Reflex arcs are evolved in animals for quick and efficient responses. Which one of the following sequences correctly represents a reflex arc?
- (a)Receptor—Sensory neuron—Relay neuron in spinal cord—Brain—Motor neuron—Effector
- (b)Receptor—Sensory neuron—Brain—Relay neuron in spinal cord—Motor neuron—Effector
- (c)Receptor—Motor neuron—Relay neuron in spinal cord—Sensory neuron—Effector
- (d)Receptor—Motor neuron—Brain—Sensory neuron—Effector
Correct — A, Receptor—Sensory neuron—Relay neuron in spinal cord—Brain—Motor neuron—Effector. Read the sequence for direction of travel and the answer separates itself from the field. A reflex begins at a receptor — the pain endings in a fingertip that touches something hot. The signal travels inward along a sensory neuron to the spinal cord, where a relay neuron passes it straight across to a motor neuron, and the motor neuron carries the command outward to an effector, the muscle that pulls the hand away. Direction is everything: sensory neurons carry signals in, motor neurons carry them out, and any sequence that puts a motor neuron before the spinal cord has the wiring backwards. NCERT's Class 10 text puts the design reason plainly — '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.' That last clause is what the mention of the brain in this option reflects. The response itself does not wait for the brain, which is exactly why the arc is fast; the same input travels up so that you feel the burn a moment after your hand has already moved.
- (b)Receptor—Sensory neuron—Brain—Relay neuron in spinal cord—Motor neuron—Effector — The order of travel is right but the brain is placed before the spinal relay, which defeats the purpose of a reflex. Routing every withdrawal through the brain first is the slow path the arc exists to avoid.
- (c)Receptor—Motor neuron—Relay neuron in spinal cord—Sensory neuron—Effector — The two neurons have swapped jobs. A motor neuron cannot carry a signal from a receptor, and a sensory neuron cannot drive a muscle; the arc would run outward first and inward afterwards.
- (d)Receptor—Motor neuron—Brain—Sensory neuron—Effector — Reversed in the same way as the option above and missing the spinal relay altogether, so it describes neither a reflex nor any working pathway.
A reflex action is an automatic, rapid response to a stimulus that does not require a decision. The pathway it runs on is the reflex arc: receptor, sensory or afferent neuron, a relay neuron within the spinal cord, motor or efferent neuron, and effector. Because the connection is made in the spinal cord rather than in the brain, the response takes a fraction of the time a considered movement would, and the same sensory input also travels up to the brain, where it registers as sensation.
The four options here differ only in the order of five or six named parts, so the reliable method is to test each sequence against two rules — a receptor must come first, and sensory must precede motor. Two options fail the second rule outright. Of the two that survive, one puts the brain ahead of the spinal relay, which is the slow route. One honest observation about the keyed option: strictly, the motor command in a spinal reflex leaves from the cord and does not pass through the brain, so listing the brain in the chain reads oddly. It is best understood as the paper's way of recording that the information also reaches the brain, and it remains the only option with the neurons in the right order, which is what the item is testing.
- The reflex arc runs receptor, sensory neuron, relay neuron in the spinal cord, motor neuron, effector.
- NCERT Class 10 Science states that reflex arcs are formed in the spinal cord itself, although the information input also goes on to reach the brain.
- Sensory or afferent neurons carry impulses towards the central nervous system; motor or efferent neurons carry them away from it.
- The gap between two neurons across which the impulse is passed chemically is the synapse.
- Reflexes evolved because the deliberative processing of the brain is too slow for a protective response, and many animals have little of that processing machinery at all.
- Swapping sensory and motor neurons; the direction of travel is what most wrong options get wrong.
- Believing a reflex is processed by the brain — it is completed in the spinal cord, and the brain is informed afterwards.
- Confusing the reflex arc with the reflex action; the arc is the pathway, the action is the response.
As a sequence item on the arc, as a which-part item on the site where the reflex is completed, or as a definition item on the synapse.
Which one of the following depicts the correct circuit of a reflex arc?
- (a) Effector—sensory neuron—spinal cord—motor neuron—receptor
- (b) Receptor—sensory neuron—spinal cord—motor neuron—effector
- (c) Receptor—sensory neuron—brain—motor neuron—effector
- (d) Sensory neuron—receptor—brain—effector—motor neuron
Answer(b) Receptor—sensory neuron—spinal cord—motor neuron—effector
The same question with the same distractor design, five years earlier in the sibling examination. Both papers build the wrong options by reversing the sensory and motor legs or by inserting the brain, and both reward a candidate who checks direction of travel first.
- practice — not a real PYQ
The junction across which an impulse passes from one neuron to the next is called which one of the following?
- (a)Dendrite
- (b)Synapse
- (c)Axon
- (d)Myelin sheath
Answer(b) Synapse — the gap between two neurons, where the electrical impulse is carried across by chemicals released from the axon ending.
- practice — not a real PYQ
In a reflex action such as pulling the hand away from a hot object, the response is completed at which one of the following sites?
- (a)Cerebrum
- (b)Cerebellum
- (c)Spinal cord
- (d)Medulla oblongata
Answer(c) Spinal cord — the relay is made there, which is why the response is faster than any movement that waits on the brain.