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What is Reflex Action? Reflex arc – How does the Nervous System help us Respond?
All the nerves of the body together make up the peripheral nervous system (PNS). They all enter or leave the central nervous system. The three types of nerves which make up the peripheral nervous system are spinal nerves, cranial nerves, and visceral nerves. Spinal nerves arise from the spinal cord along most of the length of the spinal cord and spread throughout the body (except the head). They all carry both sensory and motor neurons and are described as mixed nerves. Cranial nerves arise from the brain and spread throughout the head. They also carry both sensory and motor neurons. The visceral nerves are a special kind of nerve that mostly arise from the spinal cord (though some also arise from the brain). They are connected to the internal organs of the body. Visceral nerves also carry both sensory and motor neurons.
Reflex Action and Reflex Arcs
The simplest form of response in the nervous system is reflex action. This is a rapid, automatic response to a stimulus that is not under the voluntary control of the brain. It is described as an involuntary action. Thus, a reflex action is one that 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 hands away from 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 the 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 actions that we do without thinking to protect ourselves. For example, coughing is a reflex action that 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.
Yawning, blinking of eyes and sneezing are all reflex actions (which are performed by us unknowingly)
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 hands away. This sort of very fast, automatic response is called the reflex action. The figure shows the pathway taken by the nerve impulses in this reflex action. The stimulus here is the heat that we feel in our hands-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 the arm is an effector because it responds to the stimulus. This pathway along which the impulse travels is called the reflex arc.
The diagram shows the reflex action and its path (which is called reflex arc)
The reflexes of this type which involve only the spinal cord are called spinal reflexes. Though spinal reflexes are produced in the spinal cord the message of reflex action taken also goes on to reach the brain. Please note that when we lift a hot plate, then alongwith 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 flowchart given in Figure. Most of the reflex actions involve only the spinal cord. They are called spinal reflexes. The reflex action which we have shown in Figure is actually a spinal reflex. And the reflex arc given in the Figure 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.
A reflex arc (This is actually a spinal reflex arc)
Those reflex actions which involve the 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 the dim light, the pupil (a hole in the front of the eye) is large so that more light can enter the eye and make us see properly even in dim light (see Figure). 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 the eye from too much light) (see Figure). The contraction of the pupil of our eye automatically in the presence of bright light is an example of a cerebral reflex. This cerebral reflex action can be explained as follows:
When a bright light falls on the eye, the light receptors in the eye produce impulses in the sensory 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 the pupil becomes smaller, the amount of light entering the eye is reduced. All this happens very, very quickly.
In the dim light, the pupil of the eye is large so that more light can enter the eye.
In bright light, a speedy reflex action makes the pupil smaller. This reduces the light entering the eye.
A reflex arc showing cerebral reflex action
This cerebral reflex action can be shown by drawing a reflex arc given in Figure. Please note that though the pupil is a circular opening (or hole) in the centre of the iris of the eye it appears to be dark because no light is reflected from it (see Figures).
How the Effectors (or Muscles) Cause Action or Movement
When a motor nerve impulse sent by the spinal cord (or brain) reaches the effector organs (which are muscles), then, the muscles cause action or movement (such as lifting the hand away from a hot plate). We will now describe how muscles are able to move in response to electrical nerve impulses and cause action. Muscles are made up of muscle cells. Muscle cells contain special proteins which can change their arrangement when stimulated by electrical impulses, causing the muscle cells to change shape and contract.
When the muscle cells contract, the muscles also contract (and become shorter). When the muscles contract, they pull on the bones of the body part and make it move. For example, when electrical impulses sent by the spinal cord (or brain) stimulate the biceps muscle of the upper arm, they make the biceps muscle to contract. And when the biceps muscle contracts, it pulls on a bone of the lower arm and makes it move (lifting the hand away from the hot plate). Please note that the contraction of muscles (or muscle cells) caused by the action of electrical impulses is a reversible process.
The Autonomic Nervous System
The term ‘autos’ means ‘self’ and ‘nomos’ means ‘governing’, so ‘autonomic nervous system’ means ‘self-governing nervous system’. The autonomic nervous system is that part of the peripheral nervous system which controls the activities of the organs inside our body automatically even without our thinking about them. The autonomic nervous system is a specific network of nerves in the body that controls the processes like breathing, heartbeat, digestion, sweating, etc., that maintain our life and keep us alive. The nerves of the autonomic nervous system are attached to the smooth muscles of the various internal organs of the human body like the head, heart, blood vessels, alimentary canal, lungs, kidneys, urinary bladder, glands, skin, etc. Thus, the autonomic nervous system controls and regulates the functions of the internal organs of our body involuntarily (on its own).
Voluntary Nervous System
Those actions which need thinking and which are performed by us knowingly are called voluntary actions. For example, speaking to a friend, writing a letter, dancing, cycling, kicking a football, standing in a room, or sitting on a chair, are all voluntary actions. The voluntary nervous system helps us take voluntary actions which are under the conscious control of the brain. We will now give an example to understand the working of the voluntary nervous system.
Writing a letter, dancing, and kicking a football are all voluntary actions (which are performed by us knowingly).
Suppose we are walking down to school at a slow pace. After covering some distance, we look at our watch and find that we are getting late. So, we start walking very fast. We can do this because of our voluntary nervous system as follows:
- When our eyes see time on the watch, they send this information to the brain through the sensory nerves.
- The brain analyses this information and decides that since there is a risk of being late to school, so we should walk faster.
- The brain sends the instructions to walk faster to the muscles of our legs through the motor nerves.
- The muscles of the legs act accordingly and make us walk faster.
This is an example of voluntary action and the decision to take this voluntary action has been made by the voluntary nervous system.