Question
A doctor taps just below your knee with a rubber hammer. Your leg jerks forward automatically — without you thinking about it. Explain the pathway this impulse travels, naming each component of the reflex arc involved.
(CBSE 2024 Board Exam, 3 marks)
Solution — Step by Step
The tendon below the kneecap (patellar tendon) is suddenly stretched by the hammer tap. Stretch receptors embedded in the thigh muscle (quadriceps) detect this mechanical stimulus and convert it into an electrical nerve impulse.
The impulse travels along the afferent (sensory) neuron toward the spinal cord. Crucially, it goes to the lumbar region of the spinal cord — not the brain. This is the key feature of a reflex arc: the decision centre is the spinal cord, not the brain.
Inside the spinal cord, the sensory neuron synapses directly with a motor neuron (in this specific reflex, there is no interneuron — it is a monosynaptic reflex). The spinal cord acts as the relay centre and immediately generates a motor response.
The efferent (motor) neuron carries the impulse back out of the spinal cord to the quadriceps muscle (the effector). The muscle receives the signal and contracts sharply.
The quadriceps contracts, causing the leg to kick forward. The entire pathway — receptor → sensory neuron → spinal cord → motor neuron → effector — completes in milliseconds. The brain only becomes aware of the response after it has already happened.
Why This Works
Reflex arcs exist because speed matters. If every stimulus had to travel up to the brain for processing and then back down to the muscle, the response time would be too slow to prevent injury. By routing through the spinal cord only, the body can react in under 50 milliseconds.
The knee-jerk (patellar) reflex is special — it is monosynaptic, meaning the sensory neuron synapses directly onto the motor neuron with no interneuron in between. Most other reflexes (like withdrawing your hand from a hot object) are polysynaptic and involve interneurons in the spinal cord.
Doctors use this reflex clinically to test whether the neural pathway between the spinal cord and the leg muscles is intact. A missing or exaggerated knee-jerk response can indicate damage to specific spinal cord segments.
Receptor → Sensory (Afferent) Neuron → Spinal Cord → Motor (Efferent) Neuron → Effector (Muscle/Gland)
Alternative Method
For CBSE 3-mark questions, examiners often accept a labelled diagram in place of written steps. Draw the cross-section of the spinal cord showing:
- The dorsal root where sensory neurons enter
- The ventral root where motor neurons exit
- The synapse point inside the grey matter of the spinal cord
- Arrows showing the direction of impulse flow
Label all five components of the reflex arc clearly. A neat diagram with correct labels often scores full marks faster than a written paragraph in board exams.
Remember “SMART” for the reflex arc order: Stimulus → Musculo-receptor → Afferent neuron → Relay (spinal cord) → Transfer via efferent → response. Silly but it works in the exam hall.
Common Mistake
The most frequent error: students write that the impulse goes to the brain first and then comes back down. This is wrong for reflex actions. The whole point of a reflex arc is that it bypasses the brain for the actual response. The brain receives information about the reflex simultaneously via separate ascending pathways — but it does not control the reflex. Write “spinal cord” as the coordination centre, not “brain”.
A related slip: confusing afferent and efferent. Afferent = arriving at the spinal cord (sensory). Efferent = exiting the spinal cord (motor). Think: Afferent → Arriving.