(a) Mechanism of synaptic transmission is a device of synaptic transmission. Synapse is the close proximity of terminal branch of the axon of one neuron with dendrites of the next neuron in a chain. As the impulse reaches the end knob, a neurotransmitter e.g. acetylcholine is released from synaptic vesicles present in the end knob. This results in depolarisation of dendrite membrane initiating a new impulse in it. The neurotransmitter is released by syanptic membrane and received by postsynaptic membrane of dendrite of next neuron. To check continues stimulation of dendrite membrane; an enzyme acetyl cholinesterase in activates acetylcholine. (b) Mechanism of VisionFormation of Inverted Image on Retina. The rays of light coming from an object pass through cornea, aqueous humor, pupil, lens, vitreous humor and then fall on retina and a small inverted image of the object is formed. Accomodation. The ability to the eye to focus objects at various distances is know as accommodation. Changing the convexity of the lens and size of pupil does this. Photoreception. Light splits rhodopsin pigment into retinene (an aldehyde derivative of Vitamin A) and a protein opsin. This depolarises the rod cells to release a neutotransmitter, transmitting the nerve impulse to different cells and then to optic nerve fibres. Eye to Brain. Nerve impulses are then transmitted from the optic nerve to the brain. The centre for sight is in the occipital lobe of the cerebral hemisphere where an erect image is formed and perceived. (c) Mechanism of hearing: The phonoreceptors of organ of Corti of cochlea are sensitive to sound waves of frequency ranging from 20 to 20000 cycles / second or Hertz (Hz) (Maximum sensitive to 1000 cycles per second). In hearing, different parts of ear perform different functions: Internal ear (i) Vibrations of fenestra ovalis cause pressure changes in perilymph of scala vestibuli, vibration in Reissner’s membrane, pressure changes in endolymph of scala media, vibrations in basilar membrane and pressure changes in perilymph of scala tympani. (ii) Due to vibration of basilr membrane auditory hair phonoreceptors are distorted in the tectonial membrane and are stimulated. The phonoreceptoors initiated nerve impulses, which are conducted to the auditory area of cerebral cortex of occipital part of cerebral hemisphere. Auditory area interprets the nerve impulses. (iii) Fenestra rotunda membrane acts as a pressure relief valve in hearing. Its movements are always in opposite direction to that of fenestra ovalis membrane. (iv) Human ear can differentiate quality as well as quantity of sound waves. Sound waves depends the area in maximum displacement e.g. sound waves of high frequencies (high notes) cause maximum displacement at the base of basilar membrane. Quantity of sound waves depends upon the number of phonoreceptors stimulated in a particular area.