NCERT Textbook: Control & Coordination | Science & Technology for UPSC CSE PDF Download

 Page 1


Science
100
Control and
Coordination
6 CHAPTER
I
n the previous chapter, we looked at life processes involved in the
maintenance functions in living organisms. There, we had started with
a notion we all have, that if we see something moving, it is alive. Some of
these movements are in fact the result of growth, as in plants. A seed
germinates and grows, and we can see that the seedling moves over the
course of a few days, it pushes soil aside and comes out. But if its growth
were to be stopped, these movements would not happen. Some
movements, as in many animals and some plants, are not connected
with growth. A cat running, children playing on swings, buffaloes
chewing cud – these are not movements caused by growth.
Why do we associate such visible movements with life? A possible
answer is that we think of movement as a response to a change in the
environment of the organism. The cat may be running because it has
seen a mouse.  Not only that, we also think of movement as an attempt
by living organisms to use changes in their environment to their
advantage. Plants grow out into the sunshine. Children try to get pleasure
and fun out of swinging. Buffaloes chew cud to help break up tough
food so as to be able to digest it better. When bright light is focussed on
our eyes or when we touch a hot object, we detect the change and respond
to it with movement in order to protect ourselves.
If we think a bit more about this, it becomes apparent that all this
movement, in response to the environment, is carefully controlled. Each
kind of a change in the environment evokes an appropriate movement
in response. When we want to talk to our friends in class, we whisper,
rather than shouting loudly. Clearly, the movement to be made depends
on the event that is triggering it. Therefore, such controlled movement
must be connected to the recognition of various events in the
environment, followed by only the correct movement in response. In other
words, living organisms must use systems providing control and
coordination. In keeping with the general principles of body organisation
in multicellular organisms, specialised tissues are used to provide these
control and coordination activities.
6.1 6.1 6.1 6.1 6.1 ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM 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
2024-25
Page 2


Science
100
Control and
Coordination
6 CHAPTER
I
n the previous chapter, we looked at life processes involved in the
maintenance functions in living organisms. There, we had started with
a notion we all have, that if we see something moving, it is alive. Some of
these movements are in fact the result of growth, as in plants. A seed
germinates and grows, and we can see that the seedling moves over the
course of a few days, it pushes soil aside and comes out. But if its growth
were to be stopped, these movements would not happen. Some
movements, as in many animals and some plants, are not connected
with growth. A cat running, children playing on swings, buffaloes
chewing cud – these are not movements caused by growth.
Why do we associate such visible movements with life? A possible
answer is that we think of movement as a response to a change in the
environment of the organism. The cat may be running because it has
seen a mouse.  Not only that, we also think of movement as an attempt
by living organisms to use changes in their environment to their
advantage. Plants grow out into the sunshine. Children try to get pleasure
and fun out of swinging. Buffaloes chew cud to help break up tough
food so as to be able to digest it better. When bright light is focussed on
our eyes or when we touch a hot object, we detect the change and respond
to it with movement in order to protect ourselves.
If we think a bit more about this, it becomes apparent that all this
movement, in response to the environment, is carefully controlled. Each
kind of a change in the environment evokes an appropriate movement
in response. When we want to talk to our friends in class, we whisper,
rather than shouting loudly. Clearly, the movement to be made depends
on the event that is triggering it. Therefore, such controlled movement
must be connected to the recognition of various events in the
environment, followed by only the correct movement in response. In other
words, living organisms must use systems providing control and
coordination. In keeping with the general principles of body organisation
in multicellular organisms, specialised tissues are used to provide these
control and coordination activities.
6.1 6.1 6.1 6.1 6.1 ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM 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
2024-25
Control and Coordination 101
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. 6.1 (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. 6.1 (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. 6.1 (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.
Activity 6.1 Activity 6.1 Activity 6.1 Activity 6.1 Activity 6.1
n Put some sugar in your mouth. How does it taste?
n Block your nose by pressing it between your thumb and index
finger. Now eat sugar again. Is there any difference in its taste?
n While eating lunch, block your nose in the same way and notice if
you can fully appreciate the taste of the food you are eating.
Is there a difference in how sugar and food taste if your nose is
blocked? If so, why might this be happening? Read and talk about
possible explanations for these kinds of differences. Do you come across
a similar situation when you have a cold?
Figure 6.1 Figure 6.1 Figure 6.1 Figure 6.1 Figure 6.1 (a) Structure of neuron, (b)     Neuromuscular
junction
(a)
(b)
2024-25
Page 3


Science
100
Control and
Coordination
6 CHAPTER
I
n the previous chapter, we looked at life processes involved in the
maintenance functions in living organisms. There, we had started with
a notion we all have, that if we see something moving, it is alive. Some of
these movements are in fact the result of growth, as in plants. A seed
germinates and grows, and we can see that the seedling moves over the
course of a few days, it pushes soil aside and comes out. But if its growth
were to be stopped, these movements would not happen. Some
movements, as in many animals and some plants, are not connected
with growth. A cat running, children playing on swings, buffaloes
chewing cud – these are not movements caused by growth.
Why do we associate such visible movements with life? A possible
answer is that we think of movement as a response to a change in the
environment of the organism. The cat may be running because it has
seen a mouse.  Not only that, we also think of movement as an attempt
by living organisms to use changes in their environment to their
advantage. Plants grow out into the sunshine. Children try to get pleasure
and fun out of swinging. Buffaloes chew cud to help break up tough
food so as to be able to digest it better. When bright light is focussed on
our eyes or when we touch a hot object, we detect the change and respond
to it with movement in order to protect ourselves.
If we think a bit more about this, it becomes apparent that all this
movement, in response to the environment, is carefully controlled. Each
kind of a change in the environment evokes an appropriate movement
in response. When we want to talk to our friends in class, we whisper,
rather than shouting loudly. Clearly, the movement to be made depends
on the event that is triggering it. Therefore, such controlled movement
must be connected to the recognition of various events in the
environment, followed by only the correct movement in response. In other
words, living organisms must use systems providing control and
coordination. In keeping with the general principles of body organisation
in multicellular organisms, specialised tissues are used to provide these
control and coordination activities.
6.1 6.1 6.1 6.1 6.1 ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM 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
2024-25
Control and Coordination 101
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. 6.1 (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. 6.1 (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. 6.1 (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.
Activity 6.1 Activity 6.1 Activity 6.1 Activity 6.1 Activity 6.1
n Put some sugar in your mouth. How does it taste?
n Block your nose by pressing it between your thumb and index
finger. Now eat sugar again. Is there any difference in its taste?
n While eating lunch, block your nose in the same way and notice if
you can fully appreciate the taste of the food you are eating.
Is there a difference in how sugar and food taste if your nose is
blocked? If so, why might this be happening? Read and talk about
possible explanations for these kinds of differences. Do you come across
a similar situation when you have a cold?
Figure 6.1 Figure 6.1 Figure 6.1 Figure 6.1 Figure 6.1 (a) Structure of neuron, (b)     Neuromuscular
junction
(a)
(b)
2024-25
Science
102
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.
2024-25
Page 4


Science
100
Control and
Coordination
6 CHAPTER
I
n the previous chapter, we looked at life processes involved in the
maintenance functions in living organisms. There, we had started with
a notion we all have, that if we see something moving, it is alive. Some of
these movements are in fact the result of growth, as in plants. A seed
germinates and grows, and we can see that the seedling moves over the
course of a few days, it pushes soil aside and comes out. But if its growth
were to be stopped, these movements would not happen. Some
movements, as in many animals and some plants, are not connected
with growth. A cat running, children playing on swings, buffaloes
chewing cud – these are not movements caused by growth.
Why do we associate such visible movements with life? A possible
answer is that we think of movement as a response to a change in the
environment of the organism. The cat may be running because it has
seen a mouse.  Not only that, we also think of movement as an attempt
by living organisms to use changes in their environment to their
advantage. Plants grow out into the sunshine. Children try to get pleasure
and fun out of swinging. Buffaloes chew cud to help break up tough
food so as to be able to digest it better. When bright light is focussed on
our eyes or when we touch a hot object, we detect the change and respond
to it with movement in order to protect ourselves.
If we think a bit more about this, it becomes apparent that all this
movement, in response to the environment, is carefully controlled. Each
kind of a change in the environment evokes an appropriate movement
in response. When we want to talk to our friends in class, we whisper,
rather than shouting loudly. Clearly, the movement to be made depends
on the event that is triggering it. Therefore, such controlled movement
must be connected to the recognition of various events in the
environment, followed by only the correct movement in response. In other
words, living organisms must use systems providing control and
coordination. In keeping with the general principles of body organisation
in multicellular organisms, specialised tissues are used to provide these
control and coordination activities.
6.1 6.1 6.1 6.1 6.1 ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM 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
2024-25
Control and Coordination 101
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. 6.1 (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. 6.1 (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. 6.1 (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.
Activity 6.1 Activity 6.1 Activity 6.1 Activity 6.1 Activity 6.1
n Put some sugar in your mouth. How does it taste?
n Block your nose by pressing it between your thumb and index
finger. Now eat sugar again. Is there any difference in its taste?
n While eating lunch, block your nose in the same way and notice if
you can fully appreciate the taste of the food you are eating.
Is there a difference in how sugar and food taste if your nose is
blocked? If so, why might this be happening? Read and talk about
possible explanations for these kinds of differences. Do you come across
a similar situation when you have a cold?
Figure 6.1 Figure 6.1 Figure 6.1 Figure 6.1 Figure 6.1 (a) Structure of neuron, (b)     Neuromuscular
junction
(a)
(b)
2024-25
Science
102
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.
2024-25
Control and Coordination 103
Can you now trace the sequence of events which occur when a bright
light is focussed on your eyes?
6.1.2 Human Brain
Is reflex action the only function of the spinal cord? Obviously not, since
we know that we are thinking beings. Spinal cord is made up of nerves
which supply information to think about. Thinking involves more
complex mechanisms and neural connections. These are concentrated
in the brain, which is the main coordinating centre of the body.  The
brain and spinal cord constitute the central nervous system (Fig. 6.3).
They receive information from all parts of the body and integrate it.
We also think about our actions. Writing, talking, moving a chair,
clapping at the end of a programme are examples of voluntary actions
which are based on deciding what to do next.  So, the brain also has to
send messages to muscles.  This is the second way in which the nervous
system communicates with the muscles.  The communication between
the central nervous system and the other parts of the body is facilitated
by the peripheral nervous system consisting of cranial nerves arising
from the brain and spinal nerves arising from the spinal cord. The brain
thus allows us to think and take actions based on that thinking.  As you
will expect, this is accomplished through a complex design, with different
parts of the brain responsible for integrating different inputs and outputs.
The brain has three such major parts or regions, namely the fore-brain,
mid-brain and hind-brain.
The fore-brain is the main thinking part of the brain. It has regions
which receive sensory impulses from various receptors. Separate areas
of the fore-brain are specialised for hearing, smell, sight and so on.  There
are separate areas of association where this sensory information is
interpreted by putting it together with information from other receptors
as well as with information that is already stored in the brain.  Based on
Figure 6.2 Figure 6.2 Figure 6.2 Figure 6.2 Figure 6.2 Reflex arc
2024-25
Page 5


Science
100
Control and
Coordination
6 CHAPTER
I
n the previous chapter, we looked at life processes involved in the
maintenance functions in living organisms. There, we had started with
a notion we all have, that if we see something moving, it is alive. Some of
these movements are in fact the result of growth, as in plants. A seed
germinates and grows, and we can see that the seedling moves over the
course of a few days, it pushes soil aside and comes out. But if its growth
were to be stopped, these movements would not happen. Some
movements, as in many animals and some plants, are not connected
with growth. A cat running, children playing on swings, buffaloes
chewing cud – these are not movements caused by growth.
Why do we associate such visible movements with life? A possible
answer is that we think of movement as a response to a change in the
environment of the organism. The cat may be running because it has
seen a mouse.  Not only that, we also think of movement as an attempt
by living organisms to use changes in their environment to their
advantage. Plants grow out into the sunshine. Children try to get pleasure
and fun out of swinging. Buffaloes chew cud to help break up tough
food so as to be able to digest it better. When bright light is focussed on
our eyes or when we touch a hot object, we detect the change and respond
to it with movement in order to protect ourselves.
If we think a bit more about this, it becomes apparent that all this
movement, in response to the environment, is carefully controlled. Each
kind of a change in the environment evokes an appropriate movement
in response. When we want to talk to our friends in class, we whisper,
rather than shouting loudly. Clearly, the movement to be made depends
on the event that is triggering it. Therefore, such controlled movement
must be connected to the recognition of various events in the
environment, followed by only the correct movement in response. In other
words, living organisms must use systems providing control and
coordination. In keeping with the general principles of body organisation
in multicellular organisms, specialised tissues are used to provide these
control and coordination activities.
6.1 6.1 6.1 6.1 6.1 ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM ANIMALS – NERVOUS SYSTEM 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
2024-25
Control and Coordination 101
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. 6.1 (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. 6.1 (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. 6.1 (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.
Activity 6.1 Activity 6.1 Activity 6.1 Activity 6.1 Activity 6.1
n Put some sugar in your mouth. How does it taste?
n Block your nose by pressing it between your thumb and index
finger. Now eat sugar again. Is there any difference in its taste?
n While eating lunch, block your nose in the same way and notice if
you can fully appreciate the taste of the food you are eating.
Is there a difference in how sugar and food taste if your nose is
blocked? If so, why might this be happening? Read and talk about
possible explanations for these kinds of differences. Do you come across
a similar situation when you have a cold?
Figure 6.1 Figure 6.1 Figure 6.1 Figure 6.1 Figure 6.1 (a) Structure of neuron, (b)     Neuromuscular
junction
(a)
(b)
2024-25
Science
102
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.
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Can you now trace the sequence of events which occur when a bright
light is focussed on your eyes?
6.1.2 Human Brain
Is reflex action the only function of the spinal cord? Obviously not, since
we know that we are thinking beings. Spinal cord is made up of nerves
which supply information to think about. Thinking involves more
complex mechanisms and neural connections. These are concentrated
in the brain, which is the main coordinating centre of the body.  The
brain and spinal cord constitute the central nervous system (Fig. 6.3).
They receive information from all parts of the body and integrate it.
We also think about our actions. Writing, talking, moving a chair,
clapping at the end of a programme are examples of voluntary actions
which are based on deciding what to do next.  So, the brain also has to
send messages to muscles.  This is the second way in which the nervous
system communicates with the muscles.  The communication between
the central nervous system and the other parts of the body is facilitated
by the peripheral nervous system consisting of cranial nerves arising
from the brain and spinal nerves arising from the spinal cord. The brain
thus allows us to think and take actions based on that thinking.  As you
will expect, this is accomplished through a complex design, with different
parts of the brain responsible for integrating different inputs and outputs.
The brain has three such major parts or regions, namely the fore-brain,
mid-brain and hind-brain.
The fore-brain is the main thinking part of the brain. It has regions
which receive sensory impulses from various receptors. Separate areas
of the fore-brain are specialised for hearing, smell, sight and so on.  There
are separate areas of association where this sensory information is
interpreted by putting it together with information from other receptors
as well as with information that is already stored in the brain.  Based on
Figure 6.2 Figure 6.2 Figure 6.2 Figure 6.2 Figure 6.2 Reflex arc
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all this, a decision is made about how to respond and the information is
passed on to the motor areas which control the movement of voluntary
muscles, for example, our leg muscles.  However, certain sensations are
distinct from seeing or hearing, for example, how do we know that we
have eaten enough? The sensation of feeling full is because of a centre
associated with  hunger, which is in a separate part of the fore-brain.
Study the labelled diagram of the human brain. We have seen that
the different parts have specific functions. Can we find out the function
of each part?
Let us look at the other use of the word ‘reflex’ that we have talked
about in the introduction. Our mouth waters when we see food we like
without our meaning to. Our hearts beat without our thinking about it.
In fact, we cannot control these actions easily by thinking about them
even if we wanted to. Do we have to think about or remember to breathe
or digest food? So, in between the simple reflex actions like change in
the size of the pupil, and the thought out actions such as moving a
chair, there is another set of muscle movements over which we do not
have any thinking control. Many of these involuntary actions are
controlled by the mid-brain and hind-brain. All these involuntary actions
including blood pressure, salivation and vomiting are controlled by the
medulla in the hind-brain.
Think about activities like walking in a straight line, riding a bicycle,
picking up a pencil. These are possible due to a part of the hind-brain
called the cerebellum. It is responsible for precision of voluntary actions
and maintaining the posture and balance of the body. Imagine what
would happen if each of these events failed to take place if we were not
thinking about it.
Figure 6.3 Figure 6.3 Figure 6.3 Figure 6.3 Figure 6.3 Human brain
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