NDA (II) 2018 General Ability Test · Question 86 of 146

Correct — B, receptor to sensory neuron to spinal cord to motor neuron to effector. A reflex arc is the shortest possible route from a stimulus to a response. The receptor — a pain ending in the skin, say — detects the stimulus and fires the sensory (afferent) neuron, which carries the impulse into the spinal cord through the dorsal root. Inside the grey matter of the cord a relay neuron passes the signal straight across to the motor (efferent) neuron, which leaves by the ventral root and reaches the effector, a muscle or a gland, which acts. The whole point of the arrangement is that the decision is taken in the spinal cord and not in the brain, which is why your hand leaves a hot plate before you consciously feel the heat.

• (a)Effector—sensory neuron—spinal cord—motor neuron—receptor — The two ends have been swapped. This has the muscle detecting the stimulus and the sense organ carrying out the response, and it also sends the sensory neuron away from the receptor. Information in a reflex arc always begins at the receptor and ends at the effector.
• (c)Receptor—sensory neuron—brain—motor neuron—effector — Correct order but the wrong processing centre, and this is the sharpest distractor in the set. Routing the signal through the brain describes a voluntary or a conscious response; a reflex is defined by bypassing the brain so that the response is faster and automatic. The brain is informed afterwards, which is why you feel the pain a moment after you have already pulled back.
• (d)Sensory neuron—receptor—brain—effector—motor neuron — Scrambled at both ends. It starts with the neuron rather than the receptor, uses the brain instead of the spinal cord, and puts the effector before the motor neuron that is supposed to reach it — so the muscle would have to act before receiving any instruction.

The nervous system handles urgent responses by delegating them. In a spinal reflex the sensory neuron enters the spinal cord through the dorsal root, synapses with a relay (interneuron) in the grey matter, and that relay synapses with a motor neuron leaving by the ventral root. Only three neurons and two synapses stand between the stimulus and the muscle, so the delay is a few hundredths of a second. A branch of the same sensory signal does travel up the cord to the brain, but it arrives after the movement has already begun — the brain registers what happened rather than ordering it.

Every wrong option here is built by breaking one link in a chain the student can rebuild from first principles: something must sense, something must carry in, something must decide, something must carry out, something must act. Fix the two ends first — a receptor senses and an effector acts — and options (a) and (d) collapse immediately. That leaves only the choice between spinal cord and brain, and the word 'reflex' itself settles it: a reflex action is defined as one that does not go through the brain. Familiar examples worth carrying are the knee-jerk, blinking when something approaches the eye, and the withdrawal of a hand from a hot object.

• A reflex arc runs receptor, sensory neuron, spinal cord, motor neuron, effector.
• The sensory neuron enters the spinal cord by the dorsal root and the motor neuron leaves by the ventral root.
• A relay or interneuron inside the grey matter of the cord connects the two, so no brain involvement is needed.
• Effectors are muscles or glands; receptors are sense organs or specialised nerve endings.

• Putting the brain in the loop. A response that goes through the brain is not a reflex.
• Swapping receptor and effector; the receptor detects, the effector acts.
• Assuming the brain never learns of the event. It does — but only after the response has been made.

NDA asks for the order of components in a reflex arc, for an example of a reflex action, or for which structure takes the decision in one.

No directly related past PYQ was found.

• practice — not a real PYQ

  In a reflex action, the decision to respond is taken by the

  • (a)cerebrum
  • (b)cerebellum
  • (c)spinal cord
  • (d)medulla oblongata

  Answer(c) spinal cord — the relay neuron there connects sensory to motor directly, which is what makes the response so fast.

• practice — not a real PYQ

  Which one of the following is an effector in a reflex arc?

  • (a)A pain receptor in the skin
  • (b)A sensory neuron
  • (c)A muscle or a gland
  • (d)The dorsal root of a spinal nerve

  Answer(c) A muscle or a gland — the effector is the structure that carries out the response.