Give a brief account of
$(a)$ Mechanism of synaptic transmission
$(b)$ Mechanism of vision
$(c)$ Mechanism of hearing

(N/A) nerve impulse is transmitted from one neuron to another through junctions called synapses,which are formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron,which may or may not be separated by a synaptic cleft. There are two types of synapses: electrical synapses and chemical synapses.
At electrical synapses,the membranes of pre- and post-synaptic neurons are in very close proximity,so electrical current can flow directly from one neuron into the other. Transmission across electrical synapses is very similar to impulse conduction along a single axon and is faster than chemical synapses,though it is rare in the human body.
At a chemical synapse,the membranes are separated by a fluid-filled space called the synaptic cleft. Chemicals called neurotransmitters are involved in the transmission of impulses.
$(b)$ Light rays in the visible wavelength are focused on the retina through the cornea and lens,generating impulses in rods and cones. The photosensitive compounds (photopigments) in human eyes are composed of opsin (a protein) and retinal (an aldehyde of vitamin $A$). Light induces the dissociation of retinal from opsin,resulting in changes in the structure of opsin. This causes membrane permeability changes,generating potential differences in the photoreceptor cells. This produces a signal that generates action potentials in the ganglion cells through the bipolar cells. These impulses are transmitted by the optic nerves to the visual cortex of the brain,where they are analyzed and the image is recognized based on memory and experience.
$(c)$ The external ear receives sound waves and directs them to the eardrum (tympanic membrane). The eardrum vibrates in response to sound waves,and these vibrations are transmitted through the ear ossicles to the oval window. The vibrations are passed through the oval window into the fluid of the cochlea,where they generate waves in the lymphs. These waves induce a ripple in the basilar membrane. The movements of the basilar membrane bend the hair cells,pressing them against the tectorial membrane,thereby generating nerve impulses in the associated afferent neurons. These impulses are transmitted by the auditory nerves to the auditory cortex of the brain,where they are analyzed and the sound is recognized.