The following are involved in reflex action EXCEPT

Step 1: Analyze the question We need the one structure that is NOT part of a reflex action.

Step 2: The concept A reflex action is a rapid, automatic, involuntary response to a stimulus. It does not need conscious thought, so the signal is processed in the spinal cord and does not have to go to the brain first.

The reflex arc pathway: Stimulus → Receptor → Sensory neuron → Intermediate (relay) neuron in the spinal cord → Motor neuron → Effector (muscle or gland) → Response

Step 3: Check each option

• A. Brain cells: not part of the basic reflex arc. The brain is involved in voluntary actions, though it is informed afterward.
• B. Intermediate neuron: it is in the spinal cord and connects sensory to motor neurons, so it is involved.
• C. Motor neuron: it carries the impulse to the effector, so it is involved.
• D. Receptor: it detects the stimulus, so it is involved.
• E. Sensory neuron: it carries the impulse to the spinal cord, so it is involved.

Step 4: Answer The only structure not involved in the reflex pathway is A. brain cells.

Tip: Remember reflex = spinal cord, voluntary = brain. A quick way to recall the arc is "R-S-I-M-E": Receptor, Sensory, Intermediate, Motor, Effector. The brain isn't on the list. An example is pulling your hand away from a hot object before you even feel the pain.