216 SECTION III Central & Peripheral Neurophysiology
Although most of the responses to stimulation of the macu-
lae are reflex in nature, vestibular impulses also reach the
cerebral cortex. These impulses are presumably responsible
for conscious perception of motion and supply part of the
information necessary for orientation in space. Vertigo is the
sensation of rotation in the absence of actual rotation and is a
prominent symptom when one labyrinth is inflamed.SPATIAL ORIENTATION
Orientation in space depends in part on input from the vestib-
ular receptors, but visual cues are also important. Pertinent in-
formation is also supplied by impulses from proprioceptors in
joint capsules, which supply data about the relative position of
the various parts of the body, and impulses from cutaneous
exteroceptors, especially touch and pressure receptors. These
four inputs are synthesized at a cortical level into a continuous
picture of the individual’s orientation in space. Clinical Box
13–3 describes some common vestibular disorders.CHAPTER SUMMARY
■ The external ear funnels sound waves to the external auditory
meatus and tympanic membrane. From there, sound waves pass
through three auditory ossicles (malleus, incus, and stapes) in
the middle ear. The inner ear, or labyrinth, contains the cochlea
and organ of Corti.
■ The hair cells in the organ of Corti signal hearing. The stereocil-
ia provide a mechanism for generating changes in membrane
potential proportional to the direction and distance the hair
moves. Sound is the sensation produced when longitudinal vi-
brations of air molecules strike the tympanic membrane.
■ The activity within the auditory pathway passes from the eighth
cranial nerve afferent fibers to the dorsal and ventral cochlear
nuclei to the inferior colliculi to the thalamic medial geniculate
body and then to the auditory cortex.
■ Loudness is correlated with the amplitude of a sound wave,
pitch with the frequency, and timbre with harmonic vibrations.
■ Conductive deafness is due to impaired sound transmission in
the external or middle ear and impacts all sound frequencies.
Sensorineural deafness is usually due to loss of cochlear hair
cells but can also occur after damage to the eighth cranial nerve
or central auditory pathways.
■ Rotational acceleration stimulates the crista in the semicircular,
displacing the endolymph in a direction opposite to the direc-
tion of rotation, deforming the cupula and bending the hair cell.
The utricle responds to horizontal acceleration and the saccule
to vertical acceleration. Acceleration in any direction displaces
the otoliths, distorting the hair cell processes and generating
neural.
■ Spatial orientation is dependent on input from vestibular recep-
tors, visual cues, proprioceptors in joint capsules, and cutaneous
touch and pressure receptors.CLINICAL BOX 13–3
Vestibular Disorders
Vestibular balance disorders are the ninth most common
reason for visits to a primary care physician. It is one of the
most common reasons elderly people seek medical advice.
Patients often describe balance problems in terms of ver-
tigo, dizziness, lightheadedness, and motion sickness. Nei-
ther lightheadedness nor dizziness is necessarily a symp-
tom of vestibular problems, but vertigo is a prominent
symptom of a disorder of the inner ear or vestibular system,
especially when one labyrinth is inflamed. Benign parox-
ysmal positional vertigo is the most common vestibular
disorder characterized by episodes of vertigo that occur
with particular changes in body position (eg, turning over
in bed, bending over). One possible cause is that otoconia
from the utricle separate from the otolith membrane and
become lodged in the cupula of the posterior semicircular
canal. This causes abnormal deflections when the head
changes position relative to gravity.
Ménière disease is an abnormality of the inner ear caus-
ing vertigo or severe dizziness, tinnitus, fluctuating hearing
loss, and the sensation of pressure or pain in the affected ear
lasting several hours. Symptoms can occur suddenly and
recur daily or very rarely. The hearing loss is initially transient
but can become permanent. The pathophysiology likely in-
volves an immune reaction. An inflammatory response can
increase fluid volume within the membranous labyrinth,
causing it to rupture and allowing the endolymph and peri-
lymph to mix together. There is no cure for Ménière disease
but the symptoms can be controlled by reducing the fluid re-
tention through dietary changes (low-salt or salt-free diet, no
caffeine, no alcohol) or medication.
The nausea, blood pressure changes, sweating, pallor, and
vomiting that are the well-known symptoms of motion sick-
ness are produced by excessive vestibular stimulation and
occurs when conflicting information is fed into the vestibular
and other sensory systems. The symptoms are probably due
to reflexes mediated via vestibular connections in the brain
stem and the flocculonodular lobe of the cerebellum. Space
motion sickness— the nausea, vomiting, and vertigo expe-
rienced by astronauts—develops when they are first ex-
posed to microgravity and often wears off after a few days of
space flight. It can then recur with reentry, as the force of
gravity increases again. It is believed to be due to mis-
matches in neural input created by changes in the input
from some parts of the vestibular apparatus and other grav-
ity sensors without corresponding changes in the other spa-
tial orientation inputs.