Vestibular System

Introduction

The vestibular system is the somatosensory system part of the inner ear. It’s responsible for translating the sensory stimuli into information about motion (kinesthesia), the head’s position in space (proprioception), sudden changes of the position of the head, and spatial orientation. 

 

This system also regulates and controls the motor functions that help us maintain balance, stabilize our head and body during movement, and maintain our posture.

What Is the Structure of the Vestibular System?

The vestibular system comprises a central and a peripheral part, containing two structures of the bony labyrinth of the inner ear – the vestibule, the semicircular canals, and the structures located in the membranous labyrinth.

The Inner Ear

The inner ear is located in the temporal bone and comprised of two functional structures:

  • Bony and membranous labyrinths (involved in the process of hearing and maintenance of posture);
  • Vestibular and cochlear systems (involved in the process of equilibrium sensation).

The inner ear structures convert the mechanical energy relayed in waves of neuronal impulses (transduction of olfactory signals) to sound. 

The inner ear also has a significant role in maintaining the postural balance and the focus of our eyes (gaze fixation). 

The Cochlea

This inner ear structure has the characteristic shape of a snail’s shell, hence its name. Its central element is the cochlear cupula. The cochlear cupula provides a sense of spatial orientation. 

 

The inner ear performs several special functions. Namely, it transforms the sound waves into electrical signals (nerve impulses), which allows the brain to understand sounds. The inner ear also has an essential contribution to the process of balance maintenance.

The Otolith Organs (Vestibules)

The otolith organs are the two void spaces located in the inner ear’s bony vestibule:

  • The utricle is located in the back part of the vestibule. It is connected to the semicircular canals, while on the other end, it forms a utricular-saccular duct together with the structure labeled as saccule. It continues to the back of the skull’s temporal bone.
  • Saccule is located in front of the utricle. It’s much smaller in size. Apart from communicating with the utricular-saccular duct, this structure is also linked to the cochlea via the ductus reuniens structure.

These otolithic organs can perceive both vertical and horizontal gravity and linear acceleration (for instance, initiation of movement in a straight line). Head motions provoke the otolith organs to activate the hair cells. In this way, they stimulate the vestibular nerve, which signals the head’s position against the other body parts.

What Is Macula and What’s Its Structure?

The macula is a small sensory organ, i.e., the gravity receptor of the vestibular system. This patch of sensory cells is located within the utricle and saccule structures. Each of the macula’s cells contains 40 to 70 stereocilia, and one true cilium is called a kinocilium.

The cells comprising the macula are also enveloped in a special layer labeled as an otolithic membrane. It contains numerous small molecules of calcium carbonate in the form of crystals, labeled as statoconia.

Semicircular Canals

The semicircular canals are three membranous channels located within the bony semicircular ducts of the labyrinth:

  • Anterior (superior) canal – located in the sagittal plane;
  • Lateral (horizontal) canal –  located in the transverse plane;
  • Posterior (inferior) canal – located in the frontal plane.

These canals of the vestibular system are located in three planes of the body. Each of the canals ends with dilation at their terminal ends, called an ampulla. 

The ampulla of each semicircular canal contains a cluster of mechanoreceptors labeled as crista ampullaris. They are composed of the individual sensory receptor cells, called hair cells. 

As the head makes motions, the fluid in the canal lags, flows past the cupula, bending it in the process. The movement innervates the hairs and informs their cells to fire nerve signals.

Vestibulocochlear Nerve

The vestibulocochlear nerve refers to the cranial nerve 8 (CN VIII). While the vestibular nerve is responsible for maintaining body balance and eye movements, the cochlear nerve takes part in the process of hearing. 

Vestibular Nuclei

The vestibular nuclei are comprised of 4 nuclei:

  • The Bechterew’s superior vestibular nucleus;
  • The Deiters’ lateral vestibular nucleus;
  • The Roller’s inferior vestibular nucleus; 
  • The Schwalbe’s medial vestibular nucleus. 

 

They contain second-order neurons of the vestibular tract that synapse with the vestibular part of the vestibulocochlear nerve. 

The vestibular nuclei consolidate fibers from the following structures: 

  • the peripheral vestibular structures, 
  • contralateral vestibular nuclei, 
  • cerebellum, 
  • the visual and somatosensory systems.

Hair Cell Receptors

The ears provide not only a sense of hearing, but they also send signals about the position of our head and its motion. In this way, they are essential to balance. Both hearing and balance rely upon the activity of the hair cell receptors.

 

Hair cells are the motion receptors of the inner ear. They are located within the walls of the semicircular canals and otolith organs and are responsible for the transduction of sound signals. 

 

The semicircular canal cells project their fibers from the cristae ampullary towards the canal cavity. A cupula’s gelatinous mass covers the hair cells’ apical lining in the semicircular canals. This structure provides a sense of spatial orientation.

Which Reflexes Are Controlled by the Vestibular System?

The central vestibular system’s neuron fibers branch to the ocular muscles, skeletal muscles, and spinal cord. Hence, they support several important reflexes. 

 

  • The vestibular-ocular reflex (VOR)  
  • The vestibular-colic reflex (VCR)
  • The vestibulo-spinal reflex (VSR) 
  • The cervical-ocular reflex (COR)
  • The cervical-spinal reflex (CSR) 
  • The cervical-colic reflex (CCR) 

 

However, the most important reflex among them is the vestibular-ocular reflex (VOR).

The Vestibular-Ocular Reflex (VOR) 

The vestibular-ocular reflex refers to an unconsciously performed motor activity. It’s regulated and controlled with the help of the vestibular system in order to support the eye movement adjustments. 

As the head moves in the body’s horizontal plane, this reflex supports gaze fixation. The reflex arc is controlled by the medial longitudinal fasciculus (MLF).

How Is the Vestibular-Ocular Reflex (VOR) Triggered?

The endolymph in the vestibular organs descends on the right side when we turn our head sideways. Inertia stimulates the opposite horizontal semicircular canal hair cells. The sensory information is transmitted to the vestibular nuclei via the vestibulocochlear nerve on the same side of the body.

The neuron cells from the vestibular nuclei located on the middle and the upper side join the medial longitudinal fasciculus. From here, they get to the nucleus of the abducens nerve on the opposite side of the body. Hence, the opposite side’s rectus muscle contracts and directs the gaze in a direction opposite to the head’s movement.

How Do We Move Both Eyes Simultaneously?

During this process, some of the medial longitudinal fasciculus axons pass the middle area and create synapses with the neuron cells of the oculomotor nucleus. This activates the opposite eye’s medial rectus muscle, resulting in a simultaneous movement of both eyes in the body’s horizontal plane. 

Simultaneously, the medial rectus muscle located on the same side of the body is obstructed, so it becomes unable to interact with the muscle located on the other side of the body.

To sum up, once a person turns their head to secure and focus the gaze on an immovable object, the vestibular-ocular reflex supports eye movement in the opposite direction. In this way, we can keep our gaze on the visually targeted object.

How Does the Vestibular System Work (How Do We Keep Our Balance)?

Balance includes analyses of the data received by sensory stimuli from the eyes, skin, and muscles and an adjustment of the body’s position by regulating and controlling the motor neuron outputs. 

 

The vestibule and semicircular canals of the inner ear also play a highly significant role in the process. Namely, the vestibule responds mainly to the head’s position relative to gravity (static equilibrium). Simultaneously, the semicircular canals react primarily to the speed and direction of head movements (dynamic equilibrium). 

What Triggers Vestibular Balance Disorders?

Disorders of the vestibular system result from damage to either the peripheral or central system that regulates and controls our ability to maintain balance. 

 

 

The triggers that often lead to a dysfunction of the vestibular balance include:

 

  • Medicines;
  • Infections;
  • Inner ear problems (such as poor circulation in the ear);
  • Calcium waste collected in the semicircular canals;
  • Brain rooted medical conditions (such as traumatic brain injury).

 

These disorders are characterized by symptoms like dizziness, difficulty maintaining balance, proprioception problems, vision problems, vertigo, or hearing changes.

 

Fun Facts

Did you know?

 

  1. Kinesthesia refers to the ability to detect changes in body position and movements without relying on information received from the five senses. 
  2. Proprioception is the ability of the body to perceive its position in space. 
  3. The otolithic organs are also labeled as gravity receptors because of their response to the gravitational forces. 
  4. The Latin name of the inner ear is Auris interna.
  5. The transduction of olfactory signals refers to a series of events in which cells located in the nose bind to the scent-bearing molecules, after which they send electrical signals to the brain. In the dedicated processing centers of the brain, these signals translate as smells/odors.
  6. The word cochlea originates from the Greek word cochlos, meaning “snail.” 
  7. Motion sickness occurs due to fluctuations in the maculae. It’s a medical condition with main symptoms like nausea and vomiting that occur during traveling by any means of transport, but mostly when the individual travels by water. 
  8. Damage to the vestibulocochlear nerve results in a medical condition called vestibular neuritis (labyrinthitis). It can result in nausea, excessive sweating, and vertigo in the affected individual. 
  9. Meniere’s disease occurs due to obstruction in the cochlear aqueduct. Symptoms like ringing in the ears and dizziness, as well as sensitivity to loud noises, can be recognized in the affected individuals. 
  10. The caloric reflex test examines the vestibular-ocular reflex. The test begins with cold or warm water being poured into the outer auditory canal by a syringe. The measured difference between the water’s temperature and the body’s temperature serves to discover the degree to which the vestibular system is responsive and how symmetric the responses are between the ears.