Human Ear Structure, Functions and its Parts Notes

Human Ear Structure, Functions and its Parts Notes

External Ear

The ear consists of three parts namely, the external ear, the middle ear, and the internal ear. The external ear is formed by two parts:

1. Auricle or pinna
2. External auditory meatus.

1. Auricle Or Pinna:

• The auricle or pinna of the external ear consists of a fibrocartilaginous plate covered by connective tissue and skin. The plate is characteristically folded and ridged. The skin covering the plate is thin and contains many fine hairs and sebaceous glands. On the posterior surface of the auricles, many sweat glands are present.

Read And Learn More: Medical Physiology Notes

• In many animals, the auricle can be moved and turned to locate the source of sound, or, the auricle can be folded to avoid unwanted sound. But in man, the extrinsic and intrinsic muscles of auricles are rudimentary and the movement is not possible. The depression of the auricle, which forms the orifice of the external auditory meatus, is called concha.

2. External Auditory Meatus: The external auditory meatus starts from the concha and extends inside as a slightly curved canal with a length of about 55 mm.

The meatus consists of two parts:

1. The outer cartilaginous part
2. The inner bony part.

1. The Outer Cartilaginous Part:

• It is the initial part of the external auditory meatus and is made up of cartilage. It is covered by thick skin, which contains stiff hairs. The hairs prevent the entry of foreign particles.
• Large sebaceous glands and ceruminous glands are also present in the skin covering this portion. These glands are coiled and tubular in nature and open on the surface of the skin.
• The columnar epithelial cells of the glands contain brown pigment granules and fat droplets. The secretions of ceruminous glands, sebaceous glands, and desquamated epithelial cells form the earwax.

2. The Inner Bony Part:

• The inner part of the external auditory meatus is also covered by skin, which adheres closely to the periosteum. Only sebaceous glands are present here.
• Small fine hairs are present on the superior wall of the canal. The skin covering this portion is continuous with a cuticular layer of the tympanic membrane.

Middle Ear

The middle ear or tympanic cavity is a small, narrow, irregular, laterally compressed chamber situated within the temporal bone. It is also known as the tympanum. It is separated from the external auditory meatus by the tympanic membrane.

• The middle ear consists of the following structures:
  • Auditory ossicles
  • Auditory muscles
  • Eustachian tube.
• Tympanic Membrane: The tympanic membrane is a thin, semitransparent membrane, which separates the middle ear from the external auditory meatus. The periphery of the membrane is fixed to the tympanic sulcus in the surrounding bony ring by means of the fibrocartilage.
• Structure of Tympanic Membrane: The tympanic membrane is formed by three layers:

1. Lateral cutaneous layer: It is the skin layer, which is the continuation of the skin of the auditory meatus
2. Intermediate fibrous layer: It contains collagenous fibers
3. Medial mucous layer: It is also called tympanic mucosa and it is composed of a single layer of cuboidal epithelial cells.

1. Auditory Ossicles: The auditory ossicles are the three miniature bones, which are arranged in the form of a chain extending across the middle ear from the tympanic membrane to the oval window. The three ossicles are:

1. Malleus
2. Incus
3. Stapes.

1. Malleus: It is otherwise called a hammer. It has a handle, head, and neck. The handle is otherwise known as manubrium. It is attached to the tympanic membrane. The neck extends from the handle to the head. The head or capitulum articulates with the body of the next bone incus.

2. Incus: Incus is also known as an anvil. It looks like a premolar tooth. Incus has a body, one long process, and one short process. The anterior surface of the body articulates with the head of the malleus. The short process is attached to a ligament. The long process runs parallel to the handle of the malleus. The tip of the long process is like a knob, called the lenticular process and it articulates with the next bone, stapes.

3. Stapes: Stapes is also called stirrup. It is the smallest bone present in the body. It has a head, neck, anterior crus, posterior crus, and footplate. The head articulates with the incus. The footplate fits into the oval window.

2. Auditory Muscles: Two skeletal muscles are attached to the ossicles:

1. Tensor tympani
2. Stapedius.

1. Tensor Tympani: The tensor tympani is the larger of the two muscles of the tympanic cavity.

• Origin, insertion, and nerve supply:
  • Tensor tympani arises from the cartilaginous portion of the eustachian tube (see below), adjacent to the great wing of the sphenoid bone and osseous canal. Its tendon is inserted on the manubrium of the malleus which is in turn attached to the tympanic membrane.
  • Thus, the tensor tympani is attached to the tympanic membrane through the malleus. It is supplied by the mandibular division trigeminal nerve.
• Ensor Tympani Function: The tensor tympani muscle pulls and keeps the tympanic membrane stretched or tensed constantly. This constant stretching of the tympanic membrane is essential for the transmission of sound waves, which may reach any part of the tympanic membrane. Paralysis of tensor tympani causes hearing impairment.

2. Stapedius: Stapedius is the smallest skeletal muscle in the human body with a length of just over 1 mm. It lies in a conical bony cavity on the posterior wall of the tympanic cavity. Origin, insertion, and nerve supply: Stapedius arises from the interior pyramid of the tympanic cavity. Its tendon is inserted into the posterior surface of the neck of the stapes. It is supplied by a branch of the facial nerve.

• Stapedius Function:
  • The Stapedius muscle prevents excess movements of the stapes. When the stapedius contracts, it pulls the neck of the stapes backward and reduces the movement of the footplate against the fluid in the cochlea.
  • Paralysis of stapedius allows a wider range of oscillation of stapes leading to hyper-reaction of auditory ossicles to sound vibrations. This condition is called hyperacusis. Paralysis of stapedius occurs in the lesion of the facial nerve.

Tympanic Reflex:

• Tyrnpd lie reflex is the attenuation reflex characterized by involuntary contraction of tensor tympani and stapedius muscles in response to loud noise. It has a latent Period of 40-80 milliseconds.
• When both the muscles contract, the manubrium of the malleus moves inward, and the stapes is pulled outward. These two actions result in the stiffness of auditory ossicles so that the transmission of sound is decreased.

Significance of tympanic reflex:

• The tympanic reflex protects the tympanic membrane from being ruptured by a loud sound
• It also prevents the fixation of the footplate of stapes, against an oval window during exposure to loud sound
• It helps to protect the cochlea from the damaging effects of loud sounds. The contraction of tensor tympani and stapedius during exposure to loud sound develops stiffness of the auditory ossicles so that, the transmission of sound into the cochlea is decreased.

3. Eustachian Tube:

The eustachian tube or the auditory tube is the flattened canal extending from the anterior wall of the middle ear to the nasopharynx. Its upper part is surrounded by a bony wall and the lower part is surrounded by a fibrocartilaginous plate. The eustachian tube connects the middle ear with the posterior part of the nose and forms the passage of air between the middle ear and the atmosphere. Thus, the pressure on both sides of the tympanic membrane is equalized.

Internal Ear

The internal ear or labyrinth is a membranous structure, enclosed by a bony labyrinth in the petrous part of the temporal bone. It consists of the sense organs of hearing and equilibrium. The sense organ for hearing is the cochlea. And, the sense organ for equilibrium is the vestibular apparatus. The vestibular apparatus is already explained.

1. Cochlea: Cochlea is a coiled structure like a snail’s shell (cochlea = snail’s shell).

• It consists of two structures:

1. The central conical axis is formed by spongy bone called modiolus
2. Bony canal or tube, which winds around the modiolus.

• In man, the canal makes two and a half turns, starting from the base of the cochlea and ending at the top (apex) of the cochlea. The end of the canal is called the cupula. The base of the modiolus forms the bottom of the internal auditory meatus through which the cochlear nerve fibers pass and enter the modiolus.
• Thus, a section through the axis of the cochlea reveals the central bony pillar, li-n modiolus, and the periotic or osseous canal, which is colic around the modiolus.
• From the modiolus, a bony ridge called osseous spiral lamina projects into the canal, winding around the modiolus like the thread of a screw. The spiral lamina follows the spiral turns of the cochlea and ends at the cupula in a hook-shaped process called hamulus.

2. Compartments Of Cochlea:

Two membranous partitions extend between the osseous spiral lamina and the outer wall of the spinal canal. Both membranes divide the spiral canal of the cochlea into three compartments. The membranes are:

1. Basilar membrane
2. Vestibular membrane.

1. Basilar Membrane:

• It is a connective tissue membrane. It stretches from the tip of the osseous spiral lamina to a tough dense fibrous band called spiral ligament, which lines the outer wall of the canal. The basilar membrane is also called the membranous spiral lamina.
• Along the basilar membrane are twenty thousand to thirty thousand tiny fibers that are called basilar fibers. Each fiber has a different size and shape. The fibers near the oval window are short and stiff.
• While approaching helicotrema (see below) the basilar fibers gradually become longer and soft.

2. Vestibular Membrane:

• It is also known as Reissner’s membrane, and it is a thin membrane. It is placed obliquely between the upper surface of the osseous spiral lamina and the upper part of the spiral ligament.
• The basilar membrane and vestibular membrane divide the spiral canal of the cochlea into three compartments called scalae. The compartments of the spiral canal of the cochlea are:
  1. Scala vestibule
  2. Scala tympani
  3. Scala media.

All three compartments are filled with fluid. Scala vestibuli and scala tympani contain perilymph. The scala media is filled with endolymph.

1. Scala vestibuli: Scaia vestibuli lies above the scala media. It arises 5o’V: oval window (fenestra vestibuli) which is closed by ‘footplate of stapes. It follows the osseous canal up to its apex. At the apex, it communicates with the dough a small canal called helicotrema.

2. Scala tympani: It lies below the scala media. It is parallel to the scala vestibule and ends at the round window. The round window is closed by a strong thin membrane known as the secondary tympanic membrane.

3. Scala media:

• Scala media is otherwise called cochlear duct, membranous cochlea, or otic cochlea. It is a triangular compartment enclosed by basilar and vestibular mem¬branes. It ends blindly at the apex and at the base of the cochlea. A slender ductus reuniens arises from the basal end and connects scala media with the saccule of the otolith organ.
• Scala media is formed by upper, outer, and lower walls. The upper wall or vestibular wall is formed by a vestibular membrane. The outer wall is formed by a spiral ligament, which is the thickening of the periosteum. The lower wall is called a tympanic wall. It is formed by a basilar membrane (membranous spiral lamina) and part of the osseous spiral lamina. The scala media stretches between the tip of the osseous spiral lamina and the spiral ligament.
• The basilar membrane consists of straight unbranched connective tissue fibers, which are called basilar fibers or auditory fibers. On the upper surface of the basilar membrane, the arrangement of epithelial cells forms a special structure called the organ of Corti. It is the sensory part of the organ for hearing.

3. Organ Of Corti: The organ of the Corti is the receptor organ for hearing. It is the neuroepithelial structure in the cochlea.

Situation and Extent: It rests upon the lip of the osseous spiral lamina and the basilar membrane. It extends throughout the cochlear duct, except for a short distance on either end. The roof of the organ of Corti is formed by a gelatinous tectorial membrane.

Organ Of Corti Structure: The organ of the Corti is made up of sensory elements, called hair cells and various supporting cells. All the cells of the organ of Corti are arranged in order from the center towards the periphery of the cochlea:

1. Border cells
2. Inner hair cells
3. Inner phalangeal cells
4. Inner pillar cells
5. Outer pillar Cells
6. Outer phalangeal cells
7. Outer hair cells
8. Cells of Hensen
9. Cells of Claudius
10. Tectorial membrane and lamina reticularis.

1. Border Cells: Border cells are slender columnar cells, arranged in a single layer on the tympanic lip along the inner side of inner hair cells. The surfaces of the border cells have cuticles.

2. Inner Hair Cells:

• The inner hair cells are flask-shaped cells and are broader than the outer hair cells. The inner hair cells are arranged in a single row and occupy only the upper part of the epithelial layer. The rounded base of each cell rests on the adjacent supporting cells called the inner phalangeal cell.
• The surface of the inner hair cell bears a cuticular plate and a number of short stiff hairs which are called stereocilia. Each hair cell has about 100 stereocilia. One of the stereocilia is larger and it is called kinocilium. The stereocilia are in contact with the tectorial membrane.
• The inner hair cells and the outer hair cells together form the receptor cells. Sensory nerve fibers are distributed around the hair cells. Both inner hair cells and outer hair cells have afferent and efferent nerve fibers.

3. Inner Phalangeal Cells:

• The inner phalangeal cells are the supporting cells of inner hair cells and are arranged in a row along the inner surface of inner pillar cells. Their bases rest on the basilar membrane.
• The cuticular plate of cells (formed by the lower portion of cells) looks like the finger bones — the phalanges.

4. Inner and Outer Pillar Cells – Rods of Corti:

• The inner and outer pillar cells are called rods of Corti. Each pillar cell has a broader base, an elongated body or pillar, and a head at the tip of the pillar. The bases of inner pillar cells are close to the lip of the osseous spiral lamina (tympanic lip) whereas, the bases of outer pillar cells are close to the basilar membrane.
• The pillars of inner and outer pillar cells slope towards each other and their heads articulate. Thus, the pillars of the cells form a series of arches, which enclose a triangular tunnel called the inner tunnel or Corti’s tunnel.

5. Outer Phalangeal Cells:

• The outer phalangeal cells or the cells of Deiters are the supporting cells of outer hair cells. The outer phalangeal cell is the tall columnar cell.
• It sends stiff phalangeal processes upward between the hair cells to form the part of the lamina reticularis. The rows of outer phalangeal cells vary in different regions of the cochlear duct like the outer hair cells, i.e. from 3-5 rows. Between the innermost outer phalan¬geal cells and outer pillar cells, is a fluid space known as the space of Nuel.

6. Outer Hair Cells: The outer hair cells are the columnar cells occupying the superficial part of the epithelium of the organ of Corti. Their bases are supported by outer phalangeal cells. The structure of outer hair cells is similar to that of inner hair cells (see above).

7. Ceils of Hensen: The cells of Hensen are tall columnar cells forming the outer border cells of the organ of Corti. These cells are arranged in several rows on the basilar membrane lateral to outer phalangeal cells. The space between the outer phalangeal cells and the cells of Hensen is called the outer tunnel.

8. Cells of Claudius: The cells of Claudius are cuboidal in nature and line the lower surface of the external spiral sulcus. In certain areas, some groups of cells are present between the cells of Claudius and the basilar membrane. These cells are called Boettcher cells.

9. Tectorial Membrane and Lamina Reticularis:

• The tectorial membrane extends from the vestibular lip to the level of cells of Hensen. It forms the roof of the Organ of Corti. It is in contact with the processes of hair cells. It is assumed that the processes of hair cells are stimulated by the movements of the tectorial membrane, in relation to vibrations in endolymph.
• The cuticular plates of all the supporting cells collectively form a reticular membrane, which is known as the lamina reticularis. It covers the organ of Corti. It looks like a mosaic and has rows of holes through which the heads of hair cells are inserted.