The human body contains a large number of cells A which are very long and branched, and look like electric wires. The longest branch of this cell is B whereas there are many small branches C. Any two A cells do not join to one another completely in the human body. There is a microscopic gap D between every pair of adjacent A cells through which electric impulses can pass by the release of a chemical substance.

(a) What are cells A?

(b) What is the name of (i) branch B, and (ii) branches C?

(c) What is the microscopic gap D known as?

(d) What is the function of cells like A in the human body?

(e) The cells A are of three types. Name the three types.

Which of the following are often called glands of emergency?

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?

When we touch a hot plate unknowingly, then this heat is sensed by a receptor P present in our fingers. The receptor triggers an impulse in neuron Q which transmits the message to an organ R which is a part of the central nervous system. Here the impulse is passed on to a neuron S which in turn passes it to a yet another neuron T. The neuron T passes the impulse to a tissue U in our arm. The tissue U then contracts and pulls our hand away from the hot plate.

'(a) What is the name of (i) receptor P (ii) neuron Q, and (iii) organ R?

(b) What is (i) neuron S, and (ii) neuron T?

(c) Name the tissue U.

(d) What name is given to the phenomenon in which hand is pulled away quickly from the hot plate?

(e) Name the effector in this whole process.

The gland X which is located just below the brain in the human head secretes a chemical substance Y which controls the development of bones and muscles in the body of a person. Secretion of too little of substance Y as well as the secretion of too much of substance Y by the gland X leads to abnormal development of the body of a person.

(a) Name the gland X.

(b) What is the chemical substance Y?

(c) What happens if too little of substance Y is secreted?

(d) What happens if too much of substance Y is secreted?

(e) Name the system of glands in the human body of which gland X is a part.