Macula of Utricle.
Presence of:
Otolithic membrane and otoconia;
Hair and sustentacular cells.
2. Semicircular ducts
Semicircular ducts are three in number, small in size and found within
the semicircular canals.
These ducts have the same confi guration as the semicircular canals.
Sensory receptors are found in the ampullae of semicircular ducts as
transverse thickening called crista ampullaris.
Crista ampullaris is similar in structure to macula, but the glycoprotein
layer is very thick and conical in shape in cross section. It is called
cupula (Fig.). There are no crystalline bodies in the cupula. Crista is
sensitive to angular movement of head (changes in rotational velocity).
It is an organ of kinetic balance (movement sense).
fig. : structure of crista aampullaris
Crista Ampullaris.
Presence of:
Cupula;
Conical-shaped sense organ comprising hair and sustentacular cells.
3. Cochlear duct
Cochlear duct (scala media) lies within the bony cochlear canal and is
triangular in cross section. The base (outer wall) is formed by stria
vascularis, which is a stratifi ed vascular epithelium lining the spiral
ligament. Stria vascularis is responsible for maintaining the correct
ionic composition of endolymph. The apex of cochlear duct is at the
osseous spiral lamina.
The roof is formed by vestibular (Reissner’s) membrane which
separates the scala media from scala vestibuli. The fl oor is formed by
basilar membrane which separates the scala media from scala tympani.
The basilar membrane supports the organ of Corti, which is the receptor
sense organ of hearing (Fig.).
Fig. :cross section of coch lear duct in situe
Organ of Corti
The organ of Corti consists of two groups of cells. These are:
(a) Hair (sensory) cells
 Inner hair cells
 Outer hair cells
(b) Supporting cells
 Inner and outer pillar cells
 Inner and outer phalangeal cells
 Border cells
 Hensen’s cells
In the centre of the organ of Corti there is a canal, the tunnel of Corti,
bounded by inner and outer rows of pillar cells. The pillar cells are rodlike cells containing tonofi brils.
On the inner aspect of the inner row of pillar cells is a single row of
columnar cells, the inner phalangeal cells. They support the bases of
inner hair cells, arranged in a single row. The inner hair cells are fl askshaped cells. They bear stereocilia (hair) which are arranged in the form
of letter ‘U’.
On the outer aspect of the outer row of pillar cells, there are three to fi
ve rows of outer phalangeal cells. They support the same number of
rows of outer hair cells. Outer hair cells are much taller than the inner
hair cells and their hair (stereocilia) are arranged in the form of letter
‘W’.
Both inner and outer hair cells do not reach the basilar membrane and
they are supported by the inner and outer phalangeal cells through their
apices. The hair cells have no kinocilium and have only stereocilia
(hair). The hair project into a gelatinous layer, the tectorial membrane,
which overhangs the organ of Corti from the limbus spiralis. The hair
cells are innervated by peripheral processes of bipolar neurons whose
cell bodies are situated at the base of the osseous spiral lamina as spiral
ganglion. Their central processes pass through the modiolus as cochlear
nerve.
The organ of Corti is limited internally by the border cells. These are
columnar cells arranged in a single row on the inner
aspect of inner phalangeal cells.
Hensen’s cells limit the outer boundary of the organ of Corti. They are
arranged in several rows on the outer aspect of the outer phalangeal
cells.
Stereocilia are agitated due to vibration of basilar membrane caused by
conduction of sound from bone to fl uid. This results in excitation of
the cochlear nerve that supplies them. (Refer to a textbook of
Physiology for details.)
Cochlea.
Presence of:
o Organ of Corti;
o Scala vestibuli, scala media, scala tympani;
o
Spiral ganglion.