7 CHAPTER VIIAUTONOMIC NERVOUS SYSTEM. METHODS OF INVESTIGATION. SIGNS OF LESION
Figure 9.1. Anatomical Differences in Sympathetic and Parasympathetic Divisions (Leslie Hendon, Universitty of Alabama)
The autonomic nervous system (ANS) regulates physiologic, metabolic processes and functions of internal organs (blood vessels, sweat glands, and salivary and lachrymal glands). ANS realizes its functions independently of consciousness. It consists of two major parts: the sympathetic and parasympathetic systems. According to the anatomy it divides into central part, which includes suprasegmental level (limbic system, hypothalamus, reticular formation) and segmental level and peripheral part (all the other structures).
Each peripheral division of the autonomic nervous system is characterized by a two-neuron chain and consists of two anatomic elements: the preganglionic neuron terminates in a peripheral ganglion, and the post-ganglionic neuron, which carries impulses to their destination in the viscera.
The sympathetic and parasympathetic systems each consist of two types of nerve cells: one of them sits in the CNS and connects to cells in ganglia, which locate outside the CNS. Efferent fibers from the ganglia (postganglionic fibers) lead to effector organs.
The preganglionic cell bodies of the sympathetic system are located in the intermediolateral horn of the spinal cord between T1 and L2 or L3. The sympathetic ganglia are adjacent to the spine and consist of the vertebral (sympathetic chain) and prevertebral ganglia, including the superior cervical, celiac, superior mesenteric, and aorticorenal ganglia. Long fibers from these ganglia pass to effector organs, including the smooth muscle of blood vessels, viscera, lungs, scalp (piloerector muscles), and pupils; the heart; and glands (sweat, salivary, and digestive).
The preganglionic cell bodies of the parasympathetic system are located in the brain stem and sacral portion of the spinal cord. Preganglionic fibers exit the brain stem with the 3rd, 7th, 9th, and 10th cranial nerves. Parasympathetic ganglia are located within the effector organs, and postganglionic fibers are only 1 or 2 mm long. Thus, the parasympathetic system can produce specific, localized responses in effector organs, including blood vessels of the head, neck, and thoracoabdominal viscera; lacrimal and salivary glands; smooth muscle of viscera and glands (liver, spleen, colon, kidneys, bladder, genitals); and ocular muscles.
The ANS activity is regulated by CNS (the hypothalamus, nucleus of the solitary tract, reticular formation, amygdala, hippocampus, and olfactory cortex).
Physiology
The ANS controls BP, heart rate, body temperature, weight, digestion, metabolism, fluid and electrolyte balance, sweating, urination, defecation, sexual response, and other processes. Many organs are controlled by either the sympathetic or parasympathetic system (although they may receive input from both, e.g., sympathetic input increases heart rate; parasympathetic decreases it).
The sympathetic nervous system is catabolic. It increases heart rate and contractility, bronchodilation, hepatic glycogenolysis and glucose release, and muscular strength; it also causes sweaty palms. Less immediately life-preserving functions (e.g., digestion, renal filtration) are decreased.
The parasympathetic nervous system is anabolic. Gastrointestinal secretions and motility (including evacuation) are stimulated, heart rate is slowed, and blood pressure decreases.
Two major neurotransmitters in the ANS are acetylcholine and norepinephrine. Fibers that secrete acetylcholine are termed cholinergic; they include all preganglionic fibers and all postganglionic parasympathetic fibers. Fibers that secrete norepinephrine are termed adrenergic; they include most postganglionic sympathetic fibers, except for those that innervate piloerectors, sweat glands, and blood vessels, which are cholinergic.
