Anatomy of the ear

The organs of hearing and equilibrium convert mechanical energy to receptor (action) potential. Therefore, they are called mechanoreceptors. “The mechanoreceptors contain hair cells that generate receptor potential when the hair are bent by moving fluid or by particles.

 The study of structure, function and diseases of the ear is called otology (G. ous = ear, logos = study).

 What is location of ear ? 

The ears are located on the sides of the head. 

Anatomy of the ear  

Anatomy of the ear shows three parts : external, middle and internal. 

  1. Anatomy of External Ear. The external ear further has two regions : pinna, auricle or ear lobe and external auditory canal, or meatus.

 (i) Pinna. It is an oval, somewhat funnel- shaped, skin-covered flap of elastic cartilage and muscles. Its outer stiff ridge is called helix; lower flexible lobe is termed lobule and its cavity is known as concha. The latter collects sound waves and directs them into the external auditory canal.

(ii) External Auditory canal 

It is an S-shaped tube leading inward from the pinna . It follows an  oblique course so to prevent hard objects from hitting the tympanum directly. The canal is supported by elastic cartilage in the outer portion and by temporal bone in the inner portion. It is lined with skin continuous with that covering the pinna. The outer region of the canal bears hair that serve to keep out the dust particles. Its inner region has ceruminous or wax glands, that are modified sweat glands. They secrete a brownish, semi- solid, fatty substance, the cerumen, or earwax, which lubricates and protects the lining of the meatus. The warm, humid air enclosed in the meatus is essential for proper functioning of the tympanic membrane. 

Tympanic membrane (Tympanum, Eardrum). The tympanic membrane is a thin, oval, tightly stretched membrane closing the external auditory canal internally. It is composed of connective tissue with fibres radiating from the central region called umbo. It is covered externally by a stratified epithelium (epidermis), and internally by a simple cuboidal epithelium.

2. Anatomy of middle Ear 

The middle ear consists of an irregular, air-filled space, the tympanic cavity, enclosed in the temporal bone. It communicates with the nasopharynx by a passage called the eustachian tube (auditory tube). At the pharyngeal opening of the eustachian tube is a valve which normally remains closed. The valve opens during yawning, swallowing and during an abrupt change in altitude (ascent or descent in an aeroplane), when air enters or leaves the tympanic cavity to equalise the pressure of air on the two sides of the tympanic membrane. This enables the tympanic membrane to vibrate freely when the sound waves strike it. This also protects the tympanum from bursting due to sudden rise in air pressure caused by explosion or other loud noise. 

Anatomy of middle ear

• Blockade of Eustachian Tubes. Cold may block the eustachian tubes and the pressures outside and inside the middle ear do not equalize. For this reason, divers and fliers do not work when they have a cold. Outside pressure increases in diving and decreases in flying. In both cases, with blocked eustachian tubes, the middle ear pressure would remain the same, and the difference might injure the ear drum.

 Fenestrae. Inner wall of the tympanic cavity is formed of bone. It has two apertures, which put it in communication with a narrow space around the internal ear. Of these, the upper is called the fenestra ovalis, or oval window and the lower is termed the fenestra rotunda, or round window. Both the apertures are covered by a membrane.

 Ear Bones. A chain of three small, movably articulated bones, the ear or auditory ossicles, crosses the tympanic cavity. The outer ossicle is hammer-shaped. It is called the malleus. It is attached to the inner surface of the tympanic membrane. The inner ossicle is stirrup-shaped. It is known as the stapes. It fits into the oval window. The middle ossicle is anvil-shaped. It is called the incus. It is joined to the malleus externally and the stapes internally. The three ossicles articulate by synovial joints. 

The ear ossicles transmit the vibrations from the tympanic membrane to the internal ear and also amplify them about 20 times. 

Muscles. Two small muscles, tensor tympani and stapedius, joined to the malleus and stapes respectively, contract to prevent damage to the delicate internal ear when the ear is exposed to loud sounds. 

3. Anatomy of inner Ear . The internal ear is a delicate, irregular organ called membranous labyrinth. It is surrounded by an almost similarly shaped bony labyrinth. The membranous labyrinth is joined to the bony labyrinth at certain points, but its greater part is separated from the bony labyrinth by a narrow perilymphatic space. This space contains a watery fluid called perilymph which closely resembles cerebrospinal fluid in composition . The membranous labyrinth is itself filled with another fluid, the endolymph, It resembles the intracellular fluid in composition”. The membranous labyrinth consists of 3 parts: vestibule, semicircular ducts and cochlear duct.

(a) Vestibule. The vestibule is a central sac-like part of the membranous labyrinth. It further consists of 2 chambers : larger, somewhat oblong sac, the utricle (utriculus), which communicates with the semicircular ducts and smaller, roughly globular sac, the saccule (sacculus), that leads into the cochlear duct. From the posterior end of the saccule arises the ductus endolymphaticus which is joined by the utriculosaccular duct from the utricle. The ductus endolymphaticus passes downward and ends in a blind pouch, the saccus endolymphaticus. From the Jower part of the saccule arises a short tube, the ductus reuniens, that joins the cochlear duct. The vestibule has 2 sensory spots : macula of utricle and macula of saccule, located in the wall of the utricle and saccule respectively. A macula consists of hair cells and supporting cells. The supporting cells are columnar. The hair cells are sensory and of two types : flask-shaped and cylindrical. They bear at the free surface, nonmotile ‘hair’ or stereocilia (modified microvilli) and also have one cilium (kinocilium) arising from a centriole and form at their bases synapses with the dendrites of cells in the vestibular ganglion. Tips of the ‘hair’ and cilium project into a thick gelatinous, glycoprotein sheet, the otolithic membrane. The latter is secreted by the supporting cells, and contains numerous minute, irregular particles, called otoliths, or otoconia or ear stones, composed of protein and calcium carbonate. 

(b) Semicircular Ducts. There are 3 semicircular ducts superior, posterior and lateral, arranged in 3 mutually perpendicular planes. The superior and posterior ducts are vertical and the lateral duct is horizontal in position. Each duct opens into the utricle by both the ends. The superior and posterior ducts join to form a common duct, the crus commune, which then opens into the utricle. One (lower) end of each duct is enlarged to form an ampulla. The ampullae of the superior and lateral ducts lie nearby. Each ampulla has a sensory spot, called crista ampullaris, or simply crista, for equillibrium. A crista resembles a macula in structure except that it lies on an elevation, the acoustic ridge, its sensory cells have longer “hair”, and its gelatinous mass in dome-shaped, lacks otoliths and is called cupula .

(c) Cochlear Duct. The cochlear duct is a 35 mm. long spirally coiled tube of about two and a half turns resembling a snail’s shell. Part of the bony labyrinth that encloses the cochlear duct is called the cochlear canal. The cochlear duct and the cochlear canal are together referred to as the cochlea. Wall of the cochlear duct is fused with that of the cochlear canal on the sides, but is free above and below. With the result, the cochlea has three longitudinal chambers, called scalae, inside it: upper, middle and lower . These chambers are separated from one another by thin membranes, and become progressively narrower from the base to the apex. 

Function of the ear

The ear performs two unrelated functions : hearing and equillibrium .

  1. Hearing . The ear not only detects sound but also notes its direction, judges its loudness and determines its pitch (frequency). Sound waves are collected by the pinna and directed inward through the external auditory canal . Here they strike the tympanic membrane . The latter begins to vibrate at the same frequency as that of the sound waves. From the tympanic membrane , the vibrations are transmitted across the tympanic cavity by the ear ossicles to the membrane of rhe fenestra ovalis .
  • Audible Range

Man can hear frequencies as low as 20 cycles per second and as high as 20,000 hertz(Hz) per second . If it were more sensitive,it would pick up the random movements of the air molecules, which would result in constant hiss or buzzing . The audible range of dog reaches 40,000 Hz per second and that of bat 100,000. Bats produce and receive ultrasonic sounds of pitches much beyond human hearing to be guided during flight .

2. Equilibrium

Cristae and maculae are concerned with equilibrium of the body .

(i) Dynamic equilibrium

Cristae detect turning or rotational movements of the head (angular acceleration). When the head is turned, the endolymph in the semicircular ducts , due to its interia, does not move as fast as the head and the sensory cells of the crista, but continues to move after the head stops moving.

(ii) Static equilibrium and linear acceleration

Maculae detect changes in the head (or body) with respect to gravity (static equilibrium) and in the movement in one direction (linear acceleration) . With a change in the postion of the body , the otoliths , being heavier than the endolymph, press upon the sensory hair of the maculae.

  • Insect Tympanum .

Insects also have specialized organs for hearing . They have a tympanum , a thin region of their body covering , which vibrates in respomse to sound and stimulates special receptor cells as it is displaced. Locations of these “ears” varies in different insects .

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