The CNS coordinates the two major nervous pathways, the somatic and autonomic pathways. A neural pathway of the somatic system commonly contains one sensor neuron and one motor neuron which both usually lie outside of the CNS. The somatic nerve pairs form reflex arcs that enervate skeletal muscle. Organisms usually have some conscious control over or at least some awareness of the reflexes initiated by somatic nerve impulses. In contrast, organisms have little conscious awareness of or control over the results of stimulation by the autonomic system. Autonomic system neural pathways regulate the activities of the internal organs and differ from somatic pathways by having two motor neurons that travel peripherally rather than one.
The autonomic nervous system has two main parts, the sympathetic and the parasympathetic systems, which differ both structurally and functionally from each other. Structurally, the two systems differ as to where the cell bodies of the first motor neurons are located within the spinal chord and as to what distance from the spinal chord the first motor neurons synapse with the second motor neurons. The cell bodies of the sympathetic system's first motor neurons lie in the thoracic and lumbar sections of the spinal chord. These motor neurons synapse with dendrites of the second motor neurons in ganglia located just exterior to the ventral side of the chord. Thus, the second motor neurons have to have very long axons to reach their target organs. The cell bodies of the parasympathetic system's first motor neurons lie in the medulla of the brain and in the sacral section of the spinal chord.
