DIVISIONS OF THE NERVOUS SYSTEM
The nervous system can be divided into two major divisions: the cen-tral nervous system and the peripheral nervous system (figure 8.1). The central nervous system (CNS) consists of the brain and spi-nal cord. The peripheral nervous system (PNS) consists of all the nervous tissue outside the CNS (nerves and ganglia).
The PNS functions to link the CNS with the various parts of the body. The PNS carries information about the different tissues of the body to the CNS and carries commands from the CNS that alter body activities. The sensory division, or afferent (toward) division,of the PNS conducts action potentials from sensoryreceptors to the CNS (figure 8.2). The neurons that transmit action potentials from the periphery to the CNS are called sensory neu-rons. The motor division, orefferent(away)division,of the PNSconducts action potentials from the CNS to effector organs, such as muscles and glands. The neurons that transmit action potentials from the CNS toward the periphery are called motor neurons.
The motor division can be further subdivided based on the type of effector being innervated. The somatic (sō-mat′ik; bodily) nervous system transmits action potentials from the CNS to skel-etal muscles, and the autonomic (aw-tō-nom′ ik; self-governing) nervous system (ANS) transmits action potentials from the CNSto cardiac muscle, smooth muscle, and glands. The autonomic nervous system, in turn, is divided into sympathetic and parasym-pathetic divisions (see figure 8.2).
The enteric nervous system (ENS) is a unique subdivision of the peripheral nervous system. The ENS has both sensory and motor neurons contained wholly within the digestive tract. The ENS can function without input from the CNS or other parts of the PNS, although it is normally integrated with the CNS by sensory neurons and ANS motor neurons.