(a) What is a sense organ? (b)(i) State two main functions of the ear (ii) Explain the mechanism involved in one of the functions stated in (b) (i) above. (...

(a) What is a sense organ?

A sense organ is a specialised organ made up of a group of cells or tissues that contain receptors sensitive to a particular kind of stimulus (light, sound, heat, chemicals, pressure). The receptors detect the stimulus and convert it into nerve impulses which are transmitted along sensory nerves to the central nervous system for interpretation. Examples: the eye, ear, skin, nose and tongue.

(b)(i) Two main functions of the ear

• Hearing - the detection of sound.
• Balance (equilibrium) - the maintenance of body posture and balance.

(b)(ii) Mechanism of hearing

Sound waves in the air are collected and concentrated by the pinna and passed along the external auditory canal (meatus) to the ear drum (tympanic membrane), causing it to vibrate. These vibrations are transmitted across the middle ear and amplified by the three ear ossicles - the malleus, incus and stapes. The stapes presses on the oval window, setting up pressure waves in the fluid of the inner ear. The perilymph in the cochlea vibrates and this in turn makes the endolymph vibrate, stimulating the sensory hair cells of the organ of Corti on the basilar membrane. These cells generate nerve impulses which travel along the auditory nerve to the brain, where they are interpreted as sound.

(c) The eye defect of short-sight (myopia) and its correction

In short-sight the eyeball is too long from front to back (or the eye lens is too strongly converging). Light rays coming from a distant object are therefore brought to a focus in front of the retina; by the time the rays reach the retina they have spread out again, so a blurred image is formed and distant objects cannot be seen clearly. The defect is corrected by placing a concave (diverging) lens in front of the eye. This lens first diverges the incoming parallel rays, so that after passing through the eye lens they are brought to a focus exactly on the retina, forming a sharp image.