Page 107 - NCERT Science Class 10 English Medium

6.1.1 What happens in Reflex Actions?

‘Reflex’ is a word we use very commonly when we talk about some sudden
action in response to something in the environment. We say ‘I jumped
out of the way of the bus reflexly’, or ‘I pulled my hand back from the
flame reflexly’, or ‘I was so hungry my mouth started watering reflexly’.
What exactly do we mean? A common idea in all such examples is that
we do something without thinking about it, or without feeling in control
of our reactions. Yet these are situations where we are responding with
some action to changes in our environment. How is control and
coordination achieved in such situations?
Let us consider this further. Take one of our examples. Touching a
flame is an urgent and dangerous situation for us, or in fact, for any
animal! How would we respond to this? One seemingly simple way is to
think consciously about the pain and the possibility of getting burnt,
and therefore move our hand. An important question then is, how long
will it take us to think all this? The answer depends on how we think. If
nerve impulses are sent around the way we have talked about earlier,
then thinking is also likely to involve the creation of such impulses.
Thinking is a complex activity, so it is bound to involve a complicated
interaction of many nerve impulses from many neurons.
If this is the case, it is no surprise that the thinking tissue in our
body consists of dense networks of intricately arranged neurons. It sits
in the forward end of the skull, and receives signals from all over the
body which it thinks about before responding to them. Obviously, in
order to receive these signals, this thinking part of the brain in the skull
must be connected to nerves coming from various parts of the body.
Similarly, if this part of the brain is to instruct muscles to move, nerves
must carry this signal back to different parts of the body. If all of this is
to be done when we touch a hot object, it may take enough time for us to
get burnt!
How does the design of the body solve this problem? Rather than
having to think about the sensation of heat, if the nerves that detect heat
were to be connected to the nerves that move muscles in a simpler way,
the process of detecting the signal or the input and responding to it by
an output action might be completed quickly. Such a connection is
commonly called a reflex arc (Fig. 6.2). Where should such reflex arc
connections be made between the input nerve and the output nerve?
The best place, of course, would be at the point where they first meet
each other. Nerves from all over the body meet in a bundle in the spinal
cord on their way to the brain. Reflex arcs are formed in this spinal cord
itself, although the information input also goes on to reach the brain.
Of course, 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 neuron network needed for thinking. So it is
quite likely that reflex arcs have evolved as efficient ways of functioning
in the absence of true thought processes. However, even after complex
neuron networks have come into existence, reflex arcs continue to be
more efficient for quick responses.

102 Science

2024-25