The vestibular system
The vestibular system is a system that enables humans to maintain balance and orientation within its three dimensional environment in relation to gravity. This system allows the body to easily change orientation and maintain balance while making different types of movement simultaneously. The vestibular system helps us to maintain balance without falling down when standing and when making complex movements.
Anatomical Structures of the Vestibular System
The structures that constitute the vestibular system are located in the inner ear, they are:
- Semicircular canals – The semicircular canals are three interconnected bony tube-like structures located posteriorly and superiorly to the vestibule of the ear. The three canals are called horizontal (also called lateral), superior (also called anterior) and posterior semicircular canals.
Superior semicircular canal – It is about 15-20 millimeters long and extends across the petrous part of the temporal bone under the arcuate eminence. It has an ampulla (a sac-like enlargement) on its base that is continuous with the utricle. Its other end joins with the posterior semicircular canal at the common limb (also called crus commune ). It opens into the medial part of the vestibule. The canal contains fluid that is sensitive to movement or rotation of the head within a lateral plane.
Posterior semicircular canal – It is about 18-22 millimeters long, it curves backwards towards the posterior surface of the petrous part of the temporal bone. It has an ampulla that opens in the lower part of the vestibule and the other end is continuous with the common limb. This canal is sensitive to movements or rotation of the head within an anterior-posterior plane.
Horizontal semicircular canal – It is about 12-15 millimeters long and curves horizontally and backwards towards the posterior semicircular canal. Its ampulla is continuous with the utricle and the other end opens below the common limb. This canal is sensitive to movements within a transverse plane.
- Vestibule – The vestibule is an ovoid structure, about 5 millimeters long and lies anterior to the semicircular canals, posterior to the cochlea and medial to the tympanic cavity. Its lateral wall has an oval opening called the fenestra vestibuli where the base of the stapes inserts. There is a small spherical recess in front of the medial wall of the vestibule and this recess contains the saccule which is a collection of sensory cells. Posterior to the recess is a vestibular crest which forms the vestibular pyramid and the crest bifurcates below to surround the cochlea recess. Another recess called the elliptical recess which contain the utricle lies in the medial wall of the vestibule. The pyramid and elliptical recess contains small holes called the macula cribrosa superior, and these holes transmit branches of the vestibular nerve to the macula of the utricle and to the crista ampularis of the superior and lateral semicircular canal. The opening on the medial part of the vestibule, the vestibular aqueduct, extends to the posterior part of the petrous part of the temporal bone. The opening of the ampulla of the semicircular canals is into the posterior part of the vestibule.
Utricle – The utricle is located within the vestibule, it lies in a horizontal plane and it is sensitive to movement of the head in the horizontal plane. It is saclike and has a region called the macula of the utricle which consists of specialized hair cells that are inserted in a gelatinous membrane, these hair cells have about 70 stereocilia and one kinocilium (a receptor in the middle of the stereocillia and is probably immotile), the upper layer of the macula of the utricle contains crystalline particles called otoliths or otoconia, these crystalline particles are made up of calcium carbonate molecules. The otoliths helps in stabilizing motion in relation to gravity. The sensory hair cells in the utricle are polarized and the macula of the utricle of both ears align horizontally when the head is in its normal position.
Saccule – The saccule also lies within the vestibule, it lies in a vertical plane and just like the utricle, it has a macula (specialized area of sensory epithelia cells), the macula of the saccule is elongated, and is vertically situated on the saccule wall. The macula is also covered by an otolithic membrane and also have a striola that extends along its long axis and is polarized. The region of the macula just above the striola is called the pars interna and just below the striola is called the pars externa. The saccule is joined to the utricle by the utriculosaccular duct and to the cochlea by the ductus reuniens. The saccule and utricle are functionally similar; due to the saccule’s macula vertical orientation, it is sensitive to movement of the head in the vertical plane and this is the main sensor when the head is in a straight position.
Maintaining posture and equilibrium
Balance on the human body is achieved by the eyes (visual system), muscles and joints on the body (proprioceptive system) and the labyrinth (vestibular system). These areas relay signals to the brain to maintain equilibrium, coordination and cerebral perception.
The utricle and saccule are sensitive to linear accelerations with respect to gravity and the semicircular canals are sensitive to angular acceleration such as rotation of the head with respect to gravity. The utricle, saccule and semicircular canal possess sensory hair cells.
The cilia (stereocilia) in the hair cells are found in membranes called otholithic membrane (found in the utricle and saccule hence the name otholith organs) and cupula membrane (found in the semicircular canal).
The sensitive cells located inside the utricle, saccule and semicircular canals are the hair cells. They are used to convert mechanical energy into electrical energy by deflection of the hair cells. The hair cells have a bunch of stereocilia that project from their topmost surface. The thickest and longest stereocilia is called a kinocilium. Stereocilia within a bunch are joined together by protein strands, called tip links, which extends from the side of a taller stereocilium to the tip of its shorter neighbor in the array.
Push-Pull mechanism of the semicircular canal
The semicircular canals are oriented in a way that each canal on each side of the head is mirrored with its counterpart on the opposite side. Each of these three pairs works in a push and pull way. The stimulation of one canal (for example left side) results in the inhibition of the other on the other side of the head (for example right side). This mechanism makes it possible to be able to sense all directions of rotation when making movements with the head. It is important that both sides work in this push and pull mechanism but if there is a pathological predisposition to the canals it may result in affecting this mechanism as in those seen in vertigo.
The hair cells in the canals are arranged in such a way that they project into a gelatinous membrane called the cupula. When you turn your head in the plane of the canal, the endolymph causes it to splash against the cupula which then results in the deflection of the hair cells. But if you were to keep turning in circles without stopping, the endolymph fluid would catch up with the canal, and there would be no more pressure on the cupula. If you stopped spinning, the moving fluid would splash against a suddenly still cupula, and you would feel as though you were turning in the other direction.
The semicircular canal functions in the vestibulo-ocular reflex; the vestibulo-ocular reflex is a reflex that keeps image on the retina of the eyes stable when the head is moving. This reflex brings about eye movement when the vestibular system is stimulated. This is achieved by deflecting the eyes to the opposite direction from the direction of the head movement.
When you move your head in all directions, you can still focus on what you are looking at and this is because the semicircular canals directly control the eyes so that they can easily focus even though the head is moving. The eye is controlled by three pairs of muscles; the medial and lateral rectus, the superior and inferior rectus, and the inferior and superior oblique. A single canal controls a single muscle pair and this is called the vestibulo-ocular reflex.
Neural pathway of the vestibulo-ocular reflex
The semicircular canals is stimulated by rotation of the head and they send efferents through the vestibular nerve and through the Scarpa's ganglion and finally terminates in the vestibular nuclei in the brainstem. From the brainstem, fibers cross to the contralateral abducens nerve nucleus and synapse. They finally project to the muscle of the eye called the lateral rectus through the abducens nerve, the fibers also project to the oculomotor nucleus and activates another muscle of the eyes called the medial rectus muscle through the oculomotor nerve. When there is movement of the stereocilia in any direction, this results in an increase or decrease in rate of the pores on the tip of the cilia and this also results to a change in membrane potential (membrane voltage) of the receptor cell and this is sent to the brain by the vestibular nerve, this is further processed by visual and somatosensory signals and this finally gives the information on the orientation of the head.
When the bunch of stereocilia is deflected during movement of the head, the tip which are linked together by the protein open and close mechanically sensitive ion channels and when all cilia are deflected toward the kinocilium, the gates open and cations such as potassium ions from endolymph flow in and this makes the membrane potential of the hair cell positive hence its depolarized but the hair cell itself does not fire any action potentials. The depolarization activates voltage-gated calcium channels at the basolateral aspect of the cell which results in an influx of calcium ions into the cell and a release of neurotransmitters, such as glutamate. This is followed by diffusion across the narrow space between the hair cell and a nerve terminal, which then bind to receptors and hence activates an increase of the action potentials firing rate in the nerve.
When the stereocilia is deflected away from the kinocilium, the firing rate decreases and there is an efflux of calcium ions from the hair cells thereby causing the cell membrane potential to be more negative and hence hyperpolarized.
The vestibulo-spinal reflex uses the vestibulospinal tract to maintain balance and posture. Movement of the head stimulates both canals and their otoliths which further stimulates the vestibular nerve and its nucleus. The signals are relayed down to the lateral and medial vestibulospinal tracts and then to the spinal cord. The spinal cord stimulates an extensor effect in the muscle on the side of the neck in which the head is tilted to and also induces a flexor effect in the muscle on the contralateral side of the neck.
The lateral vestibulospinal tract are neural extrapyramidal motor fibers. These fibers are found in a bundle of nerve root in the spinal cord called lateral funiculus. They originate from the Deiters nucleus (also called the lateral vestibular nucleus) in the pons and descend to the lumbar region of laminae seven and laminae eight interneurons of the spinal cord ipsilaterally. This tract helps in stimulating extensor muscle motor neurons in the legs in other to maintain posture and balance.
The medial vestibulospinal tract are neural extrapyramidal motor fibers; these fibers are found in a bundle of nerve root in the spinal cord called the anterior funiculus. They originate from the Schwalbe's nucleus (also called the medial vestibular nucleus). The Schwalbe's nucleus is found in the inferior olivary nucleus of the medulla oblongata and the pons. This tract descends in the anterior funiculus of the spinal cord and run down to the anterior funiculus to the cervical segment of the spinal cord and stimulates ventral horn of the cervical spinal circuits and terminate in laminae seven and eight. This tract contains both crossed and uncrossed fibres. The medial vestibulospinal tract stimulates muscles that support the head and only run down to the cervical segments of the spinal cord. Other neural projections that help to maintain posture and equilibrium are the vestibulocerebellar tract, projections to the thalamus and projections to the reticular formation.
Vestibulocochlear nerve - The vestibulocochlear nerve is the VIII cranial nerve, it consists of the cochlea nerve and vestibular nerve. It arises from the margin of the cerebellum and pons called the cerebellopontine angle and passes through the posterior cranial fossa into the petrous part of the temporal bone through the internal acoustic meatus. It bifurcates into the cochlea nerve and vestibular nerve. The vestibular nerve innervates the maculae of the otoliths (utricle and saccule) and the ampulla of the semicircular canals. The cochlea nerve relays signals from the organ of Corti to the cochlea.