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Overview of Autonomic Nervous System
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Similar to the central nervous system (CNS), the peripheral nervous system (PNS) can be divided into functionally and anatomically distinct components named somatic and autonomic. The somatic nervous system (SNS) is responsible for sensory innervation from skin, joints, and muscle in addition to motor innervation to the skeletal muscle. The main function of the SNS is to regulate the body’s external environment via two modalities—first it gathers information about the environment and then it interacts with this information through voluntary motor response. The autonomic nervous system (ANS), discussed in this chapter, is a functionally distinct component of both the CNS and PNS.
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The autonomic division of the PNS contains motor and sensory axons which innervate smooth muscle, exocrine glands, and other viscera. It is best summarized as the master regulator of the body’s internal environment. The autonomic division of the CNS is mainly linked to the ANS via the hypothalamus. The hypothalamus has direct connections with various autonomic nuclei in the brainstem and spinal cord which facilitate its role as a regulator of ANS output. Additionally, the hypothalamus facilitates endocrine system functioning in two ways. Most critically, highly specialized neurons located in the posterior hypothalamic pituitary axis synthesize hormones (e.g., vasopressin and oxytocin) for transport down the axon to the posterior pituitary gland. Here, the hormones are subsequently secreted in the blood for access to target tissue. Second, specialized neurons located within the anterior hypothalamic pituitary portal system synthesize a host of hormones (e.g., thyroid-stimulating hormone, growth hormone, adrenocorticotropic hormone, luteinizing-stimulating hormone, melanocyte-stimulating hormone, prolactin) in the anterior pituitary gland. This portal system is characterized by a plexus of veins with a capillary network at each end. In conclusion, the anterior pituitary gland releases each hormone into the bloodstream to stimulate target tissue. Interestingly, the hypothalamus further regulates body temperature, hunger, thirst, and the cardiovascular system. There is no doubt the hypothalamus is the main ANS control center.
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Another important aspect of the ANS is that it is entirely involuntary, in stark contrast to the SNS which is entirely voluntary. One example of this is temperature control. Presence of inflammatory chemokines can raise body temperature and the absence lowers temperature, but the human mind cannot regulate this homeostasis. Other examples are blood pressure, blinking, and heart rate. In light of the lack of voluntary control, the ANS has very little cortical representation.
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The ANS is subdivided into three components: sympathetic, parasympathetic, and enteric. The sympathetic and parasympathetic systems each have CNS and PNS parts. Conversely, the enteric nervous system lies entirely within the PNS. In exploring the anatomy of the sympathetic and parasympathetic nervous systems, two neurons create the path from brain to effector organ. The cell body of the first neuron is located in the CNS while the second cell body is located in the PNS. In this ...