Through neuronal networks, brain directly coordinates immediate and long-term responses of the body. But many times, effects on the growth, differentiation and metabolism by specific organs are controlled by chemical messengers.
Neurons (B) are the units of nervous system. Neurons (D) or nerve cells are the basic origin and conduit of all nerve impulses. The transmission of nerve impulses along a neuron from one end to the other is believed to occur as a result of electrical changes across the membrane of the neuron. The ability of a neuron to maintain polarity (charge difference) across the plasma membrane is crucial to the conduction of nerve impulse.
Glands (C) are present in various parts of the body and some of them form the units of endocrine system. It is an organ in the human or animal body that secretes a chemical (like hormones or sebum).
Secondary messengers (D) are present within the cell, released as a response to some external signal or stimulus or molecular cascade.
Therefore, the options B, C and D are incorrect.
Chemical messengers called hormones (A), originating directly or indirectly from brain activity, are produced by endocrine glands. They control growth, differentiation, and the metabolism of specific target cells. For example- thyrotropin releasing hormone produced by the hypothalamus facilitates the release of thyroid stimulating hormone by the pituitary, which in turn, facilitates thyroxine (yet another hormone!) production by the thyroid, which in turn controls several metabolic functions.
Therefore, option (A) is correct.