Autonomic Nervous System

The autonomic nervous system consists of sympathetic and parasympathetic divisions. In the sympathetic nervous system, nerves arise from the thoracic and lumbar spinal cord and have a short preganglionic fiber and a long postganglionic fiber. Most of the ganglia are located in the paravertebral chain adjacent to the spinal cord, but a few prevertebral ganglia (the celiac, splanchnic, and mesenteric ganglia) are located more distally to the spinal cord. The parasympathetic nervous system includes portions of cranial nerves III, VII, IX, and X (the oculomotor, facial, glossopharyngeal, and vagus nerves, respectively) and some of the nerves originating from the sacral spinal cord. The parasympathetic nerves have long preganglionic fibers and short postganglionic fibers, with the ganglia often located in the innervated organs.

The origins, neurotransmitters, and receptors of the sympathetic and parasympathetic systems are shown in Figure 5-1. The sympathetic nervous system tends to discharge as a unit, producing a diffuse activation of target organs. Preganglionic, sympathetic neurons synapse with a large number of postganglionic neurons, which contributes to widespread activation of the organs during sympathetic stimulation. In addition, the release of epinephrine and norepinephrine from the adrenal medulla into the circulation enables the activation of target tissues throughout the body, including some tissues not directly innervated by sympathetic nerves. In contrast, the parasympathetic system can discretely activate specific target tissues. For example, it is possible for parasympathetic nerves to slow the heart rate without simultaneously stimulating gastrointestinal or bladder function. This is partly because of the low ratio of postganglionic fibers to preganglionic fibers in the parasympathetic system.

As shown in Figure 5-2, the sympathetic and parasympathetic nervous systems often have opposing effects on organ function. Activation of the sympathetic system produces the “fight or flight” reaction in response to threatening situations. In this reaction, cardiovascular stimulation provides muscles with oxygen and fuels required to support vigorous physical activity, while activation of glycogenolysis and lipolysis releases the necessary energy substrates. The parasympathetic system is sometimes called the “rest and digest” system, because it slows the heart rate and promotes more vegetative functions, such as digestion, defecation, and micturition. Many parasympathetic effects (including pupillary constriction, bronchoconstriction, and stimulation of gut and bladder motility) are caused by smooth muscle contraction.

Somatic Nervous System

The somatic nervous system consists of the motor neurons to the skeletal muscle. These neurons have a single nerve fiber that releases acetylcholine at the neuromuscular junction.

Enteric Nervous System

The enteric nervous system (ENS) is sometimes called the third division of the autonomic nervous system. The ENS consists of a network of autonomic nerves located in the gut wall that regulates gastrointestinal motility and secretion. The ENS, which includes the submucosal, myenteric, and subserosal nerve plexuses, is innervated by the sympathetic and parasympathetic nervous systems. The ENS, through its afferent and efferent fibers, integrates autonomic input with localized reflexes so as to synchronize propulsive contractions of gut muscle (peristalsis). Parasympathetic stimulation activates the ENS, whereas sympathetic stimulation inhibits the ENS. The ENS can function independently of autonomic innervation after autonomic denervation.

Receptors for Acetylcholine, Norepinephrine, and Epinephrine

The acetylcholine receptors have been divided into two types, based on their selective activation by one of two plant alkaloids. Muscarinic (M) receptors, which are acetylcholine receptors activated by muscarine, are primarily located at parasympathetic neuroeffector junctions. Nicotinic receptors are acetylcholine receptors activated by nicotine. They are found in all autonomic ganglia, at somatic neuromuscular junctions, and in the brain. Muscarinic receptors are subdivided based on molecular and pharmacologic criteria. Activation of the M₃ receptor produces smooth muscle contraction (except sphincters) and gland secretion. Activation of the M₂ receptor mediates cardiac slowing. The M₁ receptor is primarily concerned with modulation of neurotransmission at central and peripheral sites. Activation of nicotinic receptors in autonomic ganglia excites neurotransmission, whereas activation of these receptors in skeletal muscle causes muscle contraction.

The receptors for norepinephrine and epinephrine at sympathetic neuroeffector junctions are called adrenoceptors, a term that is derived from adrenaline, another name for epinephrine. The two types of adrenoceptors, called α-adrenoceptors and β-adrenoceptors, can be activated or blocked by drugs known as adrenoceptor agonists and adrenoceptor antagonists, respectively. These receptors have been further divided into several subtypes. The α₁-adrenoceptors mediate smooth muscle contraction, whereas β₂-adrenoceptors mediate smooth muscle relaxation. Activation of β₁-adrenoceptors produces cardiac stimulation.