About the ear and hearing loss

About the ear and hearing loss

Receptors for two sensory modalities hearing and equilibrium, are housed in the ear.The ear is an organ of hearing. It is also concerned in maintaining the equilibrium of the body. It consists of three parts: The external ear, the middle ear, and the internal ear.  

External ear

The external ear funnels sound waves to the external auditory meatus.From the meatus, the external auditory canal passes inwaed to the tympanic membrane ( eardrum ).
The external ear consists : The auricle or pinna, and the external acoustic meatus.
Pinna : The auricle is the part seen on the surface, the part the layman calls the ear. The greater part of it is made up of a single crumpled plate of elastic cartilage which is lined on both sides by skin it supports spectacles. However, the lowest part of the auricle is soft and consists only of fibro fatty tissue covered by skin. The blood supply of the auricle is derived from the posterior auricular and superficial temporal arteries.     

External acoustic meatus : The external auditory meatus conducts sound waves from the concha to the tympanic membrane. The canal is S-shaped. Its outer part is directed medially, forwards and upwards. The middle part is directed medially, backwards and upwards. In inner part is directed medially, forwards and downwards. The meatus can be straightened for examination by pulling the auricle upwards, backwards and slightly laterally. The outer part of the canal is supplied by the superficial temporal and posterior  auricular arteries, and the inner part, by the deep auricular branch of the maxillary artery.         

Middle ear 
The middle ear is an air-filled cavity in the temporal bone that opens via the auditory ( eustachain ) tube into
the nasopharynx to the exterior. The tube is usually closed, but during swallowing,chewing, and yawning it opens,keeping the air pressure on the two sides of the eardrum equalized.The manubrium ( handle of the malleus ) is attached to the back of thet tympanic membrane.Its head is attached to the wall of the middle ear, and its short process is attached the incus, which in turn articulates with the head of the stapes is named for its resemblance to a stirrup.Its foot plate is attached by an annular ligament to the walls of the oval window.

Shape and size : The middle ear is shaped like a cube.Its lateral and medial walls are large, but the other walls are narrow,because the cube is compressed from side to side. Its vertical and anteroposterior  diameters are both about 15 mm. When seen in coronal section the cavity of the middle ear is biconcave as the medial and lateral walls are closest to each other in the centre.The distance separating them are 6 mm near the roof, 2mm in the centre, and 4 mm near the floor.     

Skeletal muscles : Two small skeletal muscles, the tensor tympani and the stapedius, are also located in the middle ear. Contraction of the former pulls the manubrium of the malleus medially and decreases the vibrations of the tympanic membrane; contraction of the latter pulls the footplate of the stapes out of the oval window. 
 
Inner ear : The inner ear is made of two parts one within the other.
1.Bony labyrinth : The bony labyrinth is a series of channels in the petrous portion of the temporal bone. Inside these channels   
surrounded by a fluid called perilymph, is the membranous labyrinth.
2.Membranous labyrinth : This membranous structure more or less duplicates the shape of the bony channels . It is filled
with a fluid called endolymph, and there is no communication between the spaces filled with endolymph and those filled with perilymph.

Cochlea : The upper scala vestibuli and the lower scala tympani contain perilymph and communicate with each other at the apex of the cochlea through a small opening called the helicotrema. At the base of the cochlea, the scala vestibuli ends at the oval window, which is closed by the footplate of the stapes. The scala tympani ends at the round window, a foramen on the medial wall of the middle ear that is closed by the flexible secondary tympanic membrane. The scala media, the middle cochlear chamber, is continuous with the membranous labyrinth and does not communicate with the other two scalae. Itcontains endolymph.
 
Central auditory pathways : From the cochlear nuclei,auditory impulses pass via a variety of pathways to the inferior colliculi, the centers for auditory reflexes, and via the medial geniculate body in the thalamus to the auditory cortex.Others enter reticular formation. Information from both ears converges on each superior olive, and at all higher levels most of the neurons respond to inputs from both sides. The primary auditory cortex,brodmann's area 41, is in the superior portion of the temporal lobe.In humans, it is located in the sylvian fissure and is not normally visible on the surface of the brain.In the primary auditory cortex , most neurons respond to input from both ears, but there are also strips of cells that are stimulated by input from the contra lateral  ear and inhibited buy input from the ipsilateral rea. There are several additional auditory receiving areas just as there are several receiving areas for cutaneous sensation. The auditory association areas adjacent to the primary auditory receiving area are widespread. The olivocochlear bundle of eifferent fibers in each auditory nerve that arises from both the ipsilateral and the contralateral superior olivary
complex and ends primarily around the bases of the outer hair cells of the organ of corti.

Sound
Sound is the sensation produced when longitudinal vibrations of the molecules in the external enviroment i.e. alternate phases of condensation and rarefaction of the molecules, strike the tympanic membrane.
Auditory pathway : The ear converts sound waves in the external  environment into action potentials in the auditory nerves. The sound waves are transformed by the eardrum and auditory ossicles into movement set up waves in the fluid of the inner ear.The action of the waves on the organ of corti generates action potentials in the fibres.

Transmission : The cell bodies of the first - order neurons that innervate the hair cells are located in the spiral ganglion. The axons  of the first - order neurons from the auditory ( cochlcar ) division of the vestibulocochlear nerve and terminate in the ventral and dorsal cochlear nuclei in the medulla oblongata. From the cochlear nuclei, auditory impulses pass via variety of pathways to the inferior colliculi, the centers for auditory reflexes and via the medial geniculate body in the thalamus to the auditory cortex.

Neural pathways of equilibrium : The cell bodies of the 19,000 neurons supplying the cristae  and maculas on each side are located in the vestibular ganglion. Each vestibular nerve terminates in the ipsilateral four-part vestibular nucleus and in the flocculonodular lobe of the cerebellum. Fibers from the semicircular canals end primarily in the superior and medial divisions of the vestibular nucleus and project mainly to nuclei controlling eye movement. Fibers from the utricle and saccule and predominantly in the lateral division ,which projects to the spinal cord. They also end in the descending nucleus, which projects to the cerebellum and the
reticular formation. The vestibular nuclei also project to the thalamus and from there to two parts of the primary somato sensory cortex.             

Causes of nerve deafness : Aminoglycoside antibiotics such as streptomycin and gentamicin obstruct the meehanosensitive channels in the stereocilia of hair cells and can cause the cells to degenerate producing nerve deafness and abnormal vestibular
function. Damage to the prolonged exposure to noise is associated with hearing loss.Tumors of the vestibulocochlear nerve and cerebellopontine angle.Vascular damage in medulla.

Dissection
Clean the mastoid temporal bone off all the muscles and identify suprameatal triangle and supramastoid crest. Use a fine chisel to remove the bone of the triangle till the mastoid antrum is reached. Examine the extent of mastoid air cells.Remove the posterior and superior walls of external auditory meatus till the level of the roof of mastoid antrum.Identify the chorda tympani nerve at the posterosuperior margin of tympanic membrane. Look for arcuate eminence on the anterior face of petrous temporal bone. Identify internal acoustic meatus on the posterior face of petrous temporal bone, with the nerves in it. Try to break off the superior part of petrous temporal bone above the internal acoustic meatus. Identify the facial nerve as it passes towards the aditus. Identify the sharp bend of the facial nerve with the geniculate ganglion. Identify the facial nerve turning posteriorly into the medial wall. Trace it above the fenestra vestibuli till it turns inferiorly in the medial wall of aditus. Identify facial nerve at the stylomastoid foramen. Try to break the bone vertically along the lateral edge of the foramen to expose the whole of facial nerve canal. Break off more of the superior surface
of the petrous temporal bone. Remove the bone gently. Examine the holes in the bone produced by semicircular canals and look for the semicircular ducts lying within these canals. Note the branches of vestibulocochlear nerve entering the bone at the lateral end of  the meatus. Study the internal ear from the models in the museum.