Autonomic vs. Somatic Nervous System Pathways & Functions
Nervous System Motor Pathways
This text describes the differences between the motor pathways of the somatic and autonomic nervous systems, as might be illustrated in comparative diagrams.
Somatic vs. Autonomic Systems
One comparison illustrates the distinct pathways:
- The somatic nervous system pathway typically involves a single motor neuron from the Central Nervous System (CNS) to the effector (skeletal muscle).
- The autonomic nervous system (ANS) pathway involves two neurons: a preganglionic neuron from the CNS synapsing in a ganglion, and a postganglionic neuron from the ganglion to the effector (smooth muscle, cardiac muscle, glands).
Most organs are innervated by fibers from both the sympathetic and parasympathetic divisions of the ANS, which often act antagonistically. Both systems continually send impulses to visceral organs; the functional activity at any given time depends on the discharge rate of both. Thus, the operation of the ANS tends to maintain constant internal environmental conditions (homeostasis), even during challenges to the body.
In general:
- The sympathetic nervous system prepares the body for action in states of emergency (e.g., fighting, stress, escape). It increases blood pressure, heart rate, etc., resulting in energy consumption. This is often called the “fight-or-flight” response.
- Conversely, the action of the parasympathetic system relates to rest, recovery, and energy storage (e.g., decreased blood pressure, heart rate). This is often called the “rest-and-digest” response.
The involuntary functions controlled by autonomic neurons are closely interrelated with the somatic system. For example, while tear secretion during crying is controlled by the ANS, the somatic system determines breathing patterns and regulates the contraction of facial muscles.
Sympathetic vs. Parasympathetic Pathways
Another comparison focuses on the differences between the motor tracks of the sympathetic (often depicted superiorly) and parasympathetic (often depicted inferiorly) systems.
Autonomic Nervous System (ANS) Details
The autonomic nervous system is also called the visceral or vegetative nervous system because it regulates activities carried out involuntarily, meaning the brain does not have conscious control over them. For example, the secretion of a sweat gland or the heart rate cannot typically be controlled voluntarily.
This system consists of peripheral motor fibers originating in the CNS. Before reaching the effector organs, these fibers synapse with other neurons located in autonomic ganglia. Therefore, we distinguish between:
- Preganglionic fibers (running from the CNS to the ganglion)
- Postganglionic fibers (running from the ganglion to the effector organ)
Generally, preganglionic fibers are myelinated, while postganglionic fibers are unmyelinated.
ANS Components
Although the ANS technically includes only the motor pathways, it functionally involves:
- Nerve centers located in the CNS (hypothalamus, brainstem, spinal cord)
- Receptors that receive information
- Sensory (afferent) fibers that send impulses to the nerve centers
- Motor (efferent) fibers (preganglionic and postganglionic) that transmit commands to effectors
- Effectors, such as cardiac muscle, glands, and smooth muscle.
Functional Divisions of the ANS
Functionally, the ANS comprises two main systems: the sympathetic and the parasympathetic.
Sympathetic System
- Its preganglionic fibers emerge from the thoracic and lumbar regions of the spinal cord (thoracolumbar outflow).
- The ganglia, where synapses occur between preganglionic and postganglionic fibers, are located near the spinal column on both sides, forming the sympathetic chain or trunk.
- Consequently, sympathetic preganglionic fibers are generally short, whereas the postganglionic fibers are long.
Parasympathetic System
- Also known as the cranio-sacral system, as its preganglionic fibers emerge from cranial nerves (III, VII, IX, X) and the sacral spinal cord regions (S2-S4).
- Here, the autonomic ganglia are located close to, or even within, the effector organs. Consequently, they are scattered and not arranged in chains.
- For this reason, its preganglionic fibers are generally long, whereas postganglionic fibers are short.