Question

With reference to the nervous system, which of the following statements is false?

• A. In a divergent circuit, each axonal branch of the presynaptic neuron connects with the dendrite of a different postsynaptic neuron
• B. In a convergent circuit, axons from several presynaptic neurons meet at the dendrite(s) of a single postsynaptic neuron
• C. A three-neuron circuit consists of a sensory neuron, an interneuron in the spinal cord, and a motor neuron
• D. Reflex arc is a special type of neural circuit with only 3 neurons

Hint:

Reflex arcs are classified based on their total synaptic connections: - Monosynaptic Reflex (2 Neurons): Sensory neuron $\rightarrow$ Motor neuron. (e.g., Knee-jerk reflex). This is the fastest reflex type due to its minimal synaptic delay. - Polysynaptic Reflex ($\ge 3$ Neurons): Sensory neuron $\rightarrow$ One or more Interneurons $\rightarrow$ Motor neuron. (e.g., Withdrawal reflex from a painful stimulus).

Answer 1

The Correct Option is D

Concept: Neural circuits are functional groups of neurons processed within the central nervous system (CNS) to direct specific behavioral or physiological responses. These circuits rely on structural configurations (such as divergence and convergence) to regulate how signals propagate. A classic functional circuit model is the reflex arc, an involuntary, nearly instantaneous neural pathway that controls an action in response to a specific stimulus without requiring conscious brain input.

Step 1: Fact-checking statements (A), (B), and (C).

• Statement (A): In a divergent neural pathway, a single presynaptic neuron makes synaptic connections with multiple independent postsynaptic targets via its branching axon terminals. This architecture amplifies the signal, making statement (A) true.
• Statement (B): In a convergent neural pathway, input signals from multiple distinct presynaptic neurons gather to synapse onto a single postsynaptic neuron. This configuration allows a single cell to integrate information from diverse sources, making statement (B) true.
• Statement (C): A standard polysynaptic reflex circuit (like the withdrawal reflex) contains three sequential neurons in its pathway: a peripheral sensory receptor neuron, a central connecting interneuron within the spinal cord gray matter, and a somatic motor neuron. This confirms statement (C) is true.

Step 2: Identifying the error in statement (D).
Statement (D) claims that a reflex arc is a special circuit restricted to *only* 3 neurons. This is factually incorrect.
• Reflex arcs can be monosynaptic, containing just two neurons: a sensory neuron synapsing directly onto a motor neuron, completely bypassing interneurons. A classic clinical example is the patellar tendon stretch reflex (knee-jerk).
• Conversely, complex autonomic reflex arcs can be highly polysynaptic, incorporating dozens of interconnected interneurons between the initial sensory input and final motor output. Because reflex arcs are not structurally limited to exactly 3 neurons, Statement (D) is false. Since the question asks for the false statement, Option (D) is our correct choice.