Explain The working of Senses: Touch, Hearing, Equilibrium ?

The senses are detected by sense organs, collections of receptor cells, or cells closely associated with them. A mechanoreceptor is a cell associated with the sense of touch; photoreceptors detect intensity and color of light, thermoreceptors recognize heat and cold; chemoreceptors which are involved in taste and smell detect chemicals; and pain receptors react to tissue damage.

Receptor cells produce electrical activity which is taken by means of specialized nerve cells called conductors to the brain, where it is processed. The brain then sends signals to effectors, nerve cells that carry the messages to targets such as muscles and glands. Muscles contract and glands secrete hormones or other substances in response to the signals.

Touch :  The sensation of touch is attributed to Meissners corpuscles, found in large numbers on the skin of the fingers and the face. Pacinian corpuscles are associated with deep touch and vibration, and are located in connective tissue. Proprioreceptors provide awareness of position of the body. Hair cells in the cochlea or semicircular canals of the ear furnish information on acceleration in addition to sound.

Muscle spindle cells determine the degree of muscular contraction. Receptors located in the skin respond to various wavelengths of irradiation as do photoreceptors in the eye. Mechanoreceptors abound at the base of hair follicles. If the impulses in these receptors is constant, after a while they stop sending impulses to the brain, and send impulses again when the stimulus stops. In this way the brain is not constantly bombarded with impulses that serve no purpose.

There are two types of thermoreceptors specialized for sensing either heat or cold. Pain receptors distributed throughout the body respond to most types of energy potentially destructive to tissue. Brain chemicals called endorphins moderate the perception of pain. Endorphins are released during heavy exercise and during periods of stress, in the so-called "fight-or-flight" reaction which allows an animal to focus its attention on survival.

Hearing :  Look at the illustrations of structure of the ear and its parts. Pressure waves in the air called sound are captured by the outer ear and then pass through the auditory canal to set up vibrations in the tympanic membrane, or eardrum.

The middle ear contains three small bones that span the cavity, called the hammer, anvil, and stirrup, or in the older, or medical terminology, the malleus, incus and stapes. These respond to movement of the tympanic membrane.

Within the inner ear, the cochlea is a spiralled shell-shaped structure containing hair cells that detect pressure waves or sound. The stirrup of the middle ear presses against the oval window, a membrane that separates the cochlea from the middle ear. Vibration of the oval window moves the fluid in the cavity of cochlea.

The basilar membrane stretches across the space in the cochlea along its length, and divides the cochlea into the tympanic canal and the vestibular canal. Between these, the organ of Corti contains hair cells that are in contact with the rigid tectorial membrane, causing the hair cells to flex upon movement of the basilar membrane.

High pitched sounds carry only a short distance down the vestibular canal, to be detected by high-frequency receptors. Low pitched sounds travel farther down, activating low-frequency receptors. Intensity of the sound is determined by the number of hair cells stimulated. Repeated stimulation by loud sounds such as rock music causes damage to hair cells that cannot repair themselves, resulting permanent hearing loss.

Equilibrium

Dynamic equilibrium is the sensation of rapid movement, and is detected by structures in the vestibular apparatus of the inner ear. Mechanoreceptors associated with balance are found in hair cells at the base of each of three semicircular canals oriented at right angles to each other. Sloshing of fluid in the semicircular canals bends the hairs to sense movement in all three directions.

Static equilibrium is the sensing of body position. Below the semicircular canals are chambers containing gelatinous membranes and calcium carbonate particles called otoliths that bend hair cells according to the pull of gravity. Neurons relay the information about the position of the head to a center in the cerebellum.