(a) A nerve impulse begins when a neuron receives a chemical stimulus. A nerve impulse travels along the neuron in the form of electrical and chemical signals. The axon tip ends at a synapse. A synapse is the junction between each axon tip and the next structure. Movement of Na+ions inside the axon membrane and that of K+ions outside the membrane via ATP energised pump generates action potential across the axon membrane which in turn generates the nerve impulse.
(i) Dendrites are the small extension of nerve cells that receive signals from sensory receptors or other neurons. Nerve impulses begin in a dendrite, move towards the cell body, and then move down the axon.
(ii) An axon is a long, slender projection of a neuron that typically conducts electrical impulses away from the neuron's cell body in the form of a wave of depolarization. The function of the axon is to transmit information to different neurons, muscles and glands.
(b) Feedback mechanism is the mechanism in our body to maintain the hormonal levels in the body in desirable amount. An increase or decrease in the hormonal level in our body triggers the feedback mechanism. The body has two types of feedback mechanisms:
(i) Positive feedback mechanism.
(ii) Negative feedback mechanism.
Positive feedback stimulates and increases hormone secretion. Negative feedback stops and inhibits the hormone from being produced. For Example, if we eat carbohydrate rich food, the glucose level in the blood increase. As the blood glucose level rise, the pancreas will secrete insulin. This insulin will signal the cells to take up the blood glucose. Hence, the glucose level decrease in the blood. This is an example of positive feedback. Now, if the insulin is still present in the blood, then more and more glucose will be transported inside the cell, and there would be a scarcity of the glucose in the blood. Hence to prevent this, negative feedback is generated due to low glucose level which would inhibit the insulin secretion in the blood.