Vestibular System

You must have wondered sometimes in your life how your body maintains its balance. There is a system in your body, called the vestibular system, that is responsible for maintaining the balance of the body and performing associated functions.

It is a sensory system present in the inner ear, that provides sensory information to the brain required for maintenance of balance and performing coordinated movements.

In this article, we will discuss some important
aspects of the vestibular system including its anatomy, its mechanism, its
connections and functions. We will also discuss how the quality of life is
affected by any disease of the vestibular system.

Anatomy

In this section, we will discuss the important gross anatomical
features of the vestibular system

Location

As stated earlier, the vestibular system is present in the inner ear. It is present in a system of bony tubules and chambers present in the petrous part of temporal bone, a part of the skull. This system of bony tubules is called the bony labyrinth.

The bony labyrinth contains a system of membranous tubules and chambers, called the membranous labyrinth. This membranous labyrinth is considered to be the functional part of the vestibular system.

Structure

The inner ear, present in the petrous part of temporal bone, is made of three parts:

• Cochlea concerned with the process
  of hearing
• Vestibules concerned with the
  process of equilibrium sensation
• Semicircular Canals concerned with
  equilibrium and position sensation.

The vestibular system comprises of the vestibule and
the semicircular canals.

Vestibule

Vestibule consists of utricle and saccule. These are two parts of vestibular labyrinth (a membranous labyrinth) present within the bony labyrinth of the vestibule.

The utricle is continuous with the semicircular canals
posteriorly. Anteriorly, it communicates with the saccule through the utriculosaccular duct. The
endolymphatic duct arises from the utriculosaccular duct.

The utricle and saccule have specialized areas of
sensory epithelium concerned with the perception of equilibrium and position
sensations. This characteristic epithelium is called macula.

The macula of utricle is present on the floor of the utricle
while the macula of saccule is present in its medial wall.

Semicircular Canals

These are the bony canals that open into the vestibule of bony labyrinth. There are three semicircular canals; anterior, posterior and medial semicircular canals. They open into the vestibule via five opening because the anterior and posterior canals have one common limb.

The semicircular canals contain semicircular ducts as part of the membranous labyrinth. Each semicircular duct has an ampulla at one end. The ampulla contains the sensory area called the ampullary crest.

These ampullary crests sense the movement of head and play a role in maintaining the balance of the body.

Nervous Supply

The nerve of the vestibular system is the vestibulocochlear nerve. The vestibular part of the nerve is concerned with the sensations of position and equilibrium.

Blood Supply

The blood supply of vestibular system is derived mainly from the labyrinthine artery, a branch of basilar artery. Other arteries that provide blood to the vestibular system include the posterior auricular artery, middle meningeal artery and anterior tympanic branch of maxillary artery.

Physiology of the Vestibular System

In order to understand the physiology and functioning of the vestibular system, we first have to understand the structure of Macula and Ampulla in detail.

Structure of Macula

Macula is a small sensory organ slightly greater than 2mm in size, present inside the utricle and saccule. It has special features that help it detect then change in position and equilibrium.

Macula has thousands of hair cells present in it. Each macula is also covered by a gelatinous layer that contains many small calcium carbonate crystals. These calcium carbonate crystals are called the statoconia. They are embedded in the gelatinous layer. The cilia of hair cells project into this gelatinous layer.

The bases and sides of hair cells make synapses with the vestibular nerve. The specific gravity of the statoconia is 2 to 3 times greater than the surrounding fluid. These statoconia cause the bending of the cilia in the direction of the pull, mainly the gravitational pull.

Excitation of the Hair cells

Each hair cell in the macula has 50 to 70 small cilia, called the stereocilia. A large cilium is also present called the kinocilium. The kinocilium is always present on one side of the hair cell.

The stereocilia continuously become shorter in length as one moves to the other side of the hair cell. There are minute filamentous attachments that connect each stereocilium to next longer stereocilium and ultimately to the kinocilium.

When the kinocilium and stereocilia bend in the direction of kinocilium, the filamentous attachments pull the stereocilia in a direction outward from the cell body. A large number of fluid channels are present at the base of stereocilia.

This movement causes the opening of these fluid channels that carry a number of positive ions inside the hair cell. This causes resting membrane depolarization of hair cell.

On the other hand, the movement of stereocilia in a
direction opposite to the kinocilium causes the closing of these channels. As a
result, hyperpolarization occurs.

Under normal conditions, the nerve fibers from the hair cells conduct impulses at the rate of 100 per second. Bending of cilia towards the kinocilium increases the rate of impulses whereas the bending of cilia away from the kinocilium decreases the impulse conduction in the vestibular nerve.

Structure and Excitation of Ampulla

Ampulla are the sensory organs present in the semicircular ducts. Each semicircular duct has one ampulla at one end. Endolymph present in the semicircular as well as the ampulla.

Each ampulla has the ampullary crest called crista ampullaris. Crista ampullaris has a number of hair cells in its structure, the same cells as are present in the macula of the vestibule.

A loose gelatinous layer is present on the top of each crista, called the copula. Thousands of cilia and kinocilia project from the hair cells in crista ampullaris into the copula.

All the kinocilia in the copula are arranged in the same direction. When the head turns, the endolymph in the cochlear ducts is displaced. This displacement of endolymph causes the bending of kinocilia.

The bending of the copula in one direction causes the depolarization of hair cells whereas bending in the opposite direction causes hyperpolarization.

Transmission of Sensory Information to the Brain

The nerve impulses arising in the vestibular system are carried by the vestibular nerve to the brain.

The first-order neurons are the vestibular nuclei
present in the vestibular system of the inner ear. These vestibular ganglia
synapse with the hair cells present in the macula and ampulla. The efferent
fibers from the vestibular ganglia terminate at the vestibular nuclei.

The second-order neurons are the vestibular nuclei. They receive afferent fibers from the vestibular ganglia in the form of the vestibular nerve as well as from the cerebellum. The efferent fibers from vestibular nuclei have the following fates:

• Fibers from lateral vestibular
  nuclei travel in the spinal cord as the vestibulospinal tract
• Some fibers    form the medial longitudinal fasciculus,
  giving fibers to oculomotor, trochlear and abducent nerves
• Some fibers ascend upwards into the
  vestibular area of the cerebral cortex after relaying in the ventral posterior
  nuclei of thalamus

Functions of the Vestibular System

Here are some of the functions performed by the vestibular system.

Maintenance of Static Equilibrium

The utricle and saccule are responsible for the maintenance of static equilibrium. The hair cells in the macula of the utricle and saccule are oriented in different directions. The specific pattern of stimulation of these hair cells keep the brain informed about the position of the head.

Depending upon this information, the vestibular,
reticular and cortical systems of the brain send excitatory signals to the
appropriate muscles, so that the posture is maintained.

Prediction of Disequilibrium

When the head suddenly begins to rotate, the endolymph in the semicircular ducts is displaced. This causes hyperpolarization or depolarization of the hair cells of crista ampullaris.

These ducts send signals to the higher centers only when the head begins to rotate or stops rotation, as the copula and the hair cells rapidly adapt to the new position of the semicircular canals. This is because the endolymph becomes static. Thus, these ducts play a predictive role in the maintenance of the equilibrium.

The semicircular ducts predict that the disequilibrium is going to occur and cause the equilibrium centers to make appropriate preventive adjustments. These adjustments help the person maintain balance before the situation can be corrected.

Movement of the Eyes

The vestibular system is also responsible for the reflex movement of the eyes. It causes the eyes to rotate in the direction of movement of the head. This function is performed via the medial longitudinal fasciculus.

Conclusion/Summary

The vestibular system is the sensory system that is
responsible for the maintenance of the body positioning and equilibrium.

It is present in the inner ear, within the bony labyrinth.

It consists of two parts; the vestibule which
comprises of the utricle and the saccule, and the three semicircular canals.

The sensory organ in the utricle and saccule is the
macula, which detects any change in the linear positioning of the head.

The semicircular canals have ampulla at one end of each semicircular duct. This ampulla detects the rotatory movement of the head.

The signals arising from the vestibular system are
carried by the vestibular nerve to the brain and other centers.

The functions performed by the vestibular system include:

• Maintenance of static equilibrium
• Prediction of the disequilibrium
• Controlling the movement of eyes