Download ears: the vestibuloauditory system

EARS: THE VESTIBULOAUDITORY SYSTEM
The ear is a three-chambered sensory structure has two main systems; the auditory system,
for the perception of sound, and the vestibular system for the maintenance of balance. The
three divisions of the ear are the external, the middle, and the inner (internal) ear, within
which the auditory and vestibular systems are located. The sensory receptors transduce the
sound waves to electrical impulse in the first, and to maintain equilibrium in the second.
EXTERNAL EAR
The External Ear is composed of an Auricle and an External Auditory Meatus. The auricle
(pinna) is oval-shaped, composed of elastic cartilage with tightly adhering thin skin with hair
follicles, sebaceous glands, and sweat glands. The pinna is an essential component in sound
localization and amplification.
The external auditory meatus is an air-filled tubular space that follows slightly S-shaped
course for about 25 mm to the tympanic membrane (eardrum). The wall of the lateral one
third is cartilaginous and is continuous with the elastic cartilage of the auricle. The medial
two thirds of the canal is contained within the temporal bone. The lateral part of the canal is
lined by skin that contains hair follicles, sebaceous glands, and ceruminous glands.
The secretion of the ceruminous glands mixes with the sebaceous glands and desquamated
cells to form the cerumen or earwax. The cerumen is a brownish semisolid mixture of fats
and waxes, lubricates the skin and impedes the entry of foreign particles.
The tympanic membrane is composed of tough connective tissue contains collagen, elastic
fibers, and fibroblasts. Its external surface is covered with a thin layer of epidermis, and its
inner surface is covered with simple cuboidal epithelium.
MIDDLE EAR
The middle ear, or the tympanic cavity, is an air-filled space within the temporal
bone between the tympanic membrane and the bony surface of the internal ear. It
contains three small bones, the auditory ossicles, connected by two movable joints.
These bones are
o Malleus (hammer), attached to the tympanic membrane
o Stapes (Stirrup), whose footplate fits into the oval window
o Incus (anvil), linking the malleus to the stapes
These ossicles cross the space in the middle ear and connect the tympanic
membrane to the oval window. These bones convert sound waves in the air to
mechanical (hydraulic) vibrations in tissues and fluid filled chambers.
Anteriorly, the middle ear communicates with the pharynx via the auditory tube
(Eustachian tube), posteriorly with the air-filled cavities of the mastoid process of
the temporal bone. The middle ear is bound laterally by the tympanic membrane
and medially by the bony wall of the internal ear which contains two membranecovered openings, the oval window and the round window. In the middle ear, two
muscles, tensor tympani and stapedius, those are inserted into the stapes and
malleus.
The middle ear is lined with simple squamous epithelium, near the auditory tube and
in its interior; the simple squamous is transformed gradually into ciliated
pseudostratified columnar epithelium
INTERNAL EAR
The internal (inner) ear is located completely in the temporal bone, where the bony
labyrinth (a complicated set of spaces) houses the smaller epithelium-lined, fluid
filled spaces and tubes, the membranous labyrinth.
The membranous labyrinth: lies within the bony labyrinth, it is divided into two
major divisions
Utricle
Semicircular ducts
The membranous labyrinth
Saccule
Cochlear duct
The semicircular ducts originate from the utricle, while the cochlea
containing the cochlear duct, connected to the saccule, and provides for hearing
In each of these structures the epithelial lining contains large areas of columnar
sensory mechanoreceptors called the hair cells in the following regions
i.
Two maculae
ii.
Three cristae ampullaris in the enlarged ampullary region of each
semicircular duct
Long spiral organ of Corti in the cochlear duct
iii.
in the utricle and saccule
The bony labyrinth consists of spaces in the temporal bone, has the irregular
central cavity, the vestibule within which the saccule and the utricle are enclosed.
Behind this, three semicircular canals enclose the semicircular ducts; the
anterolateral cochlea contains the cochlear duct
Semicircular canals enclose the semicircular ducts
vestibule,
Cochlea containing the cochlear duct
The cochlea (L.cochlea, snail, screw) about 35 mm in total length, makes two-andone-half turns around a bony core known as the modiolus. The modiolus has spaces
containing blood vessels and the cell bodies and processes of the acoustic branch of
the eighth cranial nerve (spiral ganglion). Extending laterally from the modiolus is a
thin bony ridge, the osseous spiral lamina.
The bony labyrinth is filled with perilymph, which is similar in ionic composition to
cerebrospinal fluid and the extracellular fluids elsewhere but has very low protein
content. The perilymph emerges from the blood vessels of the periosteum and
drained by a perilymphatic duct into the subarachnoid space.
The membranous labyrinth contains endolymph, which is characterized by its low sodium
(16mM) and high potassium (150mM) content. The protein concentration in endolymph is
low, similar to the intracellular fluid. Endolymph is generated by capillaries in the stria
vascularis in the wall of the cochlear duct and is drained from the vestibule into the venous
sinuses of the dura mater by the endolymphatic duct.
SACCULE AND UTRICLE
The saccule and the utricle are composed of a very thin connective tissue sheath
lined with simple squamous epithelium. The two maculae in the wall of the saccule
and utricle are small sensory areas of columnar neuroepithelial cells innervated by
branches of the vestibular nerve. Both maculae are similar histologically and contain
mechanoreceptor cells (hair cells) which use gravity and endolymph movement to
detect the orientation of the stationary head and linear acceleration of the moving
head. The macular wall is composed of hair cells, supporting cells, and endings of the
vestibular branch of the eighth cranial nerve. The apical surface of the hair cells is
covered by a gelatinous otolithic layer or membrane (contains otoliths which are
crystalline structures). The hair cell contains numerous straight stereocilia and a
longer single kinocilium, a modified cilium whose tip may be slightly enlarged.
SEMICIRCULAR DUCTS
The three semicircular ducts are parts of the membranous labyrinth having the same
general form as the semicircular canals in the bony labyrinth. Each extends from and
returns to the wall of the utricle. They lie in three different spatial planes at right
angles to one another; their receptor areas are present in their ampullae and are
called cristae ampullaris. Cristae are histologically similar to maculae, with hair cells,
supporting cells, and nerve endings, with proteoglycan layer called the cupula. The cupula
here is thick, and attached to the sensory cells' hair bundles. It does not have otoliths and
extends completely across the ampulla.
COCHLEAR DUCT AND AUDITORY FUNCTIONS
The cochlear duct, connected to the saccule, is the specialized sound receptor. It is
about 35mm long and is surrounded by the perilymphtic spaces. In histological
sections, the cochlea contains three spaces: the scala vestibule, the middle cochlear
duct (or scala media), and the scala tympani. The cochlear duct contains edolymph
and ends at the apex of the cochlea. The other two spaces contain perilymph and
they are considered one long tube, beginning in the oval window and ending in the
round window. They communicate at the apex of the cochlea via the opening
helicotrema.
The cochlear duct is separated from the scala vestibule by the vestibular membrane.
It consists of basement membrane with simple squamous epithelium on both sides.
The epithelial cells have tight junctions to maintain the ionic gradient between
perilymph and endolymph.
In the lateral wall of the cochlear duct is the stria vascularis, an epithelium for the
production and maintenance of the endolymph for the entire membranous
labyrinth. Stria vascularis encloses a network of capillaries from which K ions and
fluids are released.
The spiral organ (organ of Corti) lies in the wall that separates the cochlear duct
from the scala tympani. The spiral organ rests on a thick basal lamina- the basilar
membrane. Two types of hair cells are present, outer hair cells OHC and inner hair
cells IHC, both having stereocilia. No kinocilium is present in cochlear hair cells.
The OHC are embedded in the tectorial membrane, an acellular layer extending over
the spiral organ from the modiolus.
The cell bodies of the afferent bipolar neuron are located in the bony core of the
modiolus, the spiral ganglion.
Two major types of columnar supporting cells, Pillar cells, and Phalangeal cells. The
pillar cells contain large number of microtubules and keratin, they outline a
triangular tunnel-like space between the outer and inner hair cells.
Endolymphatic Duct and Sac
The endolymphatic duct initially has a simple squamous epithelial lining. As it nears
the endolymphatic sac, it gradually changes to tall columnar epithelium composed of
two cell types; one of these cell types has microvilli on its apical surface and
abundant pinocytotic vesicles and vacuoles. These cells may be responsible for the
absorption of endolymph and for the endocytosis of foreign material and cellular
remnants that may be present in endolymph.