The nervous system provides a network of coordinated signaling for bodily functions. Somatic nerves promote skeletal muscle activity and, therefore, gross movements. Autonomic nerves provide signals for internal organs, including regulation of their function. These signals are transmitted by nerves, which rely on membrane-bound protein pumps for conduction of the impulse. Specialized lipids form myelin sheaths, which aid in the fast and efficient transmission of these signals. Neurotransmitters, often small peptides, allow continuation of the nerve signal between neurons and from neurons to target tissues. The breakdown of this network of signaling and regulation leads to several neurological diseases.
COMPONENTS OF THE NERVOUS SYSTEM
The nervous system is anatomically and functionally composed of two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS) (Figure 19-1). The CNS consists of the brain and spinal cord and the PNS is composed of all nerves outside of the CNS, including all spinal and cranial nerves. In addition, there are further classifications for the nerves of the PNS. First, nerves are distinguished by the direction of nerve propagation. Afferent (sensory) neurons conduct action potentials toward the CNS. Efferent (motor) neurons transmit impulses away from the CNS to effectors such as muscles or glands. Second, nerves of the PNS can be further divided into the somatic (skeletal muscle) and autonomic (organs, glands, and smooth muscle) nervous systems. The autonomic nervous system (ANS) is divided into the sympathetic (SNS) and parasympathetic nervous system (PSNS). Generally, both interact with the same effectors, but often paradoxically. The SNS is involved in the activities associated with the fight-or-flight response, helping the body to cope with stress. The PSNS promotes activities that support the body while at rest, including digestion.
Overview of the Nervous System. Schematic diagram comparing some anatomic and neurotransmitter features of autonomic and somatic motor nerves. Only the primary transmitter substances are shown; parasympathetic ganglia are not indicated. See text for more details. Ach, acetylcholine; D, dopamine; Epi, epinephrine; M, muscarinic receptors; N, nicotinic receptors; NE, norepinephrine. [Reproduced with permission from Katzung BG, et al.: Basic and Clinical Pharmacology, 11th edition, McGraw-Hill, 2009.]
Neurons are the fundamental components of the nervous system. The main components of a neuron include the soma (or cell body), dendrites, axon, and the axon terminals (Figure 19-2). The cell body, or soma, is the central part of the neuron that contains the nucleus and other cell organelles. The dendrites are the branching structures of the neuron that receives stimuli or messages via chemoreceptors. The axon is a slender, cable-like extension of a neuron that carries nerve impulses away from the soma toward the axon terminal. The axon terminals are the hair-like terminals of the axon where neurotransmitters are released from the synaptic knobs. Neurons communicate with one ...