Gustatory and olfactory senses are interrelated. Sense of smell helps in enhancing the sense of taste. This is the reason we don't get proper taste of food when we suffer from common cold.

The nose contains mucus-coated receptors which are specialised for receiving the sense of smell and are called olfactory receptors. These are made up of olfactory epithelium which consists of three kinds of cells. The neurons of the olfactory epithelium extend from the outside environment directly into a pair of broad bean-sized organs called olfactory bulb. Olfactory bulbs are extensions of the brains limbic system.

The entire process of response to a peripheral neural stimulation, that occurs involuntarily, i.e., without conscious effort or thought and requires the involvement of a part of the central neural system is called a reflex action.
The reflex pathway comprises at least one afferent neuron (receptor) and one efferent (effector or excitor) neuron appropriately arranged in a series. The afferent neuron receives signal from a sensory organ and transmits the impulse via a dorsal nerve root into the CNS (at the level of spinal cord). The efferent neuron then carries signals from CNS to the effector. The stimulus and response thus forms a reflex arc as shown below in the knee jerk reflex.

A neuron is a microscopic structure composed of three major parts, namely, cell body, dendrites and axon.
Cell Body. The cell body contains cytoplasm with typical cell organelles and certain granular bodies called Nissl's granules. Dendrites. Short fibres which branch repeatedly and project out of the cell body also contain Nissl's granules and are called dendrites. These fibres transmit impulses towards the cell body.
Axon. The axon is a long fibre, the distal end of which is branched. Each branch terminates as a bulb-like structure called synaptic knob which possess synaptic vesicles containing chemicals called neurotransmitters. The axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular junction.

The parasympathetic neural system is part of autonomic neural system. This system has some sort of inhibitory effect. The inhibitory effect minimizes the over functioning of certain functions. Functions, like salivating, digestion, are under control of parasympathetic neural system.

The neural system is composed of specialized cells called neurons. It detects stimuli and transmits neural signals. The neural system of complex animals is composed of two parts, viz. central neural system and peripheral neural system. The brain and nerve cord comprise the central neural system and other nerves comprise the peripheral neural system.

Differences between cranial nerve and spinal nerve in human are given below.

Light induces dissociation of the retinol from opsin. It brings about changes in the structure of the opsin. This causes membrane permeability changes. As a result, potential difference are generated in the optic nerves to the visual cortex area of the brain, where the nerves impulses are analysed and the image formed on the retina is recognised.

(i) The daylight (photopic) vision and colour vision are functions of cones. In the human eye, there are three types of cones which possess their own characteristic photopigments that respond to red, green and blue lights. The sensations of different colours are produced by various combinations of these cones and their photopigments. When these cones are stimulated equally, a sensation of white light a is produced.
(ii) The crista and macula are the specific receptors of the vestibular apparatus of inner ear which are responsible for the maintenance of balance of the body and posture.
(iii) The diameter of the pupil is regulated by the muscle fibre of iris. Photoreceptors, rods and cones regulate the amount of light that falls on the retina.

When a stimulus is applied to a nerve becomes freely permable to Na⁺ followed by the reversal of the polarity at that site. i.e., the outer surface of the membarance becomes  negatively charged and the inner sides become positively charged. The electrical potential differenec across the plasma membarance is called the action potential, which is in fact terrmed as a nerve impluse. Thus, the  shows that Na⁺ ions play an important role in the conduction of nerve impluse.

Difference between dendrites and axons are as follows:

(a) Cerebrum is the most developed.
(b) Hypothalamus acts as a master clock.