84 Structure of the Autonomic Nervous System

Preganglionic Neurons

In the autonomic nervous system (ANS), nerve fibers that connect the central nervous system to ganglia are known as preganglionic fibers.

Preganglionic Neuron Properties

In the autonomic nervous system (ANS), fibers from the central nervous system to the ganglion are known as preganglionic fibers. All preganglionic fibers, whether they are in the sympathetic nervous system (SNS) or in the parasympathetic nervous system (PSNS), are cholinergic—that is, these fibers use acetylcholine as their neurotransmitter—and are myelinated.

The ANS is unique in that it requires a sequential two-neuron efferent pathway; the preganglionic neuron must first cross a synapse onto a postganglionic neuron before innervating the target organ. The preganglionic, or first neuron will begin at the outflow and will cross a synapse at the postganglionic, or second neuron’s cell body. The postganglionic neuron will then cross a synapse at the target organ.

Sympathetic preganglionic fibers tend to be shorter than parasympathetic preganglionic fibers because sympathetic ganglia are often closer to the spinal cord while parasympathetic preganglionic fibers tend to project to and synapse with the postganglionic fiber close to the target organ.

Outflow Sites

Properties of the SNS and PSNS preganglionic neurons also differ with respect to the spinal cord exit points. The sympathetic division has thoracolumbar outflow, meaning that the neurons begin at the thoracic and lumbar (T1–L2) portions of the spinal cord. The parasympathetic division has craniosacral outflow, meaning that the neurons begin at the cranial nerves (CN3, CN7, CN9, CN10) and sacral (S2–S4) spinal cord.

The sympathetic division (thoracolumbar outflow) consists of cell bodies in the lateral horn of the spinal cord (intermediolateral cell columns) from T1 to L2. These cell bodies are GVE (general visceral efferent) neurons and are the preganglionic neurons. There are several locations where preganglionic neurons create synapses with their postganglionic neurons:

• The paravertebral ganglia of the sympathetic chain (these run on either side of the vertebral bodies), cervical ganglia, thoracic ganglia, rostral lumbar ganglia, caudal lumbar ganglia, and pelvic ganglia.
• The prevertebral ganglia celiac ganglion, aorticorenal ganglion, superior mesenteric ganglion, inferior mesenteric ganglion.
• The chromaffin cells of the adrenal medulla. This is the one exception to the two-neuron pathway rule: they create a synapse directly onto the target cell bodies.

The parasympathetic division (craniosacral outflow) consists of cell bodies from one of two locations: the brainstem (cranial nerves III, VII, IX, X) or the sacral spinal cord (S2, S3, S4).

These are the preganglionic neurons that synapse with the postganglionic neurons in these locations:

• Parasympathetic ganglia of the head (ciliary (CN III)).
• Submandibular (CN VII).
• Pterygopalatine (CN VII).
• Otic (CN IX)).
• In or near the wall of an organ innervated by the vagus (CN X) or sacral nerves (S2, S3, S4).

Divergence

Another major difference between the two ANS systems is divergence, or the number of postsynaptic fibers a single preganglionic fiber creates a synapse with. Whereas in the parasympathetic division there is a divergence factor of roughly 1:4, in the sympathetic division there can be a divergence of up to 1:20.

The site of synapse formation and this divergence for both the sympathetic and parasympathetic preganglionic neurons does, however, occur within ganglia situated within the peripheral nervous system.

Autonomic Ganglia

Autonomic ganglia are clusters of neuron cell bodies that transmit sensory signals from the periphery to the integration centers in the CNS.

Autonomic ganglia are clusters of neuronal cell bodies and their dendrites. They are essentially a junction between autonomic nerves originating from the central nervous system and autonomic nerves innervating their target organs in the periphery.

The dorsal root ganglia lie along the vertebral column by the spine and develop in the embryo from neural crest cells, not neural tube. Therefore, the spinal ganglia can be regarded as gray matter of the spinal cord that became translocated to the periphery.

The two main categories are: sympathetic ganglia and parasympathetic ganglia. An example of parasympathetic ganglion is the ciliary ganglion, involved in pupil constriction and accommodation. A depiction of all the parasympathetic ganglia in the head and neck is shown in the following illustration.

Dorsal Root Ganglia

A dorsal root ganglion (or spinal ganglion) is a nodule on a dorsal root of the spine that contains the cell bodies of nerve cells (neurons) that carry signals from the sensory organs towards the appropriate integration center.

Nerves that carry signals towards the brain are known as afferent nerves. The axons of dorsal root ganglion neurons are known as afferents. In the peripheral nervous system, afferents refer to the axons that relay sensory information into the central nervous system (i.e., the brain and the spinal cord).

These neurons are of the pseudo-unipolar type, meaning they have an axon with two branches that act as a single axon, often referred to as a distal process and a proximal process.

Unlike the majority of neurons found in the central nervous system, an action potential in a dorsal root ganglion neuron may initiate in the distal process in the periphery, bypass the cell body, and continue to propagate along the proximal process until reaching the synaptic terminal in the dorsal horn of the spinal cord.

The distal section of the axon may either be a bare nerve ending or encapsulated by a structure that helps relay specific information to a nerve. The nerve endings of dorsal root ganglion neurons have a variety of sensory receptors that are activated by mechanical, thermal, chemical, and noxious stimuli.

In these sensory neurons, a group of ion channels thought to be responsible for somatosensory transduction have been identified. For example, a Meissner’s corpuscle or Pacinian corpuscle may encapsulate the nerve ending, rendering the distal process sensitive to mechanical stimulation, such as stroking or vibration, respectively.

Sympathetic Ganglia

Sympathetic ganglia are the ganglia of the sympathetic nervous system. They deliver information to the body about stress and impending danger, and are responsible for the familiar fight-or-flight response. They contain approximately 20,000–30,000 nerve cell bodies and are located close to and on either side of the spinal cord in long chains.

Sympathetic ganglia are the tissue from which neuroblastoma tumors arise. The bilaterally symmetric sympathetic chain ganglia —also called the paravertebral ganglia —are located just ventral and lateral to the spinal cord. The chain extends from the upper neck down to the coccyx, forming the unpaired coccygeal ganglion.

Preganglionic nerves from the spinal cord create a synapse end at one of the chain ganglia, and the postganglionic fiber extends to an effector, typically a visceral organ in the thoracic cavity. There are usually 21 or 23 pairs of these ganglia: three in the cervical region, 12 in the thoracic region, four in the lumbar region, four in the sacral region and a single, unpaired ganglion lying in front of the coccyx called the ganglion impar.

Neurons of the collateral ganglia, also called the prevertebral ganglia, receive input from the splanchnic nerves and innervate organs of the abdominal and pelvic region. These include the celiac ganglia, superior mesenteric ganglia, and inferior mesenteric ganglia.

Parasympathetic Ganglia

Parasympathetic ganglia are the autonomic ganglia of the parasympathetic nervous system. Most are small terminal ganglia or intramural ganglia, so named because they lie near or within (respectively) the organs they innervate. The exceptions are the four paired parasympathetic ganglia of the head and neck.

Efferent parasympathetic nerve signals are carried from the central nervous system to their targets by a system of two neurons. The first neuron in this pathway is referred to as the preganglionic or presynaptic neuron. Its cell body sits in the central nervous system and its axon usually extends to a ganglion somewhere else in the body, where it synapses with the dendrites of the second neuron in the chain.

This second neuron is referred to as the postganglionic or postsynaptic neuron. The axons of presynaptic parasympathetic neurons are usually long. They extend from the CNS into a ganglion that is either very close to or embedded in their target organ. As a result, the postsynaptic parasympathetic nerve fibers are very short.

Postganglionic Neurons

In the autonomic nervous system, fibers from the ganglion to the effector organ are called postganglionic fibers.

In the autonomic nervous system, fibers from the ganglion to the effector organ are called postganglionic fibers. The post-ganglionic neurons are directly responsible for changes in the activity of the target organ via biochemical modulation and neurotransmitter release.

The neurotransmitters used by postganglionic fibers differ. In the parasympathetic division, they are cholinergic and use acetylcholine as their neurotransmitter. In the sympathetic division, most are adrenergic, meaning they use norepinephrine as their neurotransmitter.

The Sympathetic Fibers

At the synapses within the ganglia, the preganglionic neurons release acetylcholine, a neurotransmitter that activates nicotinic acetylcholine receptors on postganglionic neurons. In response to this stimulus, postganglionic neurons—with two important exceptions—release norepinephrine, which activates adrenergic receptors on the peripheral target tissues. The activation of target tissue receptors causes the effects associated with the sympathetic system.

The two exceptions mentioned above are the postganglionic neurons of sweat glands and the chromaffin cells of the adrenal medulla. The postganglionic neurons of sweat glands release acetylcholine for the activation of muscarinic receptors. The chromaffin cells of the adrenal medulla are analogous to post-ganglionic neurons—the adrenal medulla develops in tandem with the sympathetic nervous system and acts as a modified sympathetic ganglion. Within this endocrine gland, the pre-ganglionic neurons create synapses with chromaffin cells and stimulate the chromaffin cells to release norepinephrine and epinephrine directly into the blood.

Presynaptic nerves’ axons terminate in either the paravertebral ganglia or prevertebral ganglia. In all cases, the axon enters the paravertebral ganglion at the level of its originating spinal nerve.

After this, it can then either create a synapse in this ganglion, ascend to a more superior ganglion, or descend to a more inferior paravertebral ganglion and make a synapse there, or it can descend to a prevertebral ganglion and create a synapse there with the postsynaptic cell. The postsynaptic cell then goes on to innervate the targeted end effector (i.e., gland, smooth muscle, etc.).

Because paravertebral and prevertebral ganglia are relatively close to the spinal cord, presynaptic neurons are generally much shorter than their postsynaptic counterparts, which must extend throughout the body to reach their destinations.

The Parasympathetic Fibers

The axons of presynaptic parasympathetic neurons are usually long. They extend from the CNS into a ganglion that is either very close to or embedded in their target organ. As a result, the postsynaptic parasympathetic nerve fibers are very short.

In the cranium, preganglionic fibers (cranial nerves III, VII, and IX) usually arise from specific nuclei in the central nervous system (CNS) and create a synapse at one of four parasympathetic ganglia: ciliary, pterygopalatine, otic, or submandibular.

From these four ganglia the postsynaptic fibers complete their journey to target tissues via cranial nerve V (the trigeminal ganglion with its ophthalmic, maxillary, and mandibular branches).

The vagus nerve does not participate in these cranial ganglia, as most of its fibers are destined for a broad array of ganglia on or near the thoracic viscera (esophagus, trachea, heart, lungs) and the abdominal viscera (stomach, pancreas, liver, kidneys). It travels all the way down to the midgut/hindgut junction, which occurs just before the splenic flexure of the transverse colon.

The pelvic splanchnic efferent preganglionic nerve cell bodies reside in the lateral gray horn of the spinal cord at the S2–S4 spinal levels. Their axons continue away from the CNS to synapse at an autonomic ganglion close to the organ of innervation. This differs from the sympathetic nervous system, where synapses between pre- and post-ganglionic efferent nerves in general occur at ganglia that are farther away from the target organ.

The parasympathetic nervous system uses acetylcholine (ACh) as its chief neurotransmitter, although peptides (such as cholecystokinin) may act on the PSNS as a neurotransmitter. The ACh acts on two types of receptors, the muscarinic and nicotinic cholinergic receptors.

Most transmissions occur in two stages: When stimulated, the preganglionic nerve releases ACh at the ganglion, which acts on the nicotinic receptors of the postganglionic neurons. The postganglionic nerve then releases ACh to stimulate the muscarinic receptors of the target organ.

Autonomic Plexuses

Autonomic plexuses are formed from sympathetic and parasympathetic fibers that innervate and regulate the overall activity of visceral organs.

Autonomic plexuses are formed from sympathetic postganglionic axons, parasympathetic preganglionic axons, and some visceral sensory axons. The nerves in the each plexus are close to each other, as in the plexuses of the somatic nervous system, but typically do not interact or synpase together.

Instead, they provide a complex innervation pattern to the target organs, since most organs are innervated by both divisions of the autonomic nervous system. The autonomic plexuses include the cardiac plexus, the pulmonary plexus, the esophageal plexus, and abdominal aortic plexus, and the superior and inferior hypogastric plexuses.

Plexuses

Cardiac

The cardiac plexus is a plexus of nerves situated at the base of the heart that innervates the heart. The superficial part of the cardiac plexus lies beneath the arch of the aorta, in front of the right pulmonary artery. It is formed by the superior cardiac branch of the left sympathetic trunk and the lower superior cervical cardiac branch of the left vagus nerve. A small ganglion, the cardiac ganglion of Wrisberg, is occasionally found connected with these nerves at their point of junction.

Pulmonary

The pulmonary plexus is an autonomic plexus formed from pulmonary branches of vagus nerve and the sympathetic trunk. It supplies the bronchial tree and the visceral pleura.

Esophageal

The esophageal plexus is formed by nerve fibers from two sources: the branches of the vagus nerve and the visceral branches of the sympathetic trunk. The esophageal plexus and the cardiac plexus contain the same types of fibers and are both considered thoracic autonomic plexus(es).

Abdominal

The abdominal aortic plexus is formed by branches derived, on either side, from the celiac plexus and ganglia, and receives filaments from some of the lumbar ganglia. It is situated on the sides and front of the aorta, between the origins of the superior and inferior mesenteric arteries.

From this plexus arise parts of the spermatic, the inferior mesenteric, and the hypogastric plexuses; it also distributes filaments to the inferior vena cava.

Superior Hypogastric Plexus

The superior hypogastric plexus (in older texts, hypogastric plexus or presacral nerve) is a plexus of nerves situated on the vertebral bodies below the bifurcation of the abdominal aorta.

Inferior Hypogastric Plexus

The inferior hypogastric plexus (pelvic plexus in some texts) is a plexus of nerves that supplies the viscera of the pelvic cavity. The inferior hypogastric plexus is a paired structure, with each situated on the side of the rectum in the male, and at the sides of the rectum and vagina in the female.

Parasympathetic (Craniosacral) Division

Parasympathetic ganglia are the autonomic ganglia of the parasympathetic nervous system that lie near or within the organs they innervate.

Parasympathetic ganglia are the autonomic ganglia of the parasympathetic nervous system, blue fibers). Most are small terminal ganglia or intramural ganglia, so named because they lie near or within (respectively) the organs they innervate.

The exceptions are the four paired parasympathetic ganglia of the head and neck. These paired ganglia supply all parasympathetic innervation to the head and neck: ciliary ganglion (spincter pupillae, ciliary muscle), pterygopalatine ganglion (lacrimal gland, glands of nasal cavity), submandibular ganglion (submandibular and sublingual glands), and otic ganglion (parotid gland).

Each has three roots entering the ganglion (motor, sympathetic, and sensory roots) and a variable number of exiting branches.

1. The motor root carries presynaptic parasympathetic nerve fibers (general visceral efferent fibers) that terminate in the ganglion by creating a synapse for the postsynaptic fibers traveling to target organs.
2. The sympathetic root carries postsynaptic sympathetic fibers (general visceral efferent fibers) that traverse the ganglion without creating a synapse.
3. The sensory root carries general sensory fibers (general somatic afferent fibers) that also do not create a synapse in the ganglion.

Some ganglia also carry special sensory fibers (special visceral afferent) for taste sensation.

The nerves that supply parasympathetic fibers to the parasympathetic ganglia of the head include the oculomotor nerve (ciliary ganglion), the facial nerve (pterygopalatine ganglion, submandibular ganglion), the glossopharyngeal nerve (otic ganglion), the vagus nerve, and the pelvic splanchnic nerves.

Because of its location, the parasympathetic system is commonly referred to as having craniosacral outflow, in contrast to the sympathetic nervous system, which is said to have thoracolumbar outflow.

Sympathetic Nervous System

Sympathetic ganglia are the ganglia of the sympathetic nervous system that initiate fight-or-flight, stress-mediated responses.

The sympathetic ganglia are the ganglia of the sympathetic nervous system (the red lines in the diagram below). They deliver information to the body about stress and impending danger, and are responsible for the familiar fight-or-flight response.

This response is also known as the sympathetico-adrenal response because the pre-ganglionic sympathetic fibers that end in the adrenal medulla—like all sympathetic fibers—secrete acetylcholine. This secretion activates the secretion of adrenaline (epinephrine) and to a lesser extent noradrenaline (norepinephrine) from the adrenal medulla.

Therefore, this response is mediated directly via impulses transmitted through the sympathetic nervous system, and indirectly via catecholamines secreted from the adrenal medulla, and acts primarily on the cardiovascular system.

An example of a sympathetic ganglion in a thoracic nerve is shown in. ganglia contain approximately 20000–30000 nerve cell bodies and are located close to and on either side of the spinal cord in long chains. Sympathetic ganglia are the tissue from which neuroblastoma tumours arise.

The bilaterally symmetric sympathetic chain ganglia, also called the paravertebral ganglia, are located just ventral and lateral to the spinal cord. The chain extends from the upper neck down to the coccyx, forming the unpaired coccygeal ganglion.

Preganglionic nerves from the spinal cord create a synapse at one end of the chain ganglia and the postganglionic fiber extends to an effector, typically a visceral organ, in the thoracic cavity. There are usually 21 or 23 pairs of these ganglia: 3 in the cervical region, 12 in the thoracic region, 4 in the lumbar region, 4 in the sacral region and a single, unpaired ganglion lying in front of the coccyx called the ganglion impar.

Neurons of the collateral ganglia, also called the prevertebral ganglia, receive input from the splanchnic nerves and innervate organs of the abdominal and pelvic region. These include the celiac ganglia, the superior mesenteric ganglia, and the inferior mesenteric ganglia.

The sympathetic nervous system is said to have thoracolumbar outflow based on its location.

Autonomic Reflexes

Autonomic reflexes are unconscious motor reflexes relayed from the organs and glands to the CNS through visceral afferent signaling.

The Autonomic Nervous System

The autonomic nervous system (ANS, visceral nervous system, or involuntary nervous system) is the part of the peripheral nervous system that acts as a control system. It functions largely below the level of consciousness, and controls visceral functions.

The ANS affects heart rate, digestion, respiratory rate, salivation, perspiration, pupillary dilation, micturition (urination), and sexual arousal. Most autonomic functions are involuntary but a number of ANS actions can work alongside some degree of conscious control. Everyday examples include breathing, swallowing, and sexual arousal, and in some cases functions such as heart rate.

Functions

Within the brain, the ANS is located in the medulla oblongata in the lower brainstem. The medulla’s major ANS functions include respiration (the respiratory control center, or RCC), cardiac regulation (the cardiac control center, or CCC), vasomotor activity (the vasomotor center or VMC), and certain reflex actions (such as coughing, sneezing, vomiting and swallowing).

These then subdivide into other areas and are also linked to ANS subsystems and nervous systems external to the brain. The hypothalamus, just above the brain stem, acts as an integrator for autonomic functions, receiving ANS regulatory input from the limbic system to do so.

Classifications

The ANS is classically divided into two subsystems: the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS) that operate independently in some functions and interact co-operatively in others. In many cases, the two have opposite actions. When one activates a physiological response, the other inhibits it.

An older simplification of the sympathetic and parasympathetic nervous systems as excitatory and inhibitory was overturned due to the many exceptions found. A more modern characterization is that the sympathetic nervous system is a quick- response, mobilizing system and the parasympathetic is a more slowly activated, dampening system—but there are exceptions, such as in sexual arousal and orgasm where both play a role.

The enteric nervous system is also sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.

The ANS is unique in that it requires a sequential two- neuron efferent pathway; the preganglionic neuron must first creat a synapse to a postganglionic neuron before innervating the target organ. The preganglionic, or first neuron will begin at the outflow and will cross a synapse at the postganglionic, or second neuron’s cell body. The postganglionic neuron will then create a synapse at the target organ.

General visceral afferent sensations are mostly unconscious, visceral motor reflex sensations from hollow organs and glands that are transmitted to the CNS (see the following illustration for a depiction of a typical nerve fiber, including general visceral afferent fibers).

While the unconscious reflex arcs are normally undetectable, in certain instances they may send pain sensations to the CNS, masked as referred pain. If the peritoneal cavity becomes inflamed or if the bowel is suddenly distended, the body will interpret the afferent pain stimulus as somatic in origin. This pain is usually non-localized. The pain is usually referred to dermatomes that are at the same spinal nerve level as the visceral afferent synapse.