The Special Senses

The special senses are hearing, sight, smell, taste and balance; all use specialized
sensory receptors to collect and transmit information to specific areas of the brain.
Incoming nerve impulses from the ears, eyes, nose and mouth are integrated and
coordinated within the brain, allowing perception of this information. Up to 80%
of what we perceive comes from the external sensory senses.

THE EAR

The ear is the organ of hearing and is also involved in balance. It is supplied by the
cochlear branch of the 8th cranial (vestibulocochlear) nerve, which responds to the
vibrations generated by sound waves and transmits this information to the brain.

Structure
The ear is divided into three distinct parts: the outer ear, middle ear (tympanic
cavity) and inner ear. The middle and inner ear are encased within the petrous part
of the temporal bone.

The outer ear collects sound waves and directs them to the middle ear, which in
turn transmits them to the inner ear. The inner ear converts the sound waves into
nerve impulses, which are transmitted to the hearing area (auditory cortex) in the
brain.

OUTER EAR

The outer ear consists of the auricle (pinna) and the external acoustic meatus
(auditory canal).

Auricle (Pinna)

The auricle is the visible part of the ear that projects from the side of the head. It is
composed of fibro-elastic cartilage and covered with skin. It is deeply grooved and
ridged; the most prominent outer ridge is the helix.

The lobule (earlobe) is the soft pliable part at the lower extremity, composed of
fibrous and adipose tissue that is richly supplied with blood.
The External Acoustic Meatus (Auditory Canal)

This is a slightly ‘S’-shaped tube, that measures about 2.5cm long, extending from
the auricle to the tympanic membrane (ear drum). The lateral third is embedded in
cartilage and the remainder lies within the temporal bone. It is lined with skin
continuous with that of the auricle. There are numerous ceruminous glands and
hair follicles, with associated sebaceous glands in the skin of the lateral third.
Ceruminous glands are modified sweat glands that secrete cerumen (earwax), a
sticky material containing protective substance including the bactericidal enzyme
lysozyme and immunoglobulins. Foreign materials, e.g dust, insects and microbes
are prevented from reaching the tympanic membrane by wax, hairs and the
curvature of the meatus. Movement of the temporomandibular joint during
chewing and speaking ‘massages’ the cartilaginous meatus, moving wax towards
the exterior.

The tympanic membrane (eardrum) completely separates the external acoustic

meatus from the middle ear. It is oval–shaped with the slightly broader edge
upwards and consists of three layers: an outer covering of hairless skin, a middle
layer of fibrous tissue and an inner lining of mucous membrane continuous with
that of the middle ear.

Middle Ear (Tympanic Cavity)

This is an irregular-shaped air-filled cavity within the petrous part of the temporal
bone. The cavity, its contents and the air sacs that open out of it are lined with
either simple squamous or cuboidal epithelium.

The lateral wall of the middle ear is formed by the tympanic membrane.

The roof and floor are formed by the temporal bone.

The posterior wall is formed by the temporal bone with openings leading to the
mastoid antrum, through which it passes to the air cells within the mastoid process.

The medial wall is a thin layer of temporal bone in which there are 2 openings:

Oval window and

Round window
The oval window is attached to one of the tiny bones in the middle ear, called the
stapes and the round window by a fine sheet of fibrous tissue.

Air reaches the middle ear through the auditory (pharyngotympanic or Eustachian)
tube, which links it to the nasopharynx. It is about 4cm long and lined with ciliated
columnar epithelium. The presence of air at atmosphere pressure on both sides of
the tympanic membrane is maintained by the auditory tube and this enables
vibration of the membrane when sound wave strikes it. The auditory tube is
normally closed but when there is unequal pressure across the tympanic
membrane, e.g. at high altitude; it is opened by swallowing or yawning and the
ears ‘pop’, equalizing the pressure again.

Auditory Ossicles

These are three very small bones, only a few millimeters in size, that form a chain
across the middle ear from the tympanic membrane to the oval window. Synovial
joints between the ossicles allow them to vibrate within the air-filed tympanic
cavity. The ossicles are held in place by fine ligaments and are named according to
their shapes.

Malleus

This is the lateral hammer-shaped bone. The handle is in contact with the tympanic
membrane and the head forms a synovial joint with the incus.

Incus

This is the middle anvil-shaped bone. Its body articulates with the malleus and the
long process with the stapes; it is stabilized by the short process, fixed by fibrous
tissue to the posterior wall of the tympanic cavity.

Stapes

This is the medial stirrup-shaped bone. Its head articulates with the incus and its
footplate fits into the oval window.
Inner Ear

The inner ear or labyrinth (meaning ‘maze’) contains the organs of hearing and
balance. It has two structural parts: the bony labyrinth and the membranous
labyrinth. It is divided into three main regions:

 the vestibule, containing the utricle and saccule

 three semicircular canals
 the cochlea

The inner ear is formed from a network of channels and cavities in the temporal
bone (the bony labyrinth). Within the bony labyrinth, like a tube within a tube, is
the membranous labyrinth, a network of fluid-filled membranes that lines and fills
the bony labyrinth.

Bony Labyrinth

This is lined with periosteum. Within the bony labyrinth lies the membranous
labyrinth suspended in watery fluid called perilymph.

Membranous labyrinth

The membranous labyrinth is filled with endolymph.

Vestibule

This is the expanded region nearest the middle ear. The oval and round windows
are located in its lateral wall. It contains two membranous sacs, the utricle and the
saccule, which are important in balance.

Semicircular canals

These are three tubes arranged so that one is situated in each of the three planes of
space. They are continuous with the vestibule and are important in balance.

Cochlea

This resembles a snail's shell in appearance. It has a broad base where it is

continuous with the vestibule and a narrow apex, and it spirals round a central
bony column.
A cross-section of the cochlea reveals three compartments:

 the scala vestibule

 the scala media, or cochlear duct
 the scala tympani.

Two of these compartments contain perilymph: the scala vestibuli, which

originates at the oval window and the scala tympani. This ends at the round
window. These two compartments are continuous with each other. The cochlear
duct, which is an extension of' the labyrinth, lies between the scala vestibule and
the scala tympani, is triangular in cross-section and contains the organs of hearing.
On the basilar membrane, the base of the triangle, are supporting cells and
specialized cochlear hair cells containing auditory receptors These cells form the
spiral organ (of Corti), the sensory organ that responds to vibration by initiating
nerve impulses which are perceived as hearing within the brain. The auditory
receptors are dendrites of afferent (sensory) nerves that combine forming the
cochlear (auditory) part of the vestibulocochlear (8th cranial) nerve, which passes
through a foramen in the temporal bone to the hearing area in the temporal lobe of
the cerebrum.

PHYSIOLOGY OF HEARING

Sound is carried as pressure (sound) waves in the air, which travel at about 340
metres per second. The shape of the auricle funnels and concentrates sound waves
and directs them along the auditory canal, causing the tympanic membrane to
vibrate.

Tympanic membrane vibrations are transmitted and amplified through the middle
ear by movement of the ossicles.

At the medial end of the ossicles, the footplate of the stapes rocks to and fro in the
oval window, setting up fluid waves in the perilymph of the scala vestibuli in the
inner ear. Some of the force of these waves is transmitted along the length of the
scala vestibuli and scala tympani, but most of the pressure is transmitted into the
cochlear duct. this causes a corresponding wave motion in the endolymph,
resulting in vibration of the basilar membrane and stimulation of the auditory
receptors in the hair cells of the spiral organ. The nerve impulses generated there
are transmitted to the brain in the cochlear (auditory) branch of the
vestibulocochlear nerve. The fluid wave is finally expended into the middle ear by
vibration of the membrane of the round window.

The vestibulocochlear nerve transmits the impulses to the auditory nuclei in the
medulla, where they synapse before they are connected to the auditory area in the
temporal lobe of the cerebrum. Because some fibres cross over in the medulla and
others do not, the left and right auditory areas of the cerebrum receive impulses
from both ears.

Sound waves have the properties of pitch and volume. Pitch is determined by the
frequency of the sound waves and is measured in hertz (Hz). Sounds of different
frequencies stimulate the basilar membrane at different places along its length,
allowing discrimination of pitch. The basilar membrane near the oval window is
sensitive to high-pitched sounds, whereas the region towards the tip of the cochlea
is responsive to lower-pitched sounds.

Communication

The volume depends on the magnitude of the sound waves and is measured in
decibels (dB). The greater the amplitude of the wave created in the endolymph, the
greater the stimulation of the auditory receptors in the hair cells in the spiral organ
and the louder the sound. Long term exposure to excessive noise causes hearing
impairment because it damages the sensitive hair cells of the spiral organ.

As well as its role in hearing, the ear is also te sense organ for balance or
equilibrium.

The Semicircular Canals and Vestibule

The semicircular canals have no auditory function although they are associated
with the cochlea. Instead, they provide information about the position of the head
in space, contributing to maintenance of posture and balance.

There are three semicircular canals, one lying in each of the three planes of space.
They are situated above, beside and behind the vestibule of the inner ear and open
into it.
The utricle is a membranous sac that is part of the vestibule and the three
membranous ducts open into it at their dilated ends, the ampullae. The saccule is a
part of the vestibule and communicates with the utricle and the cochlea.

In the walls of the utricle, saccule and ampullae are fine, specialized epithelial cells
with minute projections, called hair cells. Among the hair cells there are receptors
on sensory nerve endings, which combine, forming the vestibular branch of the
vestibulocochlear nerve.

Physiology of Balance

The semicircular canals and the vestibule (utricle and saccule) are concerned with
balance, or equilibrium. The arrangement of the three semicircular canals, one in
each plane allows perception not only of the position of the head in space but also
of the direction and rate of any movement. Any change of position of the head
cause movement in the endolymph bathing the hair cells, which distorts them and
stimulates the sensory receptors in the utricle, saccule and ampullae. The resultant
nerve impulses are transmitted by the vestibular nerve, which joins the cochlear
nerve to form the vestibulocochlear nerve. The vestibular branch passes first to the
vestibular nucleus, then to the cerebellum.

The cerebellum also received nerve impulses from the eyes and proprioceptors
(sensory receptors) in the skeletal muscles and joints. The cerebellum coordinates
incoming impulses from the vestibular nerve, the eyes and proprioceptors.

Thereafter, impulses are transmitted to the cerebrum and skeletal muscles, enabling
perception of body position and any adjustments needed to maintain posture and
balance. This maintains upright posture and fixing of the eyes on the same point,
independently of head movements.