Vestibular system

Semicircular canals

The semicircular canals are the three membranous channels located within the bony semicircular ducts of the labyrinth. They are located in three planes, with each canal making an angle of approximately 90 degrees with the other. Thus, the semicircular canals are the:

• Anterior (superior), located in the sagittal plane
• Lateral (horizontal), located in the transverse plane
• Posterior (inferior), located in the frontal plane

The terminal part of each canal ends with a dilation called the ampulla, which opens into the vestibule. The ampulla of each semicircular canal contains a cluster of mechanoreceptor cells called the crista ampullaris. Each crista is composed of the special sensory receptor cells, called the hair cells. Given that the semicircular canals are filled with endolymph, the movements of this fluid stimulate the hair cells. In this way, each semicircular canal detects when the head moves during the rotational acceleration along its corresponding plane. In other words, the semicircular canals detect head movements such as nodding up and down, shaking side to side, or tilting left and right.

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Function

The rotational movements of the head in one of the planes causes the endolymph within the semicircular canals to move in the opposite direction by inertia. The endolymph then flows against the cupula, which distends across the apical surface of the hair cells, moving the stereocilia toward the kinocilium. These ciliary movements open the transduction cell channels, triggering the excitation of the cells. The movement of stereocilia in the opposite direction reduces the cell activity. The reason why the linear movements don’t stimulate the cristae ampullares is that the linear movements happen in the planes in which the endolymph always hits the cupula from both sides, eventually not causing any displacement nor hair cell stimulation.

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The hair cells of the otolithic organs function slightly differently. The otolithic membrane makes the layer over the hair cells a lot heavier than the cupula in the semicircular canal. Once the head is linearly tilted, the gravity causes the otolithic membrane to shear from the apical surface of the relevant macula. The shearing movement stimulates the cilia and triggers an action potential. The reason why the rotational movements don’t stimulate the maculae of the otolithic organs is that the flow of the endolymph alone is not enough to cause the shearing of the otolithic membrane, thus it never moves the cilia.

The action potentials from the cristae and maculae are carried by the corresponding branch of the vestibular nerve;

• Crista ampullaris of the anterior duct transmits signals via the anterior ampullary nerve.
• Crista ampullaris of the posterior duct transmits signals via the posterior ampullary nerve.
• Crista ampullaris of the lateral duct transmits signals via the lateral ampullary nerve.
• Macula of the utricle transmits signals via the utricular nerve.
• Macula the saccule transmits signals via the saccular nerve.

The utricular, anterior ampullary, and lateral ampullary nerves join to form the utriculo-ampullary nerve. Ultimately, the utriculo-ampullary, saccular and posterior ampullary nerves synapse within the vestibular ganglion. 

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Connections with the nuclei of the cranial nerves

The superior and medial vestibular nuclei send fibers that join the medial longitudinal fasciculus (MLF). Via this pathway, they synapse with the motor nuclei of the oculomotor (CN III), trochlear (CN IV) and abducens (CN VI) nerves, including the interstitial nucleus of Cajal and the nucleus of Darkschewitsch. 

This way, the vestibular system mediates the reflexive activity of the extraocular muscles. More precisely, the vestibular system mediates the vestibulo-ocular reflex, in which the movements of the eyes are adjusted to the movements of the head.

Vestibulo-ocular reflex

The vestibulo-ocular reflex is an involuntary motor activity mediated by the vestibular system which serves for adjusting the eye movements while the head moves in the horizontal plane. It serves for fixing the gaze during head repositioning. The reflex arc is mediated by the medial longitudinal fasciculus (MLF) in the following way;

1. Once the head is turned sideways (e.g. to the left), the endolymph in the vestibular organs falls behind to the right due to inertia and stimulates the hair cells of the left horizontal semicircular canal.
2. The information is conveyed to the vestibular nuclei via the ipsilateral vestibulocochlear nerve.
3. The neurons from the superior and medial vestibular nuclei join the MLF and reach the nucleus of the contralateral abducens nerve. This way, the contralateral lateral rectus muscle contracts and pulls the eyes in the opposite direction to that of the head.
4. Simultaneously to the abducens stimulation, some MLF fibers cross the midline and synapse with the oculomotor nucleus. This stimulates the medial rectus muscle on the opposite eye, resulting with the conjugated movement of both eyes in the horizontal plane. 
5. At the same time, the ipsilateral medial rectus muscle is inhibited so that it doesn’t compete with its contralateral counterpart.

To reiterate, once a person turns their head while wanting to fix the gaze on a static object, the vestibulo-ocular reflex enables the eyes to turn in the opposite direction so that the person can maintain focus on the perceived object.

Connections with the spinal cord

The lateral vestibular nucleus sends axons via the lateral vestibulospinal tract. This tract synapses with interneurons along the entire length of the spinal cord, adjusting the posture of the body and tone of extensors according to the vestibular stimuli (vestibulo-spinal reflex).

The medial and inferior vestibular nuclei project their fibers via the medial vestibulospinal tract. This tract terminates within the cervical spinal cord, adjusting the posture of the head and neck (vestibulo-cervical reflex).

Connections with the cerebellum

The vestibular inputs connect with the cerebellum in two ways;

• The second-order neurons from the vestibular nuclei project to the inferior olivary nucleus via the vestibulo-olivary tract. From here, the vestibular inputs are relayed via the lateral part of the inferior cerebellar peduncle (restiform body) into the ipsilateral cerebellar vermis, flocculus and nodulus. This connection enables the joint modulation of balance by the cerebellum and vestibular system.
• A portion of the first-order neurons from the vestibular ganglion of Scarpa pass through the medial part of the inferior cerebellar peduncle (juxtarestiform body). They enter the cerebellum as the mossy fibers and synapse directly with the ipsilateral vestibulocerebellum, vermis and fastigial nucleus. These connections enable cerebellar awareness of the vestibular sensations and promote necessary movement modifications by the cerebellum.

Connections with the cortex

The superior and lateral vestibular nuclei project to the ventral posterior nuclei of the thalamus to synapse with the third-order neurons of the vestibular pathway. The thalamus then relays the signals to the primary vestibular cortex (Brodmann area 3a), located in the cortex of the parietal lobe adjacent to the primary motor cortex. This area integrates the information from the vestibular system with other proprioceptive systems and passes that information directly to the primary motor cortex (Brodmann area 4). From here, the motor response to the proprioceptive stimuli is generated.

Other connections between the vestibular system and the cortex are still being investigated. So far, the two particularly interesting cortical areas have been identified;

• One of them lies posterior to the primary somatosensory cortex;
• The second one lies between the somatosensory cortex and motor cortex

These cortical areas process the vestibular and visual stimuli, and they are particularly active when a person undergoes rotatory movements with eyes closed.