anybody pls explain control and cordination chapter pls
Answers
7.1 ANIMALS – NERVOUS SYSTEM
In animals, such control and coordination are provided by nervous and
muscular tissues, which we have studied in Class IX. Touching a hot
object is an urgent and dangerous
situation for us. We need to detect it,
and respond to it. How do we detect that
we are touching a hot object? All
information from our environment is
detected by the specialised tips of some
nerve cells. These receptors are usually
located in our sense organs, such as the
inner ear, the nose, the tongue, and so
on. So gustatory receptors will detect taste
while olfactory receptors will detect smell.
This information, acquired at the
end of the dendritic tip of a nerve cell
[Fig. 7.1 (a)], sets off a chemical reaction
that creates an electrical impulse. This
impulse travels from the dendrite to the
cell body, and then along the axon to its
end. At the end of the axon, the electrical
impulse sets off the release of some
chemicals. These chemicals cross the
gap, or synapse, and start a similar
electrical impulse in a dendrite of the next
neuron. This is a general scheme of how
nervous impulses travel in the body. A
similar synapse finally allows delivery of such impulses from neurons to
other cells, such as muscles cells or gland [Fig. 7.1 (b)].
It is thus no surprise that nervous tissue is made up of an organised
network of nerve cells or neurons, and is specialised for conducting
information via electrical impulses from one part of the body to another.
Look at Fig. 7.1 (a) and identify the parts of a neuron (i) where
information is acquired, (ii) through which information travels as an
electrical impulse, and (iii) where this impulse must be converted into a
chemical signal for onward transmission.
7.1.1 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. 7.2). 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.
Half chapter explained.
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