The simplest form of response in the nervous system is reflex action. This is a rapid, automatic response to a stimulus which is not under the voluntary control of the brain. It is described as an involuntary action. Thus, a reflex action is one which we perform automatically. It is a comparatively simple form of behaviour in which the same stimulus produces the same response every time.

If we unknowingly touch a hot plate, we immediately move our hand away from it. So, moving our hand away on touching a hot plate is an example of reflex action. Similarly, moving our foot away when we step on something sharp is also an example of reflex action. A knee jerk, movement of diaphragm (during respiration), coughing, yawning, blinking of eyes and sneezing are all reflex actions.

In a reflex action, we are unaware that anything is going to happen to us. Reflex actions are the actions which we do without thinking to protect ourselves. For example, coughing is a reflex action which clears our windpipe.

The pupils of our eyes get smaller in bright light. This reflex action protects the retina of our eyes from damage due to too much light. The pupils of our eyes get bigger in dim light so as to help us see properly even in dim light.

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The pathway (or route) taken by nerve impulses in a reflex action is called the reflex arc. Reflex arcs allow rapid response. We will explain the meaning of a reflex arc by taking an example.

A reflex action is an automatic response to a stimulus. An example of the way in which we respond to a stimulus is our reaction to touching a hot object (like a hot plate). Very quickly, and without thinking about it, we pull our hand away. This sort of very fast, automatic response is called the reflex action. It shows the pathway taken by the nerve impulses in this reflex action.

The stimulus here is the heat which we feel in our hand on touching the hot plate. This heat is sensed by a heat receptor (or thermoreceptor) in our hand. The receptor triggers an impulse in a sensory neuron, which transmits the message to the spinal cord.

Here, the impulse is passed on to a relay neuron, which in turn, passes it to a motor neuron. The motor neuron passes the impulse to a muscle in our arm. The muscle then contracts and pulls our hand away from the hot plate. The muscle of arm is an effectors because it responds to the stimulus. This pathway along which the impulse travels is called the reflex arc.

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The reflexes of this type which involve only the spinal cord are called spinal reflexes. Though spinal reflexes are produced in the spinal cord but the message of reflex action taken also goes on to reach the brain. Please note that when we lift a hot plate, then along with heat, the pain produced by heat also acts as a ‘stimulus’.

The reflex arc described in the above example can be shown in the form of a flow-chart given. Most of the reflex actions involve only the spinal cord. They are called spinal reflexes. The reflex action which we have shown in Figure 30 is actually a spinal reflex. And the reflex arc is actually a spinal reflex arc. Some reflex actions, however, involve the brain rather than the spinal cord. Such reflex actions are known as cerebral reflexes. This is described below.

Those reflex actions which involve brain are called cerebral reflexes. Cerebral reflexes occur in the organs present in the head because these organs are directly connected to the brain. This will become clear from the following example. Our eyes are present in the head. In dim light, the pupil (a hole in the front of eye) is large so that more light can enter into the eye and make us see properly even in dim light.

Now, when a bright light shines into our eye, then the pupil of our eye automatically becomes smaller (and prevents the damage to the retina of eye from too much light). The contraction of pupil of our eye automatically in the presence of bright light is an example of cerebral reflex. This cerebral reflex action can be explained as follows:

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When a bright light falls on the eye, the light receptors in the eye produce impulses in the sensor nerves. The sensory nerves carry this message of bright light in the form of electrical impulses to the brain. The brain produces the response (that the amount of light entering the eye must be reduced).

The response produced by the brain is carried by motor nerves to the circular muscles of the iris of the eye. The circular muscles of the iris of the eye contract and reduce the size of the pupil (or hole) of the eye. As the size of pupil becomes smaller, the amount of light entering the eye is reduced. All this happens very, very quickly.

This cerebral reflex action can be shown by drawing a reflex arc given in Figure 34. Please note that though the pupil is a circular opening (or hole) in the centre of the iris of the eye but it appears to be dark because no light is reflected from it.