The multicellular animals (except sponges) have specialised cells called nerve cells (or neurons) to respond to stimuli and coordinate their activities. A system made up of nerve cells is called nervous system.
The coordination in simple multicellular animals takes place through nervous system only. For example, Hydra is a simple multicellular animal. The nervous system of Hydra consists of a network of nerve cells joined to one another and spread throughout its body.
The control and coordination in higher animals called vertebrates (including human beings) takes place through nervous system as well as hormonal system called endocrine system. Before we describe the control and coordination in humans, it will be good to know something about sense organs, receptors and effectors. These are described below.
There are five sense organs in our body: eyes, ears, nose, tongue and skin. We receive a variety of information from the environment around us through the sense organs. The sense organs contain receptors. A receptor is a cell (or a group of cells) in a sense organ which is sensitive to a particular type stimulus (or a particular type of change in the environment) such as light, sound, smell, taste, heat, pressure, etc. The different sense organs contain receptors for detecting different stimuli.
The eyes have light receptors (which can detect light), ears have sound receptors (which can detect sound), nose has smell receptors (which can detect smell), tongue has taste receptors (which can detect taste) whereas skin has receptors for detecting touch, pressure, heat (or cold) and pain, etc.
The common type of receptors also has special names such as photoreceptors, phonoreceptors, olfactory receptors, gustatory receptors and thermoreceptors. Photoreceptors detect light (they are present in eyes), phonoreceptors detect sound (they are present in inner ears), olfactory receptors detect smell (they are present in nose), gustatory receptors detect taste (they are present in tongue) whereas thermoreceptors detect heat or cold (they are present in skin).
Just like us, a dog has also five sense organs. A dog has an excellent sense of smell. Due to this, dogs are used as detectives to trace criminals and detect bombs.
A stimulus is a kind of energy such as light, sound, smell, taste, heat, or mechanical pressure, etc. Receptors contain groups of cells which are sensitive to the energy provided by the stimulus.
At a receptor, the energy provided by a stimulus sets off a chemical reaction which converts the energy of stimulus into an electrical signal called ‘electrical impulse’ (nerve impulse or just impulse). So, all the receptors in the sense organs receive stimuli from the surrounding environment and send the message conveyed by them to the spinal cord and brain in the form of electrical impulses through the sensory nerves.
Another type of nerves called motor nerves transmits the response from the brain and spinal cord to the ‘effectors’, again in the form of electrical impulses. An effectors is a part of the body which can respond to a stimulus according to the instructions sent from the nervous system (spinal cord and brain). The effectors are mainly the muscles and glands of our body. All our muscles and glands respond to electrical impulses sent from the nervous system through motor nerves.
There are two systems of coordination of activities in humans. These are: (i) Nervous system, and (ii) Endocrine system (or Hormonal system).
In human beings, nervous system and endocrine system work together to control and coordinate all our activities such as our physical actions, our thinking processes and our emotional behaviour. Both the systems of coordination, nervous system and endocrine system, consist of a number of organs working together in a systematic way. We will now describe the nervous system and endocrine system in humans in detail, one by one. Let us discuss the nervous system first.