The General Reflex and the Reflex Arc
The pathway followed by the stimulus (impulse) from beginning to end is the reflex arc.
The general reflex arc of the human nervous system has a minimum of five components:
• Receptor organ
• Afferent (sensory) neuron
• Internuncial neuron
• Efferent (motor) neuron
• Effector organ
The stimulus is received by a receptor organ specific to that stimulus.
From the receptor organ, the stimulus is carried to the CNS by way of an afferent (sensory) neuron within the appropriate peripheral nerve.
The cell body of this afferent neuron is located in the posterior root ganglion of a spinal nerve or the
individual ganglion of a cranial nerve.
Within the spinal cord or brainstem, the terminal of the afferent neuron synapses with the interneuron, or internuncial neuron.
INTER = between
NUNCIA = messenger
In turn, the internuncial neuron synapses with the cell body of the efferent (motor) neuron.
In the spinal cord, the cell bodies of the efferent (motor) neurons make up the anterior column of the gray matter. In the brainstem, the motor neurons make up the individual nuclei of the cranial nerves.
The axon of the motor neuron passes out of the CNS by way of the appropriate peripheral nerve.
Command information is thus carried away from the CNS.
The information is then delivered by the motor neuron to the effector organ.
Somatic motor neurons lead to striated muscle fibers, particularly in skeletal muscles.
Autonomic (visceral) motor neurons lead to smooth muscle tissue, cardiac muscle tissue, or glands.
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