The effect of a motor neuron is always excitation.
The autonomic nervous system contains both autonomic sensory and motor neurons.
Autonomic sensory neurons are associated with interoceptors.
Autonomic sensory input is not consciously perceived.
The ANS also receives sensory input from somatic senses and special sensory neurons.
The autonomic motor neurons regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities of cardiac muscle, smooth muscle, and glands.
Most autonomic responses can not be consciously altered or suppressed.
All autonomic motor pathways consists of two motor neurons in series (Figure 17.1).
The axon of the first motor neuron of the ANS extends from the CNS and synapses in a ganglion with the second neuron.
The second neuron synapses on an effector. Preganglionic fibersrelease acetylcholine and postganglionic fibers release acetylcholine or norepinephrine.
The output (efferent) part of the ANS is divided into two principal parts: the sympathetic and the parasympathetic divisions. Organs that receive impulses from both sympathetic and parasympathetic fibers are said to have dual innervation.
Table 17.1 summarizes the similarities and differences between the somatic and autonomic nervous systems.