The Special Senses
Te vestibule contains the saccule and utricle. Te semicircular
canals extend posteriorly from the vestibule in three planes. Tey
contain the semicircular ducts.
Te cochlea houses the cochlear duct (scala media), containing the
spiral organ, the receptor organ for hearing. Within the cochlear
duct, the hair (receptor) cells rest on the basilar membrane, and
their hairs project into the gelatinous tectorial membrane.
Physiology of Hearing
Sound originates from a vibrating object and travels in waves
consisting of alternating areas of compression and rarefaction of
Te distance from crest to crest on a sine wave is the sound’s
wavelength; the shorter the wavelength, the higher the frequency
(measured in hertz). Frequency is perceived as pitch.
Te amplitude of sound is the height of the peaks of the sine
wave, which reﬂect the sound’s intensity. Sound intensity is
measured in decibels. Intensity is perceived as loudness.
Sound passing through the external acoustic meatus sets the
tympanic membrane into vibration at the same frequency. Te
ossicles amplify and deliver the vibrations to the oval window.
Pressure waves in cochlear ﬂuids set speciﬁc locations on
the basilar membrane into resonance. At points of maximal
membrane vibration, the vibratory motion alternately depolarizes
and hyperpolarizes the hair cells of the spiral organ. Movements
of the stereocilia toward the kinocilium depolarize the hair cells
and increase the rate of impulse generation in the auditory nerve
ﬁbers. Movements away from the kinocilium have the opposite
eﬀect. High-frequency sounds stimulate hair cells near the oval
window; low-frequency sounds stimulate hair cells near the apex.
Inner hair cells send most auditory inputs to the brain. Outer hair
cells amplify the inner hair cells’ responsiveness.
Impulses generated along the cochlear nerve travel to the
cochlear nuclei of the medulla and from there through several
brain stem nuclei to the medial geniculate nucleus of the
thalamus and then the auditory cortex. Each auditory cortex
receives impulses from both ears.
Auditory processing is analytic; each tone is perceived separately.
Perception of pitch is related to the position of the excited hair
cells along the basilar membrane. Intensity perception reﬂects the
fact that as sound intensity increases, basilar membrane motion
increases, and the frequency of impulse transmission to the
cortex increases. Cues for sound localization include the intensity
and timing of sound arriving at each ear.
Equilibrium and Orientation
Te equilibrium receptor regions of the internal ear are called the
Te receptors for linear acceleration and gravity are the maculae
of the saccule and utricle. A macula consists of hair cells with
stereocilia and a kinocilium embedded in an overlying otolith
membrane. Linear movements cause the otolith membrane to
move, pulling on the hair cells and changing the rate of impulse
generation in the vestibular nerve ﬁbers.
Te receptor within each semicircular duct, the crista ampullaris,
responds to angular or rotational acceleration in one plane. It
consists of a tu± of hair cells whose stereocilia are embedded in
the gelatinous ampullary cupula. Rotational movements cause the
endolymph to ﬂow in the opposite direction, bending the cupula
and either exciting or inhibiting the hair cells.
Impulses from the vestibular apparatus are sent via vestibular
nerve ﬁbers mainly to the vestibular nuclei of the brain stem and
Each eye receives a slightly diﬀerent view of the visual ﬁeld. Te
visual cortices fuse these views to provide depth perception.
Retinal processing involves the selective destruction of inputs so
as to emphasize bright/dark or color contrasts (edges). Talamic
processing subserves high-acuity color vision and depth perception.
Cortical processing involves neurons of the striate (primary)
cortex, which receive inputs from the retinal ganglion cells, and
neurons of the prestriate (association) cortices, which receive inputs
from striate cortical cells and mostly integrate inputs concerned
with color, form, and movement. Visual processing also proceeds
anteriorly in the “what” and “where” processing streams via the
temporal and parietal lobes, respectively.
The Chemical Senses: Smell and Taste
The Olfactory Epithelium and the Sense of Smell
Te olfactory epithelium is located in the roof of the nasal cavity.
Te olfactory sensory neurons are ciliated bipolar neurons. Teir
axons are the ﬁlaments of the olfactory nerve (cranial nerve I).
Individual olfactory neurons show a range of responsiveness to
diﬀerent chemicals. Olfactory neurons bearing the same odorant
receptors synapse in the same glomerulus type.
Olfactory neurons are excited by volatile chemicals that bind to
receptors in the olfactory cilia.
Action potentials of the olfactory nerve ﬁlaments are transmitted
to the olfactory bulb where the ﬁlaments synapse with mitral
cells. Te mitral cells send impulses via the olfactory tract to the
olfactory cortex. Fibers carrying impulses from the olfactory bulb
also project to the limbic system.
Taste Buds and the Sense of Taste
Te taste buds are scattered in the oral cavity and pharynx but are
most abundant on the tongue papillae.
Gustatory epithelial cells, the receptor cells of the taste buds, have
gustatory hairs (microvilli) that serve as the receptor regions. Te
gustatory epithelial cells are excited by the binding of tastants
(food chemicals) to receptors on their microvilli.
Te ﬁve basic taste qualities are sweet, sour, salty, bitter, and
Te taste sense is served by cranial nerves VII, IX, and X, which
send impulses to the solitary nucleus of the medulla. From there,
impulses are sent to the thalamus and the gustatory cortex.
Homeostatic Imbalances of the Chemical Senses
Most chemical sense dysfunctions are olfactory disorders
(anosmias). Common causes are trauma, inﬂammation, or
The Ear: Hearing and Balance
Structure of the Ear
Te auricle and external acoustic meatus compose the external
ear. Te tympanic membrane, the boundary between the outer
and middle ears, transmits sound waves to the middle ear.
Te middle ear is a small chamber within the temporal bone,
connected by the pharyngotympanic tube to the nasopharynx.
Te ossicles, which help amplify sound, span the middle
ear cavity and transmit sound vibrations from the tympanic
membrane to the oval window.
Te internal ear consists of the bony labyrinth, within which
the membranous labyrinth is suspended. Te bony labyrinth
chambers contain perilymph; the membranous labyrinth ducts
and sacs contain endolymph.