Sensory Neuron Damage Blocks Sensation but Preserves Spinal Reflex

Steps:

• Reflex arc involves sensory neurons carrying signals from receptors to spinal cord for reflex and to brain for conscious sensation.
• Cutting sensory neurons at X (in arm) prevents signals from reaching brain, eliminating conscious feeling of touch or heat.
• Spinal reflex synapses occur locally in cord before signals ascend, allowing motor response without brain input.
• Thus, no sensation occurs, but finger withdraws automatically via intact reflex pathway.

Why A is correct:

• Reflex arc definition: Sensory input triggers spinal motor output independently of brain, so damage blocks conscious perception but enables withdrawal.

Why the others are wrong:

• B: Implies touch sensation intact (only heat lost), but cut blocks all sensory signals from finger.
• C: Reverses effect; sensory damage impairs input, not motor output.
• D: Receptors at finger remain functional; injury in arm affects neurons, not peripheral endings.

Final answer: A