P is a cell (or group of cells) in the human body which is sensitive to a particular type of stimulus and conveys the messages to CNS through nerves Q. On the other hand, R is a part of the human body which can respond to a stimulus according to the instructions sent from the CNS through nerves S.(a) What is P? Name five organs which contain cells (or group of cells) like P.(b) Name the nerves Q.(c) What is R? Give two examples of R.(d) Name nerves S.(e) How do messages travel through the nerves Q and S?

(a) P is a receptor. The five organs which contain cells like P (receptors) are eyes, ears, nose, tongue and skin.

(b) Nerves Q are sensory nerves.

(c) R is an effector organ. Two examples of R (effector organ) are the muscles and glands.

(d) Nerves S represents motor nerves.

(e) Messages travel through nerves Q (sensory nerves) and S (motor nerves) in the form of electrical impulses or nerve impulses.