Comparison between the Somatic and Autonomic Nervous System
The nervous system can be divided into two functional parts: the somatic nervous system (SNS) and the autonomic nervous system (ANS). The major difference between these two systems is based on whether you are conscious of its process. The somatic nervous system consciously detects sensory stimuli from the special senses, skin and proprioceptors. The autonomic nervous system reflexively responds to visceral sensory stimuli, such as levels of carbon dioxide concentration in the blood or stretch caused by blood pressure, that you are not consciously aware of. Moreover, the motor efferent branches of these two systems innervate different target effectors. While the somatic motor neurons innervate and cause contraction of skeletal muscles, autonomic motor neurons innervate and control cardiac and smooth muscle, as well as glandular tissue. Thus, the motor response of the somatic nervous system is voluntary while the one of the autonomic nervous system is involuntary.
Another major difference between these two systems lies within the number of lower motor neurons that are involved in the response. In the somatic nervous system, a single lower somatic motor neuron of the brainstem or spinal cord extends from the CNS towards a skeletal muscle through a cranial or spinal nerve, respectively (Figure (PageIndex{1})). These somatic motor neurons have large myelinated axons that release acetylcholine (ACh) at neuromuscular junctions.
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In comparison, the autonomic nervous system is composed of a chain of two lower motor neurons (Figure (PageIndex{1})). The cell body of the first of the two ANS motor neurons is located in the brainstem or spinal cord and is called a preganglionic neuron. The axon of the preganglionic neuron extends outside of the CNS through cranial or spinal nerves forming a preganglionic fiber. This fiber projects to an autonomic ganglion of the peripheral nervous system. Preganglionic neurons have small myelinated axons that release acetylcholine (ACh) to excite a second motor neuron. The second motor neuron is called a ganglionic neuron. The cell body of the ganglionic neuron resides within the autonomic ganglion and its axon extends to an effector (cardiac muscle, smooth muscle, or gland) forming a postganglionic fiber. Ganglionic neurons have small unmyelinated axons that release either acetylcholine (ACh) or norepinephrine (NE) to either excite or inhibit an effector, depending on the type of receptors present on the effector.
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Comparison of Somatic and Visceral Motor Systems Feature Somatic Motor Visceral Motor Type of control Voluntary Involuntary Number of neurons in pathway One: extends from CNS to effector Two: preganglionic from CNS to ganglion, ganglionic from ganglion to effector Ganglia None Autonomic ganglia: sympathetic chain (paravertebral), prevertebral, intramural or terminal Effectors Skeletal muscle fibers Smooth muscle fibers, cardiac muscle fibers, glands Effector response Excitation Excitation or inhibition Neurotransmitter(s) released Acetylcholine (Ach)
Preganglionic: Acetylcholine Ach)
Postganglionic: Acetylcholine (Ach) or norepinephrine
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Myelination Myelinated
Preganglionic: myelinated
Postganglionic: unmyelinated
Simplified Pathway (Image credit: “Somatic Motor Pathway” by Gabrielle Spurlock is licensed under CC BY-NC-SA 4.0, modification of original by BlueLink licensed under CC BY-NC 4.0 with notification of the original authors.) (Image credit: “Autonomic Motor Pathway” by Gabrielle Spurlock is licensed under CC BY-NC-SA 4.0, modification of original by BlueLink licensed under CC BY-NC 4.0 with notification of the original authors.)
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