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Full text of "Diseases Of The Nose Throat And Ear"

CHAPTER 48

PHYSIOLOGY OF THE VESTIBULAR
APPARATUS

The balance of the body is maintained by co-ordination of information from
three systems: (1) proprioception: i.e. sensation from muscles, joints, tendons
and ligaments; (2) the eyes; (3) the vestibular system.

The vestibular system consists of the semicircular canals, the utricle and the
saccule. The utricle and saccule respond to linear acceleration. The greatest
linear acceleration to which the body is normally subjected is gravity (32 ft per
sec2) and it is alterations in the position of the head in relation to the direction
of gravity which stimulate selective parts of the utricle and saccule. Impulses
from the utricle and saccule not only give information about the position of
the head in space but initiate reflexes which tend to keep the head in the
upright position, and are contributory to the maintenance of muscle tonus. In
modern life large horizontal linear accelerations may be added to the ever-
present gravity so that the body reacts to the resultant force which is no longer
vertical—a dangerous situation in aircraft take-offs and landings. The
semicircular canals respond to angular (rotatory) acceleration, and stimulation
of the semicircular canals gives rise to the sensation of rotation and to reflex
movements of the eyes and body to counter the movement. Angular accelera-
tion around any axis will stimulate at least one pair of semicircular canals,
e.g. the horizontal canal on each side or the superior canal on one side and the
posterior canal on the other side. The mechanism can most easily be explained
: by considering rotation in a rotating chair (Fig. 150) about a vertical axis
(Fig. 151). Acceleration to the right will cause movement to the left of
endolymph within the membranous horizontal canal and deflection of the
cupola on the crista in the expanded ampulla of the canal. There is a constant
impulse rate of 10-20 impulses per second in the fibres of the nerves leaving
the crista. Movement of the endolymph and cupola towards the ampulla
causes an increase in this impulse rate. Movement away from the ampulla
causes a reduction in the impulse rate. In Fig. 151 it wiU be seen that accelera-
tion to the right will cause an increase in the impulse rate from the crista of the
right horizontal canal and a reduction of the rate on the left side. This
difference in impulse rate is interpreted by the central nervous system and
gives rise to the sensation of rotation to the right. The eyes will move to the
left at a rate proportional to the degree of stimulation, but as the eyes can
only move a limited amount laterally, a central reflex will return them to
midposition and the vestibular stimulation will again move them to the left.
This constitutes nystagmus with a relatively slow vestibular component and a
very much faster central component. Similar limb reflexes, e.g. past pointing,
can also be demonstrated. Whea the acceleration stops, rotation will continue