Chapter 6 · Control and Coordination

NCERT Class 10 Science Chapter 6 Intext Questions (Page 105)

All 5 questions of NCERT Class 10 Science Chapter 6 Intext Questions (Page 105) — Control and Coordination — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   What ​is ​the ​difference ​between a ​reflex action and walking?
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   A reflex is sudden, involuntary and decided in the spinal cord; walking is voluntary and decided in the brain.

   A reflex action is a sudden, automatic and involuntary response to a stimulus. It is carried out through a reflex arc formed in the spinal cord — receptor, sensory neuron, relay neuron, motor neuron, effector — so the response is made in a fraction of a second, before the thinking part of the brain has been consulted; the brain is only informed afterwards. It is never learnt, it needs a stimulus, and it cannot be started or stopped at will, as when the hand is jerked away from a flame. Walking is a voluntary action. It begins with a decision taken in the motor area of the fore-brain, which sends the order down through the spinal cord to the motor neurons and the leg muscles, while the cerebellum keeps the posture and the balance. It is learnt in childhood, needs no stimulus, is much slower because the message has to travel up to the brain and back, and can be begun, changed or stopped whenever we wish. In short, a reflex is decided in the spinal cord and walking is decided in the brain.

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2. Q2
   What ​happens ​at ​the ​synapse ​between ​two neurons?
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   The impulse reaching the axon terminal releases chemicals that cross the gap and start a new electrical impulse in the next neuron.

   The synapse is the tiny gap between the axon terminal of one neuron and the dendrite of the next; the two neurons do not touch. When the electrical impulse travelling along the axon reaches the axon terminal, it sets off the release of some chemicals there. These chemicals cross the gap and start a similar electrical impulse in the dendrite of the next neuron. So at a synapse the message is changed from an electrical form to a chemical form and back to an electrical form. Because the chemicals are released only on the axon side and detected only on the dendrite side, the impulse can pass in one direction only. A similar synapse delivers the impulse from a neuron to other cells, such as a muscle cell or a gland.

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3. Q3
   Which ​part of ​the ​brain ​maintains posture and ​equilibrium ​of the ​body?
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   The cerebellum of the hind-brain maintains posture and equilibrium of the body.

   The cerebellum, which is a part of the hind-brain, maintains the posture and the equilibrium of the body. It is also responsible for the precision of voluntary actions, which is why activities such as walking in a straight line, riding a bicycle and picking up a pencil depend on it.

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4. Q4
   How do we ​detect the smell ​of ​an ​agarbatti ​(incense stick)?
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   The agarbatti's molecules dissolve in the nasal lining, olfactory receptors start an impulse, and it passes by neurons to the brain's smell area.

   The burning agarbatti gives off tiny chemical molecules which spread through the air and are carried into the nose when we breathe in, where they dissolve in the moist lining of the nasal cavity. There they are detected by the olfactory receptors, the specialised tips of certain nerve cells that lie in the nose and answer only to smell. The information acquired at the dendritic tip sets off a chemical reaction which creates an electrical impulse. This impulse travels from the dendrite to the cell body and along the axon to its end; there chemicals are released, cross the synapse and start a similar impulse in the next neuron, and so the message is carried to the brain. It reaches the area of the fore-brain that is specialised for smell, where it is interpreted along with information already stored in the brain, and we recognise it as the smell of an agarbatti.

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5. Q5
   What ​is ​the ​role ​of ​the ​brain ​in reflex action?
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   The spinal cord, not the brain, decides a reflex; the brain only receives the information later, so the pain is felt and remembered.

   In an ordinary reflex action the brain does not take the decision. The reflex arc is formed in the spinal cord, where a relay neuron connects the sensory neuron directly to the motor neuron, so the response is ordered by the spinal cord and made in a fraction of a second — long before the slow work of thinking could be finished. The brain's role is that of an informed observer and a later planner. The information input also goes on to reach the brain, so the brain comes to know what has happened, makes us feel the heat and the pain, and stores the event in memory so that we are more careful in future; it then decides the deliberate actions that follow, such as putting the hand under cold water or moving the pan away. The brain also acts as the relay centre for those reflexes in which the receptor and the effector both lie in the head — the narrowing of the pupil in bright light, blinking, sneezing — because the nerves concerned meet in the brain rather than in the spinal cord; even in these the thinking part of the brain takes no decision.

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