Page 355 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Sense of Balance membrane (! A4) with high density (! 3.0)
calcite crystals called statoconia, statoliths or
Anatomy. Each of the three semicircular canals otoliths. They displace the membrane and
(! A1) is located in a plane about at right angles to thereby bend the embedded cilia (! A4) due
the others. The ampulla of each canal contains a to changes of the direction of gravity, e.g. when
ridge-like structure called the crista ampullaris the head position deviates from the perpen-
(! A2). It contains hair cells (secondary sensory dicular axis. The maculae respond also to other
cells), the cilia of which (! A3) project into a gelat- linear (translational) accelerations or decelera-
Central Nervous System and Senses resting state, the hair cells release a transmit- the vestibular ganglion synapse with the vesti-
inous membrane called the cupula (! A2). Each hair
tions, e.g. of a car or an elevator.
cell has a long kinocilium and ca. 80 stereocilia of vari-
Central connections. The bipolar neurons of
able length. Their tips are connected to longer adja-
cent cilia via the ”tip links” (! A3).
bular nuclei (! A, B). Important tracts extend
Semicircular canals. When the cilia are in a
from there to the contralateral side and to ocu-
lar muscle nuclei, cerebellum (! p. 326), mo-
ter (glutamate) that triggers the firing of action
toneurons of the skeletal muscles, and to the
potentials (AP) in the nerve fibers of the vesti-
postcentral gyrus (conscious spatial orienta-
bular ganglion. When the head is turned, the
tion). Vestibular reflexes (a) maintain the
semicircular canal automatically moves with
balance of the body (postural motor function,
it, but endolymph in the canal moves more
! p. 328) and (b) keep the visual field in focus
ence thus develops between the two sides of
(oculomotor control, ! B and p. 360).
the cupula. The resultant vaulting of the
cupula causes the stereocilia to bend (! A2)
12 sluggishly due to inertia. A brief pressure differ- despite changes in head and body position
Example (! C): If a support holding a test subject is
and shear against each other, thereby changing tilted, the activated vestibular organ prompts the
the cation conductance of the hair cell mem- subject to extend the arm and thigh on the declining
brane. Bending of the stereocilia towards the side and to bend the arm on the inclining side to
kinocilium increases conductivity and allows maintain balance (! C2). The patient with an im-
paired equilibrium organ fails to respond appro-
the influx of K and Na along a high electro- priately and topples over (! C3).
+
+
chemical gradient between the endolymph
and hair cell interior (see also pp. 366 and Since the vestibular organ cannot determine
369 C). Thus, the hair cell becomes de- whether the head alone or the entire body
2+
polarized, Ca channels open, more glutamate moves (sense of movement) or changed posi-
is released, and the AP frequency increases. tion (postural sense), the vestibular nuclei
The reverse occurs when the cilia bend in the must also receive and process visual informa-
other direction (away from the kinocilium). tion and that from propriosensors in the neck
The semicircular canals function to detect muscles. Efferent fibers project bilaterally to
angular (rotational) accelerations of the head the eye muscle nuclei, and any change in head
in all planes (rotation, nodding, tilting side- position is immediately corrected by opposing
ways). Since normal head movements take less eye movement (! B). This vestibulo-ocular re-
than 0.3 s (acceleration ! deceleration), flex maintains spatial orientation.
stimulation of the semicircular canals usually Vestibular organ function can be assessed by testing
reflects the rotational velocity. oculomotor control. Secondary or postrotatory nys-
The pressure difference across the cupula disappears tagmus occurs after abrupt cessation of prolonged
when the body rotates for longer periods of time. rotation of the head around the vertical axis (e.g., in
Deceleration of the rotation causes a pressure an office chair) due to activation of the horizontal
semicircular canals. It is characterized by slow hor-
gradient in the opposite direction. When bending of izontal movement of the eyes in the direction of rota-
the cilia increased the AP frequency at the start of ro-
tation, it decreases during deceleration and vice tion and rapid return movement. Rightward rotation
versa. Abrupt cessation of the rotation leads to ver- leads to left nystagmus and vice versa (! p. 360). Ca-
tigo and nystagmus (see below). loric stimulation of the horizontal semicircular canal
by instilling cold (30 !C) or warm water (44 !C) in the
342 The saccule and utricle contain maculae (! A1, auditory canal leads to caloric nystagmus. This
A4) with cilia that project into a gelatinous method can be used for unilateral testing.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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