Ear - Structure

Man has a pair of ears that function as the organs of hearing and the organs for equilibrium. It has two different structures for each of these functions. Both these structures have receptors that convert the vibrations into nervous impulses that are transmitted via the auditory nerve to the brain.

Structure

Internal Structure of a Human Ear

Each ear has three parts - outer, middle and inner ear.

Outer ear

The outer ear has three parts. They are:

Pinna (Auricle)

It is a cartilagenous structure that is formed of skin and elastic cartilage. In other animals, it functions as structures that collect sound waves and direct them into the auditory canal. However, in man, the pinnae do not move and are vestigial organs.

Auditory canal

It is an oblique, narrow canal that is also called the external auditory meatus. The outer portion of the canal is lined by fine hairs and the inner portion is lined by wax glands (cerumenous glands) that secrete ear wax. The auditory canal collects and concentrates the sound waves.

Tympanum

The external ear is separated from the middle ear by a membranous structure called the ear drum or the tympanum. The sound waves striking on the tympanum produce vibrations that are transmitted to the inner parts of the ear.

Middle ear

The middle ear consists of

Tympanic cavity

It is the air-filled cavity inner to the ear drum, tympanum. It is separated from the inner ear by a partition with two membrane-bound apertures. The upper aperture is oval and is called fenestra ovalis (oval window). The lower one is round and called the fenestra rotunda (round window).

Auditory ossicles

In the tympanic cavity are three small bones called the auditory ossicles. They are all movably joined to each other. The three ossicles are

Malleus

- It is the largest and hammer-shaped. It is in contact with the tympanum and the next ossicle, incus.

Incus

- It is anvil-shaped and joined to the next ossicle, stapes.

Stapes

- It is the smallest and stirrup-shaped. One end is joined to the incus and the other touches the fenestra ovalis.

Eustachian tube

It is the narrow canal connecting the tympanic cavity to the pharynx. It maintains the air pressure in the tympanic cavity. Its opening into the pharynx is guarded by a valve.

Inner ear

It is the innermost part of the ear within the skull. It is surrounded by the periotic bone of the skull. Within the inner ear are canals and cavities. They are formed by membranes and bones. The structures formed by the bones and the membranes are similar. Within the cavities formed by the bones are the canals formed by the membranes. the space between the bone and the membrane is filled with the watery fluid called the perilymph and the membranous canals are filled with a similar watery fluid called the endolymph. The inner ear has two parts carrying out two distinct functions. Both are essentially a system of canals. They are

Vestibular apparatus

It is the organ of equilibrium. The entire apparatus consists of a bony cavity called the vestibule and three semi-circular bony canals. Within the bony part is the membranous part with two sac-like structures in the vestibule and three semi-circular canals within the bony ones. The three membranous structures are

Utricule (sing. Utriculus)

It is a larger sac-like structure containing the sensory macula (pl. maculae).

Saccule (sing. Sacculus)

It is a smaller sac-like structure present below the utricule, also containing a sensory macula.

Structure of maculae

Each macula has a set of sensory hair cells embedded in a gelatinous mass. At one end the tuft of hairs can come in contact with calcareous ear stones or otoliths, that are embedded in the gelatinous layer. At the other end, the cells synapse with the neurones of the vestibular branch of the auditory nerve.

Functioning of maculae

Macula detects the vertical and lateral movements of the head. With the movement of the head, the endolymph causes the otoliths to displace which are brought in touch with the sensory hairs. This disturbance is then transmitted to the nerve endings.

Semi-circular canals

There are three semi-circular canals, oriented perpendicular to each other (i.e., along the three axes- x, y and z). Each canal begins with a swollen portion called the ampulla. Each ampulla contains the sensory structure called the crista.

Structure of cristae

Cristae are similar to maculae except that they do not contain the ear stones. The sensory hair cells are arranged in a cone-like manner. The sensory hairs are embedded in the cone-shaped gelatinous material. They are sensitive to the direction (rotational movement) and the rate of change of position of the head.

Functioning of cristae

With each movement the endolymph in the semi-circular canals also moves and strikes against the gelatinous material. This moves the hairs which transmit the sensation to the neurones.

(i) Semicircular duct showing position of a cristae with head in still position and [ ii ] position of cristae when head moves

Cochlea

It is the organ of hearing. It is a coiled bony structure called the cochlear duct that has three canals running parallel to each other.

Inside view of Cochlea

They are:

Median canal

(Scala media)

It is the central canal formed by longitudinal membranes present along the length of the cochlea. It is filled with endolymph. The dorsal membrane is called the Reissner's membrane and the ventral membrane is called the basilar membrane. The median canal has the organ of Corti where the sound waves are transformed into action potentials. Organ of Corti - Along the basilar membrane are arranged the longitudinal rows of sensory hair cells.

The rows of hair cells are overlapped by the tectorial membrane in such a manner that the hairs can come in contact with it. At the other end, the sensory hair cells are in touch with the neurons of the auditory nerve.

Stages of axon membrane during resting, depolarisation, action potential and repolarisation, showing changes in membrane potentials and molecular events. (a) Resting state: voltage-gated Na⁺ channels are in resting state and voltage-gated K⁺  channels are closed. (b) When stimulated, depolaristion opens Na⁺ channel activation gates, K⁺ channels still closed. (c) Action potential peaks and repolarisation begins, K⁺ channels open and Na+ channel gates close. (d) Repolarisation complete, K⁺ ions exit and Na⁺ channels begin to open again

Vestibular canal

It is the dorsal canal formed by the cavity between the periotic skull bone and the upper membrane of the median canal. It is filled with perilymph.

Tympanic canal

It is the ventral canal formed in a similar manner as the vestibular canal. It is also filled with perilymph. It is connected to the vestibular canal at the end of the cochlear duct.