Question
What is a reflex arc? Trace the complete path of a nerve impulse in the knee-jerk (patellar) reflex.
Solution — Step by Step
A reflex action is a rapid, automatic, involuntary response to a stimulus — it happens without conscious thought. The neural pathway that carries the impulse from receptor to effector during a reflex action is called a reflex arc.
Reflex arcs are fast because the impulse doesn’t travel all the way to the brain — it is processed at the level of the spinal cord.
Every reflex arc has these five elements in order:
- Receptor — detects the stimulus
- Afferent (sensory) neuron — carries impulse toward the CNS
- Nerve centre (integration centre) — in the spinal cord (or brainstem for some reflexes)
- Efferent (motor) neuron — carries impulse away from CNS toward the effector
- Effector — muscle or gland that produces the response
The knee-jerk is a monosynaptic reflex — there is only one synapse, between the sensory and motor neuron, making it the fastest type.
Stimulus: A rubber hammer taps just below the kneecap (patella), stretching the patellar tendon and the quadriceps muscle.
Path of impulse:
- Receptor — stretch receptors (muscle spindles) in the quadriceps muscle detect the stretch
- Afferent neuron — sensory neuron carries impulse to the lumbar region of the spinal cord (L2–L4)
- Integration — in the spinal cord, the sensory neuron synapses directly with a motor neuron (no interneuron in this simple reflex)
- Efferent neuron — motor neuron carries impulse back to the quadriceps
- Effector — quadriceps contracts → leg kicks forward
The brain is informed (via ascending pathways) that the reflex occurred, but it does not control or initiate it. This is why you kick before you realise you were tapped. Doctors test the knee-jerk to assess spinal cord integrity — an absent response suggests damage.
Why This Works
Speed is the reason reflexes bypass the brain. The spinal cord processes the reflex in milliseconds, while a brain-mediated response would take 3–5 times longer due to the extra distance. For survival situations (touching a hot surface, maintaining balance), this speed difference is critical.
The knee-jerk is monosynaptic — just one synapse means minimal delay. Most other reflexes are polysynaptic (involve interneurons), but the patellar reflex is the simplest possible neural circuit.
CBSE Class 10 and NEET both love the reflex arc diagram. In your answer, label all five components and draw the arc as a loop from receptor → spinal cord → effector. Note: the brain is connected via ascending fibres but doesn’t control the reflex.
Alternative Method
For other reflexes (like the withdrawal reflex — pulling hand from fire):
- Stimulus: pain/heat
- Receptor: pain receptors in skin
- Afferent: sensory neuron to spinal cord
- Interneuron(s): in spinal cord (polysynaptic)
- Efferent: motor neuron to arm muscles
- Effector: arm muscles contract → hand withdrawn
The pathway is the same, just with added interneurons.
Common Mistake
Many students write that “the impulse goes to the brain and the brain sends a signal back.” This confuses a reflex arc with a voluntary response. In a reflex arc, the spinal cord coordinates the response — the brain receives information about the reflex but does not initiate it. Also, do not mix up afferent (sensory, carrying signals toward CNS) and efferent (motor, carrying signals away from CNS) — these directions are commonly swapped in exams.