Tag: structure and function of ear

Biosonar or sonar system helps bats to detect distant objects by echolocation. These organisms emit high-pitched sounds that bounce off objects in its path and produce an echo back.  They respond rapidly to the echo and skillfully avoid obstacles and captures prey. Since they use only sonar system, not the vision, bats can travel with eyes plugged and ears open. 

An otolith, also called statoconium or otoconium, is a structure in the saccule or utricle of the inner ear, specifically in the vestibular labyrinth of vertebrates. They are sensitive to gravity and linear acceleration. In mammals, otoliths are small particles, composed of a combination of a gelatinous matrix and calcium carbonate in the endolymph of the saccule and utricle. The inertia of these small particles causes them to stimulate hair cells, when the head moves. 

A sensory receptor such as a visual receptor or an auditory receptor, that can be stimulated by distant stimuli and that mediates a far sense is called as telereceptor. 

While semicircular canals respond to angular acceleration in specific directions, hair cells in the utricle and saccule respond to linear accelerations. The utricle and saccule are sac-like structures, that contain a patch of sensory hair cells called as the macula. The hair cells in the macula, which are similar to those in the cristae, are embedded in the otolith (ear stone) membrane, a gelatinous structure, that contains a large number of hexagonal prisms of calcium carbonate called as otoconia (ear dust). 

When the pressure variations strike the ear, they find their way through the external auditory canal to the tympanic membrane, setting it into vibration. The signal is thus converted to mechanical vibrations in solid matter. These vibrations of the tympanic membrane are transmitted to the ossicles, which in turn transmit them to the cochlea. Here, the signal undergoes a second change of nature, being converted into pressure variations within the liquid. These are then transformed again by specialised hair cells, which convert the liquid waves into nervous signals. Inside the cochlea, the tectorial membrane moves along with the pressure variations of the cochlear fluid. This membrane is in contact with the cilia on the top of the hair cells. There are two kinds of hair cells. The outer hair cells are the actual receptors. When the tectorial membrane moves, so does the hair on the outer cells. This movement is then encoded into electrical signals and goes to the brain through the cochlear nerve.

The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, in rabbit making 2.5 turns around its axis, the modiolus. A core component of the cochlea is the organ of Corti, the sensory organ of hearing.