An autonomic reflex arc is similar to the somatic kind, but differs principally in the motor output side. The sensory side is similar in that a transducer, 1, sends a signal via a nerve fiber (2, drawn in blue) into the CNS. As with the somatic arc, this sensory fiber is associated with a pseudo-unipolar neuron in a craniospinal ganglion, usually one of the dorsal root ganglia. In the autonomic arc, the sensory transducer is often located in or associated with visceral organs.
Inside the CNS, there may or may not be an interconnector neuron involved. While in the somatic arc there is always a mediating interconnector neuron that receives the sensory signal and passes it to the motor neuron, the contact between the sensory fiber and the first motor neuron of the autonomic arc may be direct. (In this sketch, the interconnector neuron—labeled "3" in the sketch of the somatic arc—has been omitted.)
The motor output in an autonomic arc involves not one, but two motor neurons. Both are shown here in red. The first (4A), as in the somatic arc, is located in the spinal cord. Its efferent fiber (i.e., its axon) leaves the CNS to form a synapse with a second motor neuron (4B). The second neuron is located in the peripheral nervous system, not the CNS. More specifically, it's located in an autonomic ganglion of some kind. Autonomic ganglia are "switching stations" in the periphery; here the signal is passed from the first neuron of the motor chain to the second neuron. More than one type of autonomic ganglion exists. A discussion of the different types of autonomic ganglia will be found elsewhere.
It's the second neuron's axon that completes the arc by synapsing with the effector organ (5). The effector is not skeletal muscle, of course. It may be smooth muscle, however. Here I have sketched it as the muscle of the gut wall. It may be something like a sweat gland, or the adrenal medulla.
In a functional sense, autonomic reflex arcs are vital to what we think of as "automatic" functions, such as gut peristalsis, sweating, etc. For the most part these activities are carried out without conscious thought or volition.
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