Neuroscience 9B - Vestibular Apparatus and Pathways

Neuroscience 9b - Vestibular Apparatus and Pathways

Anil Chopra

1. Draw a diagram of the basic arrangement of the vestibular apparatus (utricle, saccule, and semicircular canals) of the inner ear.
2. Describe the sensory organs found in the macula (utricle and saccule) and in the crista ampulla (semicircular canals).
3. Explain how these sensory organs tranduce information about static head position (with respect to gravity) and about linear and angular head movements.
4. Describe the basic pathway from the peripheral apparatus to the vestibular nuclei of the brainstem, and outline the relationship of these nuclei with the cerebellum.
5. Outline the procedures and rationale for routine tests of vestibular function.
6. Outline the main causes and consequences of disorders of the vestibular system.

Functions of the Vestibular System

•Detects changes in head position, linear acceleration, and angular acceleration.

•Controls the position of the trunk, head and limbs in space.

•Control of eye movements when the head is moving.

Vestibular Apparatus

Comprises of the components of the inner ear: (membranous labyrinth)

-Semicircular canals: lateral, posterior & anterior.

• Kinetic labyrinth: respond to angular accelerations caused by head rotation.

-Otolith organs: utricle and saccule.

• Static labyrinth: respond to static head position and linear acceleration

Bony labyrinth: this is the chamber formed by the petrous temporal bone. It is filled with perilymph.

Membranous labyrinth: essentially located within the utricle, saccule and semicircular canals. It is filled with endolymph and contains the sensory receptor cells.

Transduction Mechanism

The epithelial hair cells are located in the macula of the utricle and saccule and the cristae in the ampulla (the ampulla are the semicircular canals).

In thegelatinous matrix of the macula of the utricle and saccule, there are stereociliahair cells (the single longest one is the kinocilium) that are lined up in varying heights and crystals known as otoconia. The nerve endings in the hair cells can be one of two types:

–Type I: chalice-like endings form ribbon synapses

–Type II: simple nerve terminals

Hair cells are mechanical transducers detecting static tilt and acceleration.

When the head is moved the hairs on the hair cell move, either via the otoconia movement or due to fluid inertia. If they move:

-Toward the kinocilium depolarization

• Increase frequency of impulse

-Away from the kinocilium hyperpolarisation

• Decreased frequency of impulse

Static Labyrinth

In the layer of hair cells there are breaks called striola. These differentiate between the hair bundles of opposing polarities so that movement in any direction will stimulate a distinct subset of cells.

Because the maculae in the utricle and saccule like perpendicular to one another, which ever way the head moves, a certain bundle of cells is going to move and that bundle of cells with stimulate action potential changes. Tilting the head to one side also has opposite effects on corresponding hair cells of the other side.

When the head is upright – discharge is tonic.

When the head is tilted – tonic discharge changes for the duration of head tilt.

When the head is moved linearly – discharge is spontaneous.

Kinetic Labyrinth

This detects head movement in 3 planes. The functional unit is the crista which is located in the ampulla. The crista also contains hair cells which modulate action potential firing when moved.

When the head is moved the inertia of the endolymph causes the hairs in the crista to move in the opposite direction. The change in action potential discharge depends on which direction the head is moved in:

Head is turned left

-Firing rate of vestibular ganglion cells increases on the left side and decreases on the right side

-Push-pull arrangement operates for all three pairs of canals.

Vestibular Pathways

-The primary afferent fibres from the hair cells travel via the vestibulo-cochlear nerve (VIII) to the brainstem where they synapse in the vestibular nuclei. (there are 4, superior, inferior, medial and lateral).

• Static labyrinth (otoliths) - lateral & inferior
• Kinetic labyrinth (SCC) - superior & medial

-From the vestibular they project to 4 places:

Spinal Cord

The fibres descend in the lateral and medial vestibulospinal tracts.

Lateral Vestibulospinal Tract: descends in ventral funiculus to terminate in the ventral horn affecting motor neurons to limb muscles.

Medial Vestibulospinal Tract: descends in medial longitudinal funiculus to terminate in cervical and upper spinal cord affecting motor neurons to neck muscles.

These are used to maintain posture postural reflex.

Eyes

Superior and medial vestibular neurons project to motor nuclei supplyingextraocular muscles.This results in thevestibulo-ocular reflex. When the head rotates to the left, the eyes rotate in compensation to the right with repositioning saccades to the left (used to maintain gaze on a target).

Vestibular nystagmus is the involuntary movement of the eye as part of the vestibulo-ocular reflex. The eye slowly drifts one way and then quickly saccades back to its centre. It can be tested by warming the right ear and watching for a slow drift away from the right side. The direction of the nystagmus is named in accordance to the fast saccadic phase.

Cerebellum

Vestibular afferent neurons either directly from the vestibular ganglion or indirectly from the vestibular nuclei in the brainstem project to the flocculonodular lobe of the cerebellum. Efferent fibres from the cerebellum project onto all the vestibular nuclei. These control head movements, eye movements and posture.

Thalamus and Cortex

All of the neurons from the vestibular nuclei project onto ventral posterior and ventral lateral nuclei of the thalamus. From here they project onto cortical areas 2V and 3a, in part of the somatosensory cortex (the “head” part), and also onto superior parietal cortex: ‘vestibular cortex’ concerned with spatial orientation. These may account for the feelings of dizziness (vertigo) in certain kinds of vestibular stimulation.