4.1. DRUGS AFFECTING THE PARASYMPATHETIC NERVOUS SYSTEM
The central nervous system provides efferent innervation of the effector organs and tissues which is performed by way of:
• somatic innervation regulating voluntary movements and including the motor nerve fibers innervating the skeletal muscles;
• visceral (autonomic) innervation regulating involuntary functions of organs and systems and including autonomic nerve fibers innervating internal organs, blood vessels, glands etc.
The somatic nervous system consists of:
1) the central part: motor nerve cells (motor neurons) with the bodies located in the brain and the spinal cord;
2) the peripheral part: long extensions (ax-ons) of motor neurons, which are motor nerve fibers that make contacts with the skeletal muscles (neuromuscular junctions (synapses).
The autonomic ( vegetative) nervous system
is subdivided into:
• the sympathetic nervous system;
• the parasympathetic nervous system (fig. 4.1). The internal organs and glands are innervated by both sympathetic and parasympathet-ic nerves. Sympathetic and parasympathetic nerve fibers are not distributed equally in a particular organ or system; therefore, in most organs simultaneous excitation of both systems will result in prevalence of one of the systems. These systems often act as antagonists. For instance, in the bronchi excitation of the sympathetic nervous system causes relaxation of smooth muscles, whereas excitation of the parasympathetic nervous system leads to their contraction (table 4.1).
The structure of the autonomic nervous system differs from that of the somatic system and consists of two sequentially located neurons. Bodies of the first neurons are located in the CNS. Their axons (also termed preganglionic fibers) terminate in the autonomic ganglia where they form synaptic contacts with the second (ganglionic) neurons. The axons of ganglionic neurons, also termed postganglionic fibers,