The correct answer is Option 1.
Solution Explanation
A reflex arc is the neural pathway that mediates a reflex action. Its primary function is to allow for an immediate and involuntary response to a stimulus, often to protect the body from harm. This is achieved by processing the signal at the level of the spinal cord, bypassing the brain for a quicker reaction.
Let's break down the correct sequence as shown in Option 1:
Receptor: This is a specialized cell or nerve ending that detects a specific stimulus (e.g., heat, pain, pressure). For instance, pain receptors in the skin of your hand.
Sensory Neuron (Afferent Neuron): When the receptor is stimulated, it generates a nerve impulse. The sensory neuron transmits this impulse from the receptor towards the central nervous system (in this case, the spinal cord).
Relay Neuron (Interneuron) in Spinal Cord: The sensory neuron synapses with a relay neuron within the grey matter of the spinal cord. This relay neuron acts as an intermediary, processing the signal and passing it from the sensory neuron to the motor neuron. This is the integration center for the reflex.
Motor Neuron (Efferent Neuron): The relay neuron activates a motor neuron. The motor neuron carries the nerve impulse away from the spinal cord to the part of the body that will respond.
Effector: The effector is the muscle or gland that receives the impulse from the motor neuron and carries out the response. For example, the biceps muscle in your arm contracts to pull your hand away from a hot object.