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Chapter 11

PSL440Y1 Chapter Notes - Chapter 11: Streptomycin, Vestibulocochlear Nerve, Proprioception


Department
Physiology
Course Code
PSL440Y1
Professor
A
Chapter
11

Page:
of 2
THE VESTIBULAR SYSTEM
The vestibular system monitors the position and movement of the head, gives us a sense of
balance and equilibrium, and helps coordinate movements of the head and eyes.
The Vestibular Labyrinth
The vestibular and auditory systems both use hair cells to transduce movements. In mammals,
all hair cells are contained within sets of interconnected chambers called the vestibular
labyrinth (See Fig. 11.28) which include two types of structures with different structures:
Otolith organs detects force of gravity and tilts of the head
o Contain the saccule and the utricle
o Located near the center of the labyrinth
Semicircular canals sensitive to head rotation
o Three arcing structures
o Lie in approximately orthogonal planes
Ultimately, these structures transmit mechanical energy, derived from head movement, to its
hair cells.
Otolith Organs
The saccule and utricle detect changes of head angle as well as linear acceleration of the head.
Each otolith organ contains a sensory epithelium called a macula.
Vertically oriented within the saccule and horizontally oriented within the utricle
The vestibular macula contains hair cells (see Fig.11.29). Movements are transduced when the
hair bundles are deflected.
The otolith organs contain tiny crystals of calcium carbonate called otoliths which
encrust the surface of the macula’s gelatinous cap. This is key to the tilt sensitivity of the
macula.
When the head accelerates, a force is exerted on the otoliths. This in turn exerts a force in the
same direction on the gelatinous cap, which moves slightly, and the cilia of the hair cells bend.
Each hair cell has one especially tall cilium called the kinocillium.
The bending of hairs toward the kinocillium results in depolarizing, excitatory potential
The bending of hairs in the other direction will lead to hyperpolarization and inhibition.
If the hairs are perpendicular to the preferred direction, there is barely any response
The response saturates when the hairs are bent less than 0.5 µm.
The responses of hair cells are, therefore, direction selective. A given head movement excites
hair cells on one side while inhibiting hair cells in the corresponding location on the other side.
Thus, any tilt or acceleration of the head will excite some hair cells, inhibit others, and have no
effect on the rest.
Semicircular Organs
Semicircular organs detect turning movements of the head and also sense acceleration but of a
different nature. Angular acceleration is generated by sudden rotational movements (e.g.
spinning in circles). The hair cells of the semicircular canals are clustered within a sheet of cells
called the crista which are located within a bulge along the canal called the ampulla.
Hair cells of the ampulla have their kinocillia oriented in the same direction. Therefore,
they are all excited or inhibited together.
The semicircular canals are filled with endolymph. Bending of the cilia occurs when the canal is
suddenly rotated about its axis (like a wheel). As the wall of the canal and the cupula begin spin,
the endolymph tends to stay behind and exerts a force on the cupula. This bends the cilia (see
Fig. 11.31). The cupula bows and bends the cilia. Depending on the direction of the rotation, this
will either excite or inhibit the release of neurotransmitter.
If the head is maintained at constant velocity, the friction of the endolymph with the canal
walls eventually makes the two move together and will reduce or eliminate the bending
of the cupula after 15-30 seconds.
The three semicircular canals on one side of the head help sense all possible head-rotation
angles. These canals are paired with another on the opposite side of the head.
Each member of a pair sits within the same orientation plane and response to rotation
about the same axis.
While rotation excites the hair cells of one canal, it inhibits the hair cells of its
contralateral partner canal.
Central Vestibular Pathways and Vestibular Reflexes
The central vestibular pathway coordinate and integrate information about head and body
movement and use it to control the output of motor neurons that adjust head, eye, and body
positions.
Primary vestibular axons from cranial nerve VIII make direct connections to the
vestibular nucleus on the same side of the brain stem (see Fig. 11.33)
The vestibular nuclei also receive inputs from other parts of the brain and in turn projects
to a variety of targets above it in the brain stem, and below it into the spinal cord
The Vestibulo-Ocular Reflex (VOR) (See Fig. 11.34)
One very important function of the central vestibular system is to keep your eyes pointed in a
particular direction. This is called the vestibulo-ocular reflex. It works by sensing rotations of
the head, and commands a compensatory movement of the eyes in the opposite direction.
This helps to keep your line of sight tightly fixed on a visual target
For example, when the head turns to the left, VOR induces both eyes to turn right.
Because the VOR is a reflex triggered by vestibular input rather than visual input, it works
amazingly well even in the dark or when your eyes are closed.
Vestibular Pathology
The vestibular system can be damaged in a variety of ways, for example:
Toxicity of high doses of antibiotics (e.g. streptomycin)
Bilateral lesions of the vestibular labyrinths
When people with vestibular damages cannot stabilize an image on their moving retinas, they
may feel that the world is constantly moving around them. This can make walking and standing
difficult.
Compensatory adjustments come with time as the brain learns to substitute more visual
and proprioceptive cues to help guide smooth and accurate movements.