Autonomic Nervous System

Lori Nading

Ahmed Alshaarawi

The autonomic nervous system (ANS) is a subdivision of the peripheral nervous system, which acts autonomously, or involuntarily, to control the visceral (internal) functions of the body. In other words, its effects on the body happen automatically without conscious actions. The ANS is separated into two divisions: the sympathetic system and the parasympathetic system. The sympathetic nervous system works to prepare the body for stressful situations. Thus, the sympathetic nervous system is also referred to as the “fight-or-flight” system. The parasympathetic nervous system counteracts the sympathetic nervous system and works to return the body to normal after a stressful situation, helping to maintain homeostasis. It is sometimes referred to as the “rest-and-restore” system.

The ANS differs from the somatic nervous system in several ways. A brief review of the somatic system reveals a one-motor neuron system with a synaptic cleft. Acetylcholine is the primary neurotransmitter that allows for propagation of an impulse across the synaptic cleft. The effector site for the somatic system is skeletal muscle. The ANS is a two-motor neuron system comprised of a preganglionic neuron and a postganglionic neuron. Located between the two neurons is a ganglion, and it is within this ganglion that the synapse occurs. The postganglionic neurotransmitters (at the effector site) used in the ANS consist of acetylcholine and norepinephrine. The effector sites of the ANS are smooth muscle, cardiac muscle, and secretory glands.

Ganglia are a collection of neuronal cell bodies outside the central nervous system (CNS). Sympathetic ganglia are also called paravertebral ganglia. The sympathetic paravertebral ganglia branch off the spinal nerves anterior to the ventral roots. The paravertebral ganglia are connected vertically, forming a chain lateral to the spinal cord. This chain is referred to as the sympathetic chain or trunk.

In the sympathetic nervous system, the first-order neurons of the ANS arise from the CNS. The preganglionic fibers deliver impulses to second-order neurons or the paravertebral ganglia. These ganglia contain the cell bodies of the postganglionic fibers responsible for delivering the impulse to the effector organs. The preganglionic fibers of the ANS are myelinated, and the postganglionic fibers of the ANS are nonmyelinated. Postganglionic fibers of the sympathetic division are long and are spread throughout the body. They produce a more generalized mass response. The postganglionic fibers of the parasympathetic division are short with their terminal ganglia near the effected organ. Their effect is more localized.

▪ ANATOMY OF THE AUTONOMIC NERVOUS SYSTEM

The sympathetic nervous system preganglionic fibers originate in the thoracic (T1-T12) segments and the first three lumbar (L1-L3) segments of the spinal cord (Figure 14.1). For this reason, it is sometimes referred to as the thoracolumbar division. The myelinated effector nerves leave the spinal cord and enter the ganglia. After reaching the ganglia, the impulse may travel in one of three ways: (1) directly across the ganglion to synapse with cell bodies of the postganglionic fibers, (2) cephalad or caudad to synapse with a higher or lower postganglionic neuron, or (3) through the sympathetic chain without synapsing. Some preganglionic fibers exit the sympathetic chain and synapse with outlying ganglia such as the celiac ganglia or the superior and inferior mesenteric ganglia. Synapses with these outlying ganglia, sometimes also referred to as collateral ganglia, innervate the visceral organs below the diaphragm. Innervation of the adrenal
medulla is unique in that the secretory cells are considered modified postganglionic neurons. Therefore, preganglionic fibers do not synapse prior to reaching the adrenal gland. Because the preganglionic fibers are myelinated, the signal speed is quick, causing a rapid release of norepinephrine and epinephrine from cells within the adrenal medulla.

The parasympathetic nervous system cell bodies stem from cranial nerves III, VII, IX, and X and the sacral segment of the spinal cord Figure 14.1. It is sometimes referred to as the craniosacral division. The preganglionic fibers of the parasympathetic system differ from those of the sympathetic system in that they travel uninterrupted to their effector organ before synapsing with a short postganglionic fiber. Parasympathetic stimulation arising from the cranial nerves innervate viscera of the head, thorax, and abdomen. A large percentage of parasympathetic innervation to the thorax and abdomen stems from the vagus (X) nerve. This includes parasympathetic stimulation to the heart, lungs, stomach, small intestine, liver, gallbladder, and pancreas.
The eye receives parasympathetic stimulation via the occulomotor (III) nerve, and the lacrimal and salivary glands are stimulated through fibers from the facial (VII) nerve. Additionally, salivary glands also receive parasympathetic stimulation through the glossopharyngeal (IX) nerve. Parasympathetic nervous system fibers arising from the sacral portion of the spinal cord innervate the large intestine, rectum, and bladder.

▪ FIGURE 14.1 Spinal cord with medulla pons and midbrain, connecting parasympathetic and sympathetic nerves to the ganglion to the affected organs, glands.

▪ PHYSIOLOGY OF THE AUTONOMIC NERVOUS SYSTEM

The primary neurotransmitters released in the ANS are acetylcholine and norepinephrine. The preganglionic fibers of the sympathetic division and the preganglionic and postganglionic fibers of the parasympathetic division all release acetylcholine. Most of the postganglionic fibers of the sympathetic division release norepinephrine. There are a few exceptions such as the postganglionic fibers of the sympathetic nervous system that stimulate the sweat glands. These fibers release acetylcholine. Acetylcholine exerts its effect on cholinergic receptors found in the ganglia or in the effector organs. There are two types of cholinergic receptors, nicotinic and muscarinic. Nicotinic receptors are almost always excitatory. Acetylcholine released from preganglionic fibers act on nicotinic receptors found in the ganglia on the postganglionic fibers in both the sympathetic and parasympathetic nervous systems.

Acetylcholine that is released from postganglionic fibers in the parasympathetic nervous system exerts its systemic effects by acting on muscarinic receptors. Muscarinic receptors can exhibit excitatory or inhibitory properties. Muscarinic activation in the heart causes decreased heart rate and contractility. Muscarinic activation also causes bronchoconstriction (e.g., wheezing), increased secretion by salivary glands, and intestinal and bladder contraction with release of their sphincter tone (often resulting in urination and defecation).

Within the sympathetic nervous system, norepinephrine released from postganglionic nerves acts on adrenergic receptors. The two major classes of adrenergic receptors are alpha (α) receptors and beta (β) receptors. These receptors are further subclassified as α₁ and α₂ and β₁ and β₂. In general, stimulation of α₁ receptors that exist outside of the central nervous system (CNS) results in constriction of blood vessels (hypertension) and relaxation of bladder and bowel, while at the same time causing constriction of the sphincters of the bowel and bladder. Stimulation of α₂ receptors results in decreased release of norepinephrine from nerve terminals. Stimulation of β₁ receptors causes increased heart rate and increased cardiac contractility, whereas stimulation of β₂ receptors causes dilation of blood vessels (hypotension), dilation of bronchioles (i.e., good treatment for bronchospasm/wheezing), and increased blood glucose (from glycogenolysis and gluconeogenesis).

▪ EFFECTS OF AUTONOMIC NERVOUS SYSTEM STIMULATION

Unlike innervation of skeletal muscle, which is all excitatory, visceral organs receive both excitatory and inhibitory innervation. The two divisions of the ANS are responsible for this antagonistic innervation. As noted previously, the sympathetic system is usually excitatory and the parasympathetic system inhibitory. Stimulation of the sympathetic division of the ANS

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