Biology Topics related to disease and health provide critical insights into human physiology and medicine.
Name the Two Systems of Control and Coordination in Animals
Multicellular animals (except sponges) have specialized cells called nerve cells (or neurons) to respond to stimuli and coordinate their activities. A system made up of nerve cells is called the nervous system. The coordination in simple multicellular animals takes place through the 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 (see Figure). The control and coordination in higher animals called vertebrates (including human beings) takes place through the nervous system as well as the hormonal system called the 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 (see Figure). 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 that is sensitive to a particular type of 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.
Nervous System of Hydra. (please note the network of nerve cells throughout the body of Hydra).
The eyes have light receptors (which can detect light), ears have sound receptors (which can detect sound), the nose has smell receptors (which can detect smell), the 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, photoreceptors, olfactory receptors, gustatory receptors, and thermoreceptors.
Photoreceptors detect light (they are present in the eyes), photoreceptors detect sound (they are present in the inner ears), olfactory receptors detect smell (they are present in the nose), gustatory receptors detect taste (they are present in the tongue) whereas thermoreceptors detect heat or cold (they are present in skin).
The eyes, ears, nose, tongue, and skin are sense organs. They contain different receptors which can detect different stimuli in the environment.
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, mechanical pressure, etc. Receptors contain groups of cells that are sensitive to the energy provided by the stimulus. At a receptor, the energy provided by a stimulus sets off a chemical reaction that converts the energy of the stimulus into an electrical signal called an ‘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 nerve called the motor nerve transmits the response from the brain and spinal cord to the ‘effectors’, again in the form of electrical impulses. An effector is a part of the body that 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.