Structure and function of Ear-2

The Middle Ear

Three tiny bones, the smallest in the body, are found in the middle ear, a box-like structure. It is lined by the epithelium, connected to the inner ear by a thin bone partition that has two tiny membrane-covered holes called the oval window and the round window, and separated from the external ear by the eardrum.

Air passageways from the middle ear are provided by two holes that are not membrane-covered. In the mastoid process of the temporal bone, one opens into the mastoid air cells. The other opens into the pharynx and is called the auditory (pharyngo tympanic) tube, also called the eustachian tube. The middle ear is joined to the nasopharynx by the auditory tube (upper part of the throat). The auditory tube aids in bringing the middle ear pressure closer to that of the surrounding air. The eardrum vibrates freely as sound waves impact it when the pressures are equal. Vertigo, severe discomfort, and impaired hearing may ensue if the pressure is not balanced. The middle ear epithelium, auditory canals, and throat epithelium are all expansions of the same continuous membrane. As a result, a sore throat may spread to produce a middle ear infection called otitis media.

The names of the three tiny ear bones, or ossicles, are malleus (which means "hammer"), incus (which means "anvil"), and stapes. The ear drum and the oval window of the inner ear are connected mechanically by these bones. The inside surface of the tympanic membrane is where the malleus' "handle" is attached. The malleus' "head" is attached to the incus' body. The head of the stapes and the incus articulate. The stapes' footplate slides into the window's circular opening. The inner ear and middle ear are separated by an oval window. The movement created by the eardrum's vibration by sound waves is transmitted and increased as it goes into the middle ear by the ear ossicles. Tensor tympani and stapedius muscles, two little skeletal muscles, are also attached to the auditory ossicles.

Contents of the middle ear:

• Stapes, incus, and malleus are the three ear ossicles.
• Chorda tympani and tympanic plexus are two nerves.
• Tensor tympani and stapedius are two muscles.

[Pneumonic for ear ossicle and their shape: MIS HAS]

Functions of the Middle Ear

• The primary role of the middle ear is to send vibrations produced by the tympanic membrane's motions to the inner ear.
• Small middle ear muscles reduce hearing sensitivity while protecting the inner ear from loud noises.
• A balanced pressure between the middle ear and the atmosphere is maintained with the aid of the auditory tube in the middle ear.

The Inner (Internal) Ear

Due to its intricate shape, the inner ear is sometimes known as a labyrinth. It houses the delicate receptors for sound and balance detection, which are carefully contained within the temporal bone below the eye orbit. The bony labyrinth and the membranous labyrinth are the two main sections of the inner ear. The bony labyrinth is a rigid cavity surrounded by periosteum in the temporal bone. It is divided into three areas:

• Vestibule-1
• Semicircular canals-3
• Cochlea-1

The cochlea is involved in hearing, while the vestibule and semicircular canals are involved in balance. The membranous labyrinth is housed within the bony labyrinth. The perilymph, a liquid, separates the bony labyrinth from the membranous labyrinth. Its composition is comparable to CSF's.

• The Vestibule: The vestibule, which is medial to the tympanic cavity, behind the cochlea, and in front of the semicircular canals, is the center portion of the bone labyrinth. The oval window is located in its lateral wall. The utricle and the saccule, two sacs that make up the membrane labyrinth in the vestibule, are joined by a tiny duct. When you move your head, a nerve impulse is produced by equilibrium receptors called cristae ampullares located in the saccule and utricle. The saccule and utricle are in charge of the body's linear motion or acceleration.
• The Semicircular Canals: From the vestibule, the three semicircular canals extend superiorly and posteriorly. They are known as the lateral, anterior, and posterior semicircular canals, and they are positioned at roughly right angles to one another. They are arranged so that movement in all directions can be felt. The semicircular ducts, a membrane labyrinth inside the bone semicircular canals that also houses an endolymph, are so named. The utricle of the vestibule is connected to the semicircular ducts. Equilibrium receptors are housed in an enlarged swelling called an ampulla that protrudes from one end of each semicircular canal. Acceleration or angular movement is caused by semicircular canals. The vestibular ampulla, along with the vestibule's utricle and saccule, houses the vestibular branch's fine nerve endings.
• The Cochlea: The cochlea is the portion of the bony labyrinth with a snail-like form that houses the cochlear duct, the portion of the internal ear responsible for hearing. The modiolus is the cone-shaped, spongy bone core around which the spiral canal of the cochlea twists. The spiral canal of the cochlea starts at the vestibule and makes 2.75 revolutions. The modiolus has canals where blood vessels and cochlear nerve branches are distributed. The tympanic cavity's promontory of the labyrinthine wall is created by the cochlea's massive basal turn. The cochlear aqueduct connects the bony labyrinth at the basal turn to the subarachnoid space above the jugular foramen. Additionally, it has the circular window (L. fenestra), closed by the secondary tympanic membrane.
  Scala vestibule and scala tympani are separated from each other by Reissner's membrane and the basilar membrane of the cochlear duct. Perilymph is present in both the scala vestibule and scala tympani. Helicotrema links the two together. The Corti organ of hearing is housed within the cochlea. The membrane cochlea, also known as the cochlear duct or scala media, is the membranous tube found inside the bony cochlea and surrounds the organ of Corti. The endolymph is likewise included within the cochlear duct. When bent by the movement of the endolymph, specialized hair cells on the organ of Corti produce nerve impulses. The vestibular membrane, also known as the Reissner's membrane, forms the roof of the cochlear duct.
  The basilar membrane, a portion of the duct, and the outside edge of the osseous spiral lamina together make up the duct's floor. The spiral organ (of Corti), which is located on the basilar membrane, is the receptor for auditory impulses. The gelatinous tectorial membrane covers it. Supporting epithelial cells and hair cells—specialized sensory cells with projections that resemble hairs at their apical ends—are both present in the Corti organ. A stereocilli is the name for these projections.

The Membranous Labyrinth: Within the bony labyrinth, there is a continuous succession of membranous sacs and ducts that all have the same basic shape. It is covered in epithelium and is filled with endolymph.