Question
What is a reflex arc? Name its components in order and explain the pathway of a simple spinal reflex (e.g., knee-jerk reflex or withdrawal reflex).
(NCERT Class 11, commonly asked in NEET)
Solution — Step by Step
A reflex action is a rapid, involuntary, automatic response to a stimulus that does not involve conscious thinking. Examples: pulling your hand away from a hot surface, knee-jerk response, blinking when something approaches the eye.
A reflex arc is the neural pathway that carries a reflex action. It is the shortest route from stimulus to response.
A typical reflex arc has five components in sequence:
- Receptor: Detects the stimulus (e.g., pain receptors in the skin, stretch receptors in the muscle).
- Sensory (afferent) neuron: Carries the impulse from the receptor to the central nervous system (spinal cord).
- Integration centre: Usually an interneuron (relay neuron) in the spinal cord that processes the information and connects the sensory neuron to the motor neuron. In simple reflexes like the knee-jerk, the sensory neuron may synapse directly with the motor neuron (monosynaptic reflex).
- Motor (efferent) neuron: Carries the response impulse from the CNS to the effector.
- Effector: The organ that carries out the response (e.g., a muscle that contracts or a gland that secretes).
When you touch a hot surface:
- Heat receptors in the fingertip detect the stimulus.
- Sensory neurons carry the impulse via dorsal root ganglion into the spinal cord.
- Interneurons in the grey matter of the spinal cord relay the signal to motor neurons.
- Motor neurons carry the impulse through the ventral root to the arm muscles.
- Arm muscles (effectors) contract, pulling the hand away.
This entire process takes a fraction of a second. The brain receives the pain signal slightly later — you feel the pain after you have already pulled your hand away.
Why This Works
Reflex arcs bypass the brain for speed. If the signal had to travel to the brain, be processed consciously, and then a decision sent back down, the response would be dangerously slow for protective reflexes. The spinal cord handles the immediate response, while the brain is informed simultaneously (so you become aware of what happened).
The reflex arc is also the simplest example of neural circuit design — a sensor connected to a processor connected to an actuator, all wired for speed.
For NEET, distinguish between monosynaptic reflexes (one synapse, e.g., knee-jerk — sensory neuron directly synapses with motor neuron) and polysynaptic reflexes (multiple synapses, e.g., withdrawal reflex — involves interneurons). The knee-jerk reflex is the only common example of a monosynaptic reflex in humans.
Common Mistake
Students often write the components in the wrong order or leave out the interneuron. The correct sequence is always: receptor → sensory neuron → integration centre (interneuron/spinal cord) → motor neuron → effector. Reversing sensory and motor neurons is a surprisingly common error in exams.
Also, do not write that “reflexes do not involve the brain at all.” While spinal reflexes are processed in the spinal cord, the brain is informed simultaneously. Some reflexes (like the pupillary reflex) are actually processed in the brain stem, not the spinal cord. These are called cranial reflexes.