214 SECTION III Central & Peripheral Neurophysiologywithin the inner ear detects head motion and position and
transduces this information to a neural signal (Figure 13–3).
The vestibular nuclei are primarily concerned with maintain-
ing the position of the head in space. The tracts that descend
from these nuclei mediate head-on-neck and head-on-body
adjustments.CENTRAL PATHWAY
The cell bodies of the 19,000 neurons supplying the cristae and
maculae on each side are located in the vestibular ganglion.
Each vestibular nerve terminates in the ipsilateral four-part ves-
tibular nucleus and in the flocculonodular lobe of the cerebel-
lum (Figure 13–12). Fibers from the semicircular canals end
primarily in the superior and medial divisions of the vestibular
nucleus and project mainly to nuclei controlling eye movement.
Fibers from the utricle and saccule end predominantly in the
lateral division (Deiters nucleus), which projects to the spinal
cord. They also end on neurons that project to the cerebellum
and the reticular formation. The vestibular nuclei also project to
the thalamus and from there to two parts of the primary soma-
tosensory cortex. The ascending connections to cranial nerve
nuclei are largely concerned with eye movements.RESPONSES TO ROTATIONAL
ACCELERATIONRotational acceleration in the plane of a given semicircular ca-
nal stimulates its crista. The endolymph, because of its inertia,
is displaced in a direction opposite to the direction of rotation.
The fluid pushes on the cupula, deforming it. This bends the
processes of the hair cells (Figure 13–3). When a constant
speed of rotation is reached, the fluid spins at the same rate as
the body and the cupula swings back into the upright position.
When rotation is stopped, deceleration produces displace-
ment of the endolymph in the direction of the rotation, and
the cupula is deformed in a direction opposite to that during
acceleration. It returns to mid position in 25 to 30 s. Move-
ment of the cupula in one direction commonly causes anCLINICAL BOX 13–1
Genetic Mutations Contributing to Deafness
Single-gene mutations have been shown to cause hearing
loss. This type of hearing loss is a monogenic disorder with an
autosomal dominant, autosomal recessive, X-linked, or mito-
chondrial mode of inheritance. Monogenic forms of deafness
can be defined as syndromic (hearing loss associated with
other abnormalities) or nonsyndromic (only hearing loss).
About 0.1% of newborns have genetic mutations leading to
deafness. Nonsyndromic deafness due to genetic mutations
can first appear in adults rather than in children and may ac-
count for many of the 16% of all adults who have significant
hearing impairment. It is now estimated that the products of
100 or more genes are essential for normal hearing, and deaf-
ness loci have been described in all but 5 of the 24 human
chromosomes. The most common mutation leading to con-
genital hearing loss is that of the protein connexin 26. This de-
fect prevents the normal recycling of K+ through the sustenac-
ular cells. Mutations in three nonmuscle myosins also cause
deafness. These are myosin-VIIa, associated with the actin in
the hair cell processes; myosin-Ib, which is probably part of the
“adaptation motor” that adjusts tension on the tip links; and
myosin-VI, which is essential in some way for the formation of
normal cilia. Deafness is also associated with mutant forms of
α-tectin, one of the major proteins in the tectorial membrane.
An example of syndromic deafness is Pendred syndrome, in
which a mutant sulfate transport protein causes deafness and
goiter. Another example is one form of the long QT syn-
drome in which one of the K+ channel proteins, KVLQT1, is
mutated. In the stria vascularis, the normal form of this protein
is essential for maintaining the high K+ concentration in en-
dolymph, and in the heart it helps maintain a normal QT inter-
val. Individuals who are homozygous for mutant KVLQT1 are
deaf and predisposed to the ventricular arrhythmias and sud-
den death that characterize the long QT syndrome. Mutations
of the membrane protein barttin can cause deafness as well
as the renal manifestations of Bartter syndrome.TABLE 13–1 Common tests with a tuning fork to distinguish between sensorineural and conduction deafness.
Weber Rinne Schwabach
Method Base of vibrating tuning fork placed on
vertex of skull.Base of vibrating tuning fork placed on
mastoid process until subject no longer
hears it, then held in air next to ear.Bone conduction of patient com-
pared with that of normal subject.Normal Hears equally on both sides. Hears vibration in air after bone conduction
is over.
Conduction deaf-
ness (one ear)Sound louder in diseased ear because
masking effect of environmental noise is
absent on diseased side.Vibrations in air not heard after bone con-
duction is over.Bone conduction better than nor-
mal (conduction defect excludes
masking noise).
Sensorineural deaf-
ness (one ear)Sound louder in normal ear. Vibration heard in air after bone conduction
is over, as long as nerve deafness is partial.Bone conduction worse than
normal.