Which is the correct sequence of the five steps in a reflex arc?

Which is the correct sequence of the five steps in a reflex arc?

• A. Arrival of stimulus and activation of receptor; Activation of a sensory neuron (Afferent); Information processing in CNS (Interneurons); Activation of a motor neuron (Efferent); Response by Effector
• B. Activation of a sensory neuron (Afferent); Arrival of stimulus and Activation of receptor; Information processing in CNS (Interneurons); Activation of a motor neuron (Efferent); Response by Effector
• C. Arrival of stimulus and Activation of a receptor; Activation of a motor neuron (Efferent); Information processing in CNS (Interneurons); Activation of a sensory neuron (Afferent); Response by Effector
• D. Arrival of stimulus and Activation of receptor; Activation of a sensory neuron (Afferent); Information processing in CNS (Interneurons); Activation of a motor neuron (Efferent); Response by Effector

Answer: (D)

The sequence starts with the stimulus arriving at a receptor and activating it. Receptors convert the stimulus into an electrical signal, creating a neural impulse. That impulse then travels along a sensory (afferent) neuron to the central nervous system. Inside the CNS, the information is processed by interneurons, where integration occurs (in some reflexes a direct sensory-to-motor connection exists, but processing or integration is still part of the pathway). The processed signal then travels via a motor (efferent) neuron to the effector organ. Finally, the effector responds, such as a muscle contracting or a gland changing its activity. This order makes sense because you must first detect and transduce the stimulus, then convey it to the CNS, have it processed or integrated, send a motor command back to the body, and finally observe the response.

The sequence starts with the stimulus arriving at a receptor and activating it. Receptors convert the stimulus into an electrical signal, creating a neural impulse. That impulse then travels along a sensory (afferent) neuron to the central nervous system. Inside the CNS, the information is processed by interneurons, where integration occurs (in some reflexes a direct sensory-to-motor connection exists, but processing or integration is still part of the pathway). The processed signal then travels via a motor (efferent) neuron to the effector organ. Finally, the effector responds, such as a muscle contracting or a gland changing its activity.

This order makes sense because you must first detect and transduce the stimulus, then convey it to the CNS, have it processed or integrated, send a motor command back to the body, and finally observe the response.