt of the head that bends the hair against the axis of sensitivity has the opposite effect (inhibition of afferent neuron).
Bending of the hair cells changes the permeability of the cells to Na+ and K+. The resulting graded generator potential stimulates nerve endings of the vestibular nerve fibres to generate another graded generator potential and subsequently an action potential in the vestibular nerve.
Dynamic Equilibrium Semicircular Ducts
Each duct of the three semicircular ducts contains a bulge, the ampulla where the sensory hair cells are located which is the crista ampullaris. The hair is embedded within a gelatinous membrane called cupula that projects into the end lymph.
The hair cells respond to changes in rotational acceleration i.e. angular movements. Because of the situation of the three ducts in different planes at right angles to each other, at least one duct is affected by each head movement the one in the same plane as the head movement.
The endolymph of the semicircular ducts serves a function analogous to the otoliths membrane. It provides inertia so that the sensory processes will be bent in direct opposite to that of the angular acceleration. As the head rotates to the right for example, the endolymph causes the cupula to bend towards the left thereby stimulating the hair cells.
This is similar to our body’s jerking backwards, when the car in which we are suddenly jerks forward. If the head continues to rotate, the endolymph catches up and moves in unison (same rate and direction) with the head, so that the hairs return to their unbent position. When the head slows down and stops, reverse situation occurs.
The endolymph continues to move in the direction of the rotation while the head decelerates to a stop. The result is that the cupula and its hair are bent in the direction of the preceding rotation. When the endolymph gradually comes to a halt, the hairs straighten again. This way the semicircular ducts detect changes in rotational movement. They do not respond when the head is motionless or during circular motion at a constant speed.
Linear acceleration probably fails to displace the cupula and therefore does not stimulate the cristae. When the hairs bend in the direction of the axis of sensitivity, nerve endings are stimulated to produce a graded generator potential followed by an action potential in each nerve fibre. The brain receives the impulse and signals appropriate muscles to contract in order to maintain the body’s equilibrium.
Neural Pathways for Equilibrium
Stimulation of hair cells in the vestibular apparatus activates sensory neurons of the vestibulocochlear (8th cranial) nerve. The vestibular tracts consist of pathways to the brainstem, spinal cord, cerebellum and cerebral cortex.
The primary vestibular fibres from the vestibular nerve pass into the vestibular nuclei in the medulla oblongata and from their fibres are sent to the oculomotor centre of the brain stem and to the spinal cord. Neurons in the oculomotor centre control eye movements and those in the spinal cord stimulate movements of the head, neck and limbs.
The sensory signals from the vestibular sensors of the labyrinth are indicators of the position and movements of the head. These inputs from the vestibular receptors are critical in;
Generating compensatory mechanism to maintain balance and an erect posture in response to gravity.
Producing the conjugate movements of the eyes that compensate for changes in the position of the constantly moving head, and
Supplying information for conscious awareness of position, acceleration, deceleration and rotation. The vestibular functions are supplemented by proprioceptive inputs from the muscles and joints as well as the visual system.
Nystagmus and Vertigo
When a person first begins to spin, the inertia of endolymph within the semicircular canals causes the cupula to bend in the opposite direction. As the spin continues however, the inertia of the endolymph is overcome and the cupula and hair cells straighten up.
This time the endolymph and cupula move in the same direction and at the same speed. If the movement is suddenly stopped, the greater inertia of the endolymph causes it to continue moving in the previous direction of spin and to bend the cupula in that direction (the way your body moves forward when your car stops suddenly).
Bending the cupula after the movement has stopped affects the muscular control of the eyes and body through the neural pathways. The eyes slowly drift in the direction of the previous spin and then are rapidly jerked back to the midline position producing involuntary oscillations.
These movements are called vestibular nystagmus and people experiencing this effect may feel that they or the room are spinning. The loss of equilibrium that results is called vertigo . If the vertigo is severe or in persons who are particularly susceptible the autonomic nervous system may be involved and this produces dizziness, pallor, sweating and nausea.
Vestibular nystagmus is one of the symptoms of an inner ear disease called Meniere’s disease. The disease may occur as a result of fluid imbalances within the inner ear. The early symptoms of the disease are ringing in the ears or tinnitus. Since the endolymph of the cochlea and the endolymph of the vestibular apparatus are continuous, vestibular symptoms of nystagmus and vertigo are often accompanied by hearing problems in this disease.
Some individuals, for incompletely understood reasons, are especially sensitive to particular motions that activate the vestibular apparatus and cause symptoms of dizziness and nausea; this sensitivity is called motion sickness.
Because the inner ears of deaf mutes are not functioning, they are immune to dizziness and motion sickness. These ones have to rely on visual cues for the maintenance of normal movement and posture. Without visual cues, someone who has lost the use of the inner ear may navigate down instead of up when he wants to move up, because there is no sense of position in space (equilibrium).