Question
Draw and describe the reflex arc. Name all five components in the correct order and trace the pathway of a simple reflex action (like pulling your hand away from a hot object).
Solution — Step by Step
The reflex arc has exactly five components, and the sequence is what NEET tests — not just the names.
Receptor → Afferent nerve → Nerve centre (spinal cord) → Efferent nerve → Effector
When your finger touches a flame, pain receptors in the skin detect the stimulus and generate a nerve impulse. This impulse travels along the afferent (sensory) neuron toward the spinal cord — specifically into the dorsal horn of the grey matter.
The key here: the signal goes up to the spinal cord, not the brain. That’s what makes it a reflex.
Inside the spinal cord, a relay (interneuron) connects the sensory neuron to the motor neuron. This synapse is the decision point — the signal gets passed across without waiting for the brain’s approval.
The spinal cord acts as the integrating centre for spinal reflexes. The brain only finds out after the response has already happened.
The relay neuron passes the impulse to the efferent (motor) neuron, which exits from the ventral horn of the spinal cord. This motor neuron carries the command to the effector — in this case, the biceps muscle.
The muscle contracts, you withdraw your hand. Total time: ~50 milliseconds.
A clean labelled diagram should show this U-shaped pathway:
Stimulus → Receptor (skin) → Afferent neuron (dorsal root)
→ Interneuron (spinal cord grey matter)
→ Efferent neuron (ventral root) → Effector (muscle)
→ Response (withdrawal)Note that the dorsal root carries sensory signals in, and the ventral root carries motor signals out. This is Bell-Magendie Law — frequently tested.
Why This Works
The reflex arc bypasses the brain deliberately. If every stimulus had to travel up to the cerebral cortex, get processed, and send a response back down, the delay would be ~400–500 ms. The spinal reflex cuts this to under 100 ms — crucial for protective responses.
This is why a reflex is involuntary. The conscious brain receives the information about the stimulus (that’s why you feel pain after you’ve already pulled back), but the withdrawal had nothing to do with conscious decision-making.
The interneuron in the spinal cord is the architectural genius here — it allows a local circuit to function independently while still communicating upward to higher brain centres.
NEET frequently asks: “Which part of the nervous system coordinates a spinal reflex?” The answer is the spinal cord, not the brain. Cranial reflexes (like pupillary reflex) are coordinated by the brain stem — don’t confuse the two.
Alternative Method — Cranial Reflex Example
Not all reflex arcs go through the spinal cord. The pupillary light reflex is a cranial reflex — its nerve centre is the midbrain, not the spinal cord.
Pathway: Light hits retina → Optic nerve (CN II) → Pretectal nucleus in midbrain → Edinger-Westphal nucleus → Oculomotor nerve (CN III) → Ciliary muscle → Pupil constricts.
This shows the same five-component structure but routed through the brain stem. NEET 2022 asked specifically about this reflex — knowing both types gives you full coverage.
Common Mistake
Students write “stimulus → receptor → brain → effector” in their diagrams. This is wrong for spinal reflexes. The brain is NOT part of the reflex arc pathway — it only receives a copy of the information after the response. Writing “brain” in place of “spinal cord” as the nerve centre will cost you the full mark in NEET’s assertion-reason format.
A related trap: mixing up which root is sensory and which is motor. Remember SAME — Sensory = Afferent = dorsal (back), Motor = Efferent = ventral (front). Dorsal roots carry signals into the cord; ventral roots carry signals out.