Human Ear
The ear is the organ of hearing and is also involved in balance. It is supplied by the 8th cranial nerve, I.e the cochlear part of the Vestibule- cochlear nerve, which is stimulated by vibration caused by sound waves.
Structure of human ear
The ear is divided into three distinct parts -
1- Outer ear
2- Middle ear
3- Inner ear
Outer ear
The outer ear consist of the auricle and the external acoustic meatus.
The auricle (Pinna)
The auricle is the visible part of the ear that projects from the side of the head. It is composed of fibroelastic cartilage covered with skin. It is deeply grooved and ridged; the most prominent outer ridge is the helix.
External acoustic meatus (auditory canal)
This is slightly S shaped tube about 25 cm long extending from the auricle to the tympanic membrane. Lateral third is embedded in cartilage and the reminder lies within the temporal bone. The meatus is lined with skin continuous with that of the auricle.
Apocrine gland that secrete earwax , sticky material containing protective substance including the Cl enzyme, lysozyme and immunoglobulins.
Middle ear
This is an irregular shaped air filled cavity within the petrous portion of the temporal bone. The cavity, its content and the air sacs which open out of it are lined with 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 air passes to the air cells within the mastoid process.
The medial wall is a thin layer of temporal bone in which there are two openings ;
Oval window
Round window
The oval window is occluded by part of a small bone is called the stapes and the round window by a fine sheet of fibrous tissue.
How many bones in human ear?
Auditory ossicles
These are three very small bones only a few millimetres in size that extends across the middle ear from the tympanic membrane to a oval window
.
The malleus - this is the lateral hammer-shaped bone . The handle is in contact with the tympanic membrane and the head forms a moveable joint with the incus.
The incus - this is the middle anvil-shaped one. Its body articulates with the malleus, the long process with the stapes, and it is stablished by the short process, fixed by fibrous tissue to the posterior wall of the tympanic cavity.
The 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 contains the organs of hearing and balance. It is described in two parts , the bony labyrinth and the membranous labyrinth
The bony labyrinth - this is lined with periosteum within the bony labyrinth, the membranous labyrinth is suspended in a watery fluid called perilymph.
The membranous labyrinth - this is filled with endolymph.
The vestibule
This is the expended part nearest 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.
The 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 also important in balance
The cochlea
This resembles a snail's shell. It has a broad base where it is continuous with the vestibule and a narrow apex, and it spiral round a central bony column.
A cross section of the cochlea contains three compartments;
1- the scala vestiboli
2- the scala media
3- the scala tympani
Physiology of hearing
Every sounds produces sound waves or vibrations in the air, which travel at about 332 metres per seconds. The auricle , because of its shape, collects and concentrates the waves and directs them along the auditory canal causing the tympanic membrane to vibrate. Tympanic membrane vibration are transmitted and amplified through the middle ear by movement of the ossicles. At their medial end the footplate of the stapes rocks to and fro in oval window, setting up fluid waves in the perilymph of the Scala vestibule. 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 simulation of the auditory receptors in the hair cells of the spiral organ. The nerve impulses generated pass to the brain in the cochlear portion 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 conducted to the auditory area in the temporal lobe in the cerebrum. Because some fibres cross over in the medulla and others remain on the same side, the left and right auditory areas of the cerebrum receive impulses from both ears.
Sound waves have the properties of pitch and volume, or intensity. Pitch is determined by the frequency of the sound waves and is measured hertz. Sound of different frequencies stimulate the basilar membrane at different places along its length, allowing discrimination of pitch.
The volume depends on the magnitude of the sound waves and is measured in decibels. The greater the amplitude of the wave created in the endolymph, the greater is the stimulation of the auditory receptors in hair cells in the spiral organ, enabling perception of volume. Long term exposure to excessive noise causes hearing loss because it damage the sensitive hair cells of the spiral organ.
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