Control and CoordinationAnimals – Nervous System
In animals, control and coordination are the work of nervous tissue and muscular tissue, which you met in Class IX. Touching something hot is urgent and dangerous: we have to detect it and respond at once. But how do we detect it in the first place?
Every piece of information about our surroundings is picked up by the specialised tips of some nerve cells. These tips are called receptors, and most of them sit in our sense organs: the inner ear, the nose, the tongue, the skin and so on. Different receptors pick up different things. Gustatory receptors on the tongue detect taste, and olfactory receptors in the nose detect smell.
Activity 6.1. Riya tries an experiment at the lunch table. First she tastes a spoon of sugar. Then she pinches her nose shut between her thumb and first finger and tastes the sugar again. Then she eats her lunch with her nose pinched. Complete all three tasting missions.
Sugar stays sweet, because sweetness is picked up by the tongue alone. But lunch turns flat: much of what we call the "taste" of food is really its smell, picked up by the olfactory receptors. So when you have a cold and your nose is blocked, food tastes
The Nerve Cell
A receptor is only the beginning. The message it picks up has to travel, sometimes more than a metre, to the place where a decision is made. It travels along nerve cells, or neurons.
At the tip of a dendrite, the information sets off a chemical reaction that creates an electrical impulse. The impulse travels from the dendrite to the cell body, and then along the long axon to its end. At the nerve ending, the electrical impulse makes the cell release some chemicals, which pass the message on.
This is the textbook's Figure 6.1 (a), the structure of a neuron. Tap a label, then its pin on the diagram.
Now use your labelled neuron. Information is picked up at the
So nervous tissue is an organised network of nerve cells, specialised for carrying information as electrical impulses from one part of the body to another.
Across the Gap
Neurons are not joined end to end like one long wire. Between the nerve ending of one neuron and a dendrite of the next there is a tiny gap, called a synapse. The electrical impulse cannot jump it. Instead, the chemicals released at the nerve ending cross the gap and start a fresh electrical impulse in a dendrite of the next neuron.
A similar synapse passes the message from the last neuron to the cells that act on it, such as muscle cells or a gland. Where a nerve meets a muscle fibre, the meeting point is called a neuromuscular junction (the textbook's Figure 6.1 (b)).
Send a message. Watch it run along neuron 1, cross the synapse as chemicals, run along neuron 2 and reach the muscle fibre.
So a nerve message is electrical inside a neuron and chemical across every gap. The gap between two neurons is called a
What happens in Reflex Actions?
We use the word reflex for a sudden action in response to something around us. "I jumped out of the way of the bus without thinking." "I pulled my hand back from the flame." "I was so hungry, my mouth started watering." In each case we act without thinking, without even feeling in control of what we do. Yet these are still responses to changes in the environment. So how are they controlled?
Take one example. Aman reaches for a roti and his fingertips touch the hot tawa. Touching a flame or a hot pan is urgent and dangerous for any animal. One way to respond would be to think it through: "This is hot, it hurts, I could get burnt, so I should move my hand." How long would that take?
Thinking is a complex activity. If messages travel as nerve impulses, then thinking must involve a complicated interplay of many impulses from many neurons. That is why the thinking tissue of our body is a dense network of intricately arranged neurons. It sits at the front end of the skull and receives signals from all over the body. For the thinking part of the brain to move the hand, the message would have to travel up to it, be thought about, and travel all the way back down to the arm. If all of that happened while we touched a hot object, we might get burnt.
The Shortcut: a Reflex Arc
How does the design of the body solve this problem? Suppose the nerves that detect heat were connected to the nerves that move muscles in a simpler way, without waiting for the brain to think. Then the input could be detected and answered with an output action very quickly. Such a connection is called a reflex arc.
Where should the input nerve meet the output nerve? The best place is wherever they first meet. Nerves from all over the body meet in a bundle in the spinal cord on their way to the brain, so reflex arcs are formed right there, in the spinal cord.
Put the five stations of Aman's reflex arc in order, from the hot tawa to the moving hand. Then watch the textbook's Figure 6.2 come alive.
Follow the arc: the receptor in the skin, the sensory neuron, the relay neuron in the spinal cord, the motor neuron and the effector muscle. The information also travels on up to the brain, which is why you feel the pain a moment later, when your hand is already safe.
Reflex arcs have evolved in animals because the thinking process of the brain is not fast enough. In fact many animals have very little or none of the complex network of neurons needed for thinking, so reflex arcs probably evolved as an efficient way of working without true thought. Even in animals with a thinking brain, the reflex arc is still the faster route for quick responses.
Your Turn: Bright Light in the Eyes
You walk out of a dark cinema hall into the bright afternoon sun. Without deciding anything, your pupils shrink. Trace this reflex the same way.
The receptors are the light-sensitive cells of the
The same plan in a different place: this arc runs through the brain instead of the spinal cord, because the eyes are connected straight to the brain. It is still automatic, though. You cannot stop your pupils from shrinking just by deciding to.
Human Brain
Is making reflex arcs the only job of the spinal cord? Clearly not, because we are thinking beings. The spinal cord is made of nerves that carry information up to be thought about. Thinking needs far more complex connections, and these are concentrated in the brain, the main coordinating centre of the body. The brain and the spinal cord together make up the central nervous system (CNS). They receive information from every part of the body and integrate it.
We also think about our actions. Writing, talking, moving a chair, clapping at the end of a programme: these are voluntary actions, based on deciding what to do next. So the brain must also send messages out to the muscles. This is the second way in which the nervous system talks to the muscles, after the reflex arc.
The CNS is linked to the rest of the body by the peripheral nervous system. It is made of cranial nerves, which arise from the
So the brain lets us think and then act on that thinking. As you might expect, it does this with a complex design, in which different parts integrate different inputs and outputs. The brain has three major regions: the fore-brain, the mid-brain and the hind-brain.
Map of the Brain
The brain sits inside the skull. Seen from the side and cut down the middle, it looks like the textbook's Figure 6.3. Label its parts: tap a label, then its pin on the diagram.
The big, folded cerebrum is the main part of the
Who Does What?
The fore-brain is the main thinking part of the brain. It has regions that receive sensory impulses from the receptors, with separate areas for hearing, smell, sight and so on. Separate areas of association make sense of this information by putting it together with information from other receptors and with what is already stored in the brain. Based on all this, a decision is made about how to respond, and the information is passed to the motor areas, which control the voluntary muscles, for example our leg muscles.
Some sensations are different from seeing or hearing. How do you know that you have eaten enough? The feeling of being full comes from a centre associated with hunger, in a separate part of the fore-brain.
Now think about the other use of the word "reflex" from the start of this section. Your mouth waters when you see food you like, without your meaning it to. Your heart beats without your thinking about it. You do not have to remember to breathe or to digest your lunch, and you could not easily control these actions even if you wanted to. Between simple reflexes, like the change in the size of the pupil, and thought-out actions, like moving a chair, lies this third group: involuntary actions. Many of them are controlled by the mid-brain and the hind-brain. All of these involuntary actions, including blood pressure, salivation and vomiting, are controlled by the medulla in the hind-brain.
Finally, think about walking in a straight line, riding a bicycle or picking up a pencil. These are possible because of the cerebellum, the part of the hind-brain responsible for the precision of voluntary actions and for keeping the posture and balance of the body. Imagine having to manage all of this while thinking about every tiny movement.
Join each part to its job.
So the responses of the nervous system come in three kinds: reflex actions (sudden and automatic, often through the spinal cord), voluntary actions (decided by the fore-brain), and involuntary actions (run automatically by the mid-brain and the hind-brain).
How are these Tissues protected?
An organ as delicate and as important as the brain needs careful protection. The body is designed so that the brain sits inside a bony box, the cranium or skull. Inside the box, the brain is held in a fluid-filled balloon, which acts as a shock absorber. Run your hand down the middle of your back and you will feel a hard, bumpy column: the vertebral column, or backbone, which protects the spinal cord.
When you land from a jump, your brain does not bang against the skull because it is cushioned by
Hard bone outside, soft cushioning inside: the same idea as a cricket helmet with padding inside its shell.
How does the Nervous Tissue cause Action?
So far the nervous tissue has collected information, carried it around the body, made decisions and sent the decisions out to the muscles. The final job, the movement itself, is done by muscle tissue. When a nerve impulse reaches a muscle, the muscle fibres must move. But how does a single cell move?
The simplest way is for the cell to change its shape and become shorter. Muscle cells contain special proteins that change both their shape and their arrangement in the cell in response to nerve impulses. The new arrangement gives the cell a shorter form, and when thousands of cells shorten together, the whole muscle pulls.
Send an impulse to the biceps and watch both the arm and the inside of one of its cells. Then lower the arm again.
In Class IX you met different kinds of muscle: voluntary muscles, like the biceps, and involuntary muscles, like those of the stomach and the heart. Using what you now know, the difference is in what controls them. Voluntary muscles move only when
Speed Round: Voluntary, Involuntary or Reflex?
Decide who is in charge. Thought out and chosen: voluntary. Automatic and ongoing, run by the mid-brain and hind-brain: involuntary. Sudden and protective: reflex.
Get five right to move on.