VPB 121: Veterinary Physiology - II (2+1)

HEARING

Auditory apparatus

• The sense of hearing is the process of detecting and interpretation of sound waves.
• It is done by the organ called as the ears.
• The ear consists of three parts: The outer ear, middle ear, and inner ear.
• The outer ear is of the external pinna and the auditory canal.
• The pinna is of different shape and sizes in various species of animals and helps in focussing the sound waves to the auditory canal.
• The auditory canal conducts the sound waves into the middle ear. It is lined by waxy secretion called as cerumen. This filters the air entering the ear to avoid infections.
• There is a separation between the outer ear and the middle ear called as the tympanic membrane or commonly called as the ear drum.
• The ear drum transfers the sound waves to the vibrations which are conducted towards the inner ear by the middle ear.
• The middle ear consists of the ear ossicles. The ear ossicles are bony structures namely malleus (hammer), incus (anvil), and stapes (stirrup). They conduct the vibrations toward the inner ear.

VIEW ANIMATION...

• These ear ossicles are connected in a way to amplify the sound waves to evoke or stimulate the hair cells of the inner ear.  

• The middle ear and the inner ear is separated by the oval window
• The inner ear consists of fluid filled membranous sacs and canals.
• The whole inner ear is covered by a fluid perilymph.
• The inner ear has both vestibular apparatus, which is the organ of equilibrium and hearing apparatus.
• The hearing apparatus is the coiled tubular structure called as the cochlea.
• The cochlea is replaced by cochlear duct in birds, which is the hearing apparatus and it is small in size.
• The cochlea is filled with a fluid known as endolymph, which has high K+ concentration than the Na+ concentration.

• The cochlea is formed by the coiling of three fluid-filled tubes known as scala vestibuli, scala media (cochlear duct)‚ and scala tympani.

• The scala media is separated from the scala vestibubi by the Ressiner's membrane or vestibular membrane, while the scala media is separated from the scala tympani by the basilar membrane.
• Along the floor of the scala media, on the basilar membrane lies the hair cell receptor system, the Organ of Corti.
• The basilar membrane of the cochlea consists of the real organ of hearing called as the Organ of Corti. It contains hair cells.
• The hair cells mainly are of two types: The inner hair cells and outer hair cells.
• The inner hair cells detect sounds and outer cells amplify the sound waves.
• The cochlea contains two fluid systems perilymph (similar in composition to extracellular fluid) is the fluid in the scala tympani and scala vestibuli and endolymph (similar to intracellular fluid) is the fluid in the scala media.
• The oval window is located at the proximal end of the scala vestibuli, and the round window is located at the proximal end of the scala tympani.
• The hair cells are modified epithelial cells and they synapse with the sensory neurons.
• The hair cells contain projections called as cilia. Among these, one cilium is much longer than the other and is present in one side and is named as kinocilium. The shorter cilia are called as stereocilia. These stereocilia are arranged in increasing order of size gradually till they reach the kinocilium.
• The stereocilia and kinocilium are interconnected with tip-links and acts as one unit.
• The hair cells present within a cupula, a gelatinous mass covering over the hair cells with the synapsing the sensory neurons are known as neuromast.
• These hair cells are present both for hearing as well as maintaining equilibrium.
• The hair cells, tectorial membrane and basilar membranes with cochlear nerve terminals form the organ of Corti.
• Stereocilia that project into the endolymph filled scala media. The tectorial membrane, the specialised flap of the basilar membrane, over hangs the cilia of the hair cells. The terminals of the cochlear nerve fibres synapse with the basal ends of each hair cell. The auditory impulses are transmitted through vestibulo - cochlear nerve‚ (cranial nerve VIII) to higher brain centres.