It is difficult to understand all the complexities of the nervous system because the field of neuroscience has rapidly evolved over the past 20 years; moreover, answers to new questions are being found almost daily. A thorough knowledge of the individual components of the nervous system and their functions, however, will lead you to a better understanding of how the human body works and will facilitate your future acquisition of knowledge about the nervous system.
The nervous system consists of two parts, shown in Figure 1:
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The central nervous system (CNS) consists of the brain and spinal cord.
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The peripheral nervous system (PNS) consists of nerves outside the CNS.
Nerves of the PNS are classified in three ways. First, PNS nerves are classified by how they are connected to the CNS. Cranial nerves originate from or terminate in the brain, while spinal nerves originate from or terminate at the spinal cord.
Second, nerves of the PNS are classified by the direction of nerve propagation. Sensory ( afferent) neurons transmit impulses from skin and other sensory organs or from various places within the body to the CNS. Motor ( efferent) neurons transmit impulses from the CNS to effectors (muscles or glands).
Third, motor neurons are further classified according to the effectors they target. The somatic nervous system (SNS) directs the contraction of skeletal muscles. The autonomic nervous system (ANS) controls the activities of organs, glands, and various involuntary muscles, such as cardiac and smooth muscles.
The autonomic nervous system has two divisions:
The sympathetic nervous system is involved in the stimulation of activities that prepare the body for action, such as increasing the heart rate, increasing the release of sugar from the liver into the blood, and other activities generally considered as fight‐or‐flight responses (responses that serve to fight off or retreat from danger).
The parasympathetic nervous system activates tranquil functions, such as stimulating the secretion of saliva or digestive enzymes into the stomach and small intestine.
Generally, both sympathetic and parasympathetic systems target the same organs, but often work antagonistically. For example, the sympathetic system accelerates the heartbeat, while the parasympathetic system slows the heartbeat. Each system is stimulated as is appropriate to maintain homeostasis.
Figure 1. Two parts of the nervous system.