Input and output nerves meet at

Question

UnlockIAS · Accepted Answer

Correct — B, central nervous system. Translate the stem's everyday words into the textbook ones and the answer is almost a definition. The 'input' nerves are the sensory or afferent fibres, which carry information inward from receptors — in the skin, eye, ear, tongue, nose and the stretch receptors of muscle — towards the centre. The 'output' nerves are the motor or efferent fibres, which carry instructions outward from the centre to the effectors, meaning skeletal muscles and glands. The place where the incoming stream is read and the outgoing stream is issued is by definition the integrating centre, and in a vertebrate that is the central nervous system: the brain and the spinal cord together. The anatomy is unusually literal about this. Afferent and efferent fibres travel side by side in a mixed spinal nerve out in the limb, but they part company the moment they reach the cord — sensory fibres enter through the dorsal, or posterior, root, motor fibres leave through the ventral, or anterior, root, a separation known as the Bell–Magendie law. The two therefore physically converge inside the grey matter of the spinal cord and nowhere in the periphery. The clearest working example is the reflex arc that NCERT uses: touch something hot and the chain runs receptor → sensory neuron → dorsal root → relay neuron in the grey matter of the cord → motor neuron → ventral root → muscle of the arm. The hand is pulled back before the brain has registered pain, because the connection is made in the cord and only afterwards does the information travel up to the brain. That short circuit is the whole reason reflexes are fast. And the junction itself is not a fusion but a synapse — a gap of a few tens of nanometres across which the electrical impulse is converted into a chemical signal by a neurotransmitter such as acetylcholine, then converted back. Because transmitter is released on one side and detected on the other, the synapse acts as a one-way valve and fixes the direction of travel through the whole arc.

Why the other options are wrong — (a) The largest gland in the human body and its principal chemical factory — it secretes bile, stores glucose as glycogen, deaminates surplus amino acids and builds urea, detoxifies drugs and alcohol, and makes most of the plasma proteins. None of that is integration of nervous signals. The liver does have a nerve supply, autonomic fibres reaching it through the hepatic plexus, but that traffic is output only: the nervous system instructs the liver, and no sensory pathway terminates there to be handed on to a motor one. It is an effector organ, and this option is on the list mainly because it is the organ candidates associate with 'does many things at once'.  (c) The genuinely tempting option, because more is true of the heart than of the liver here. The heart does receive both arms of the autonomic nervous system — sympathetic fibres that raise rate and force, and the parasympathetic vagus that slows it — so two nerve supplies really do arrive there. It also has its own conducting system: the sino-atrial node, the atrioventricular node, the bundle of His and the Purkinje fibres, which make the heart myogenic, able to beat with every nerve to it cut. But that conducting tissue is specialised cardiac muscle, not nerve, and the autonomic supply is purely efferent — it carries orders in, not information out. Stretch and pressure receptors in the heart and the great vessels send their signals away to the cardiovascular centre in the medulla oblongata, and the corrective instruction is issued from there. The loop closes in the brain stem, not in the chest.  (d) Not available, because one of the three preceding options is exactly right. A candidate may be tempted here by a half-remembered fact — that some reflexes never reach the brain — and conclude that no single named structure can be the meeting point. That reasoning defeats itself: the spinal cord where those reflexes are completed is part of the central nervous system, so 'CNS' covers the reflex case and the deliberate case alike, which is precisely why it is the safe answer rather than 'brain'.

Concept: The human nervous system is built in two halves that divide the labour of control. The central nervous system — the brain and the spinal cord — receives, sorts and interprets information and decides what to do about it. The peripheral nervous system is the wiring that connects it to everything else: twelve pairs of cranial nerves leaving the brain and thirty-one pairs of spinal nerves leaving the cord, each carrying afferent fibres in and efferent fibres out. The efferent half of the peripheral system splits again into the somatic division, which drives skeletal muscle under voluntary control, and the autonomic division, which runs the viscera involuntarily and is itself divided into the sympathetic system of fight-or-flight and the parasympathetic system of rest-and-digest. The functional unit throughout is the neuron — a cell body with branching dendrites that receive signals and a single long axon that transmits them — and neurons never touch. They communicate across synapses, where a neurotransmitter carries the message chemically from one cell to the next in one direction only. Stack these facts and the architecture of every response becomes the same three-step sequence: receptor, integrating centre, effector. Only the middle step is negotiable, and it always lies within the central nervous system.

State PCS 2023 Q30 — Science & Technology PYQ with Answer

State PCS 2023 Q30 — Science & Technology PYQ with Answer