appropriate motor response. This happens in the central nervous system because it requires memory and higher cognitive functioning to generate the correct responses. The nervous system has three major parts. The first is the somatic nervous system or SNS. This is responsible for the reception of conscious stimuli, perception, and voluntary motor responses. While these are voluntary, it doesn’t mean that they have to be entirely conscious. A startle response involves the somatic nervous system but rarely does one actually think before startling, which involves movement of skeletal muscles. Other things that are part of the SNS that aren’t truly conscious are things that are done automatically. These happen because of “procedural memory” or the learning of habits. The second part is the autonomic nervous system or ANS. The autonomic nervous system is involuntary in nature and responsible for achieving and maintaining homeostasis. Homeostasis involves the regulation of the internal environment so that it achieves and maintains a steady state. Sensory input can be external or internal but the output is to cardiac and smooth muscles rather than skeletal muscles. Things like the heart, sweat glands, and certain smooth muscle functions of the GI tract are under autonomic control. The third part of the nervous system is the enteric nervous system or ENS. This controls the function of the glandular tissue in the digestive system and most of the smooth muscle function of the digestive tract. This is largely independent and does not rely on the central nervous system. There is some overlap between the functions of the autonomic nervous system and the enteric nervous system.
GLIAL CELLS OF THE PNS There are two types of glial cells found in the PNS. The first is the “satellite cell.” This type of cell is found in the autonomic and sensory ganglia, where they surround the cell bodies of neurons. They are largely supportive cells and are similar to the astrocytes in the central nervous system. In fact, the only difference is that these satellite cells do not establish the blood-brain barrier as is seen in the CNS.
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