Download 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.
o Kinetic labyrinth: respond to angular accelerations caused by head
rotation.
- Otolith organs: utricle and saccule.
o 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 the gelatinous matrix of the macula of the utricle and
saccule, there are stereocilia hair 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
o Increase frequency of impulse
- Away from the kinocilium  hyperpolarisation
o 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).
o Static labyrinth (otoliths) - lateral & inferior
o 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 supplying extraocular muscles. This results in the
vestibulo-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.