Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

21. Autonomic Nervous System

21.1 Sympathetic and Parasympathetic Nervous Systems, Organization

A Structure of the autonomic nervous system

The portion of the nervous system which innervates smooth muscle, cardiac muscle, and glands is called the autonomic nervous system. This is further subdivided into the sympathetic (red) and parasympathetic (blue) systems, each of which has a two-neuron sequence between the CNS and its target, consisting of a presynaptic neuron in the CNS, and a postsynaptic neuron in a ganglion (PNS) close to the target organ:

• Sympathetic system: presynaptic neurons located in the lateral horns of the cervical, thoracic, and lumbar spinal cords. Their axons exit the CNS via the ventral roots and synapse with postsynaptic neurons in sympathetic ganglia.

• Parasympathetic system: presynaptic neurons located in the brainstem and sacral spinal cord. Their axons exit the CNS via cranial nerves and pelvic splanchnic nerves to synapse with postsynaptic parasympathetic neurons, typically within the target organ.

The sympathetic and parasympathetic systems regulate blood flow, secretions and organ function, often acting in antagonistic ways on the same target (see C). In the abdomen, small clusters of neurons embedded in target organs form a network that can be considered a third autonomic division: the enteric nervous system (see p.324). Although this network receives some presynaptic parasympathetic innervation via the vagus nerve (CN X), it typically functions independently, responding to local reflexes.

В Synaptic organization of the autonomic nervous system

The sympathetic and parasympathetic portions of the nervous systems innervate many of the same targets, but use different transmitters, often with antagonistic effects (see C). These antagonistic systems also have differing patterns of organization, including unique paths to their targets and connections to the CNS. The cell bodies of the presynaptic motor neurons of the sympathetic system are located in the lateral horn of spinal cord segments T1 to L2 (sometimes C8 and L3). Their axons leave the spinal cord through thoracolumbar ventral roots, briefly travel in spinal nerves, and enter the paravertebral sympathetic trunk via white rami communicantes (white = myelinated). These axons terminate in synapses with postsynaptic neurons at three different levels:

1. Sympathetic ganglia along the paravertebral chain: The postsynaptic neurons send their axons back into the spinal nerves via gray rami communicantes (gray = unmyelinated). These axons travel in the spinal nerves to innervate local blood vessels, sweat glands, etc.

2. Prevertebral sympathetic ganglia: These ganglion cells send their axons along arterial plexuses to the bowel, kidneys, etc., providing innervation to both the organs and their vasculature.

3. Adrenal medulla (not shown): Adrenal medullary (endocrine) cells are develop mentally related to sympathetic ganglion cells, and receive direct innervation from presynaptic sympathetic axons.

In contrast, the presynaptic neurons of the parasympathetic system are located in the CNS in the brainstem (cranial nerves III, VII, IX, and X) and sacral spinal cord (S2-S4). The presynaptic axons leave the CNS via the cranial nerves noted above (the vagus nerve [CN X] is the example shown here), and pelvic splanchnic nerves. These presynaptic axons synapse with postsynaptic neurons in discrete cranial ganglia (ciliary, pterygopalatine, submandibular, and otic), which in turn send their axons in other cranial nerves to the target organ. Some presynaptic axons, particularly the vagus nerve, innervate scattered postsynaptic neurons that are embedded in the target organs themselves. Afferent fibers (shown in green), originating from pseudounipolar neurons in spinal (dorsal root) and cranial sensory ganglia, travel with autonomic motor axons. These sensory fibers carry information from visceral nociceptors (pain) and stretch receptors into the CNS. Efferent fibers are shown in purple, the ascending pain pathway in gray. For detailed description of the autonomic innervation of the viscera, see Volume II, Neck and Internal Organs.

C Synopsis of the sympathetic and parasympathetic nervous systems

This table summarizes the effects of the sympathetic and parasympathetic nervous systems on specific organs. 

• The sympathetic nervous system is the excitatory part of the autonomic nervous system (fight or flight).

 The parasympathetic nervous system coordinates rest and digestive processes (rest and digest).

 Although the two systems have separate nuclei, they establish close anatomical and functional connections in the periphery.

 The transmitter at the target organ is acetylcholine in the parasympathetic and norepinephrine in the sympathetic nervous system (except for the adrenal medulla).

 Stimulation of the sympathetic or parasympathetic nervous system produces the following effects in specific organs (see table):

Organ

Sympathetic nervous system

Parasympathetic nervous system

Eye

Pupillary dilation

Pupillary constriction and increased curvature of the lens

Salivary glands

Decreased salivation (scant, viscous)

Increased salivation (copious, watery)

Heart

Rise in heart rate

Fall in heart rate

Lungs

Decreased bronchial secretions and bronchodilation

Increased bronchial secretions and bronchoconstriction

Gastrointestinal

tract

Decrease in secretions and motility

Increase in secretions and motility

Pancreas

Decreased exocrine secretions

Increased exocrine secretions

Male sex organs

Ejaculation

Erection

Skin

Vasoconstriction, sweating, piloerection

No effect

21.2 Autonomic Nervous System, Actions and Regulation

A Circuit diagram of the autonomic nervous system

The central first (presynaptic) neuron uses acetylcholine as a transmitter in both the sympathetic and parasympathetic nervous systems (cholinergic neuron, shown in blue). Acetylcholine is also used as a neurotransmitter by the second (postsynaptic) neuron in the parasympathetic nervous system. In the sympathetic nervous system,  norepinephrine is used by the noradrenergic neuron (shown in red). Л/ote: the target cell membrane contains different types of receptors (= transmitter sensors) for acetylcholine and norepinephrine. Each transmitters can produce entirely different effects, depending on the type of receptor.

В Control of the peripheral autonomic nervous system (after Klinke and Silbernagl)

The peripheral actions of the autonomic nervous system are subject to control at various levels, the highest being the limbic system, whose efferent fibers act on the peripheral target organs (e.g., heart, lung, bowel; also affects sympathetic tone and cutaneous blood flow) through centers in the hypothalamus, medulla oblongata, and spinal cord. The higher the control center, the more subtle and complex its effect on the target organ. The limbic system receives signals from its target organs via afferent feedback mechanisms.

C Excitatory and inhibitory effects on sympathoexcitatory neurons in the medulla oblongata

a Cross-section through the brainstem at the level of the medulla oblongata. To generate a baseline level of sympathetic outflow, the presynaptic visceral efferent sympathetic neurons in the spinal cord (intermedio- lateral and intermediomedial nuclei, see p.271) must be stimulated by sympathoexcitatory neurons in the anterolateral part of the medulla oblongata. Numerous factors can inhibit or enhance the activity of these neurons which play a critical role in the regulation of blood pressure. If the blood pressure is too high, for example, afferent impulses from the pressoreceptors will inhibit sympathetic outflow.

b Afferent impulses from the factors listed in a are relayed in the medial nuclei of the solitary tract nucleus to secondary neurons, whose axons project back to the sympathoexcitatory neurons. When these neurons are inhibited, the peripheral resistance vessels relax and the blood pressure falls. The axons from these sympathoexcitatory neurons pass ipsilaterally through the posterolateral funiculus to presynaptic sympathetic neurons in the lateral horn of the spinal cord. Sensory neurons are shown in orange, motor neurons in green.

21.3 Parasympathetic Nervous System, Overview and Connections

A Overview: parasympathetic nervous system (cranial part)

There are four parasympathetic nuclei in the brainstem. The visceral efferent fibers of these nuclei travel along particular cranial nerves, listed below.

 Visceral oculomotor (Edinger- Westphal) nucleus: oculomotor nerve (CN III)

 Superior salivatory nucleus: facial nerve (CN VII)

 Inferior salivatory nucleus: glossopharyngeal nerve (CN IX)

 Dorsal vagal nucleus: vagus nerve (CNX)

The presynaptic parasympathetic fibers often travel with multiple cranial nerves to reach their target organs (for details see p. 81 and E, p.85). The vagus nerve supplies all of the thoracic and abdominal organs as far as a point near the left colic flexure.

Note: The sympathetic fibers to the head travel along the arteries to their target organs.

В Parasympathetic ganglia in the head

Nucleus

Path of presynaptic fibers

Ganglion

Postsynaptic fibers

Target organs

• Visceral oculomotor (Edinger-Westphal) nucleus

• Oculomotor nerve

• Ciliary ganglion

• Short ciliary nerves

• Ciliary muscle (accommodation)

• Pupillary sphincter (miosis)

• Superior salivatory nucleus

• Nervus intermedius (facial nerve root) divides into:

 

• Maxillary nerve → zygomatic nerve →anastomosis → lacrimal nerve

• Lacrimal gland

 

• Greater petrosal nerve —» nerve of pterygoid canal

• Pterygopalatine ganglion

• Orbital branches

• Lateral posterior nasal branches

• Nasopalatine nerve

• Palatine nerves

• Glands on:

- posterior ethmoid cells

- nasal conchae

- anterior palate

- hard and soft palate

 

• Chorda tympani → lingual nerve

• Submandibular ganglion

• Glandular branches

• Submandibular gland

• Sublingual gland

• Inferior salivatory nucleus

• Glossopharyngeal nerve → tympanic nerve → lesser petrosal nerve

• Otic ganglion

• Auriculotemporal nerve (CN V3)

• Parotid gland

• Dorsal vagal nucleus

• Vagus nerve

• Ganglia near organs

• Fine fibers in organs, not individually named

• Thoracic and abdominal viscera

→ = is continuous with

C Overview: parasympathetic nervous system (lumbrosacral part)

The portions of the bowel near the left colic flexure and the pelvic viscera are supplied by the sacral part of the parasympathetic nervous system. Efferent fibers emerge from the anterior sacral foramina in the ventral roots of segments S2-S4. The fibers are collected into bundles to form the pelvic splanchnic nerves. They blend with the sympathetic fibers and synapse in the ganglia in or near the organs.

D Connections of the dorsal longitudinal fasciculus

Increased salivation during eating results from stimulation of the salivary glands by the parasympathetic nervous system. To produce the coordinated stimulation of various glands, the cranial parasympathetic nuclei require excitatory impulses from higher centers (tuberal nuclei, mammillary bodies). The parasympathetic nuclei are then stimulated to increase the flow of saliva. The dorsal longitudinal fasciculus establishes the necessary connections with the higher centers. Besides the fibers that coordinate the parasympathetic nuclei, the fasciculus contains other fiber systems that are not shown in the diagram.

21.4 Autonomic Nervous System: Pain Conduction

A Pain afferents conducted from the viscera by the sympathetic and parasympathetic nervous systems (after Janig)

a Sympathetic pain fibers,

b parasympathetic pain fibers.

It was originally thought that the sympathetic and parasympathetic nervous systems conveyed only efferent fibers to the viscera. More recent research has shown, however, that both systems also carry afferent nociceptive (pain) fibers (shown in green), many running parallel to visceral efferent fibers (shown in purple). It is likely that many of these fibers (which make up only 5% of all the afferent pain fibers in the body) are inactive during normal processes and may become active in response to organ lesions, for example.

a The pain-conducting (nociceptive) axons from the viscera course in the splanchnic nerves to the sympathetic ganglia and reach the spinal nerve by way of the white ramus communicans. The cell bodies of these neurons are located in the spinal ganglion. From the spinal nerve, the neurons pass through the dorsal roots to the posterior horn of the spinal cord. There they are relayed to establish a connection with the ascending pain pathway. Alternatively, a reflex arc may be established through interneurons (see B). Note: unlike the efferent system, the afferent nociceptive fibers of the sympathetic and parasympathetic systems are not relayed in the peripheral ganglia, b The cell bodies of the pain-conducting pseudounipolar neurons in the cranial parasympathetic system are located in the inferior or superior ganglion of the vagus nerve (CN X). Those of the sacral parasympathetic system are located in the sacral spinal ganglia of S2-S4. Their fibers run parallel to the efferent vagal fibers and establish a central connection with the pain-processing systems.

В Referred pain

It is believed that nociceptive afferent fibers from dermatomes (somatic pain) and internal organs (visceral pain) terminate on the same relay neurons in the posterior horn of the spinal cord. The convergence of somatic and visceral afferent fibers (see b) confuses the relationship between the perceived and actual sites of pain, a phenomenon known as referred pain. The pain is typically perceived at the somatic site, as somatic pain is well-localized while visceral pain is not. Pain impulses from a particular internal organ are consistently projected to the same well-defined skin area (a); the pattern of pain projection is very helpful in determining the affected organ.

21.5 Enteric Nervous System

A Enteric nervous system in the small intestine

The enteric nervous system is the intrinsic nervous system of the bowel, consisting of small groups of neurons that form interconnected, microscopically visible ganglia in the wall of the digestive tube. Its two main divisions are the myenteric (Auerbach) plexus (located between the longitudinal and circular muscle fibers) and the submucosal plexus (located in the submucosa), which is subdivided into an external (Schabadasch) and internal (Meissner) submucosal plexuses. (Details on the fine lamination of the enteric nervous system can be found in textbooks of histology.) These networks of neurons are the foundation for autonomic reflex pathways. In principle they can function without external innervation, but their activity is intensely modulated by the sympathetic and parasympathetic nervous systems. Activities influenced by the enteric nervous system include enteric motility, secretion into the digestive tube, and local intestinal blood flow.

В Modulation of intestinal innervation by the autonomic nervous system

Although the parasympathetic nervous system (“rest and digest”) generally promotes the activities of the digestive tube (secretion, motility), it may also produce inhibitory effects.

a Excitatory presynaptic cholinergic parasympathetic fibers terminate on excitatory cholinergic neurons that promote intestinal motility (mixing of the bowel contents to facilitate absorption),

b An inhibitory parasympathetic fiber synapses with an inhibitory ganglion cell that uses noncholinergic, nonadrenergic (NCNA) transmitters. These NCNA transmitters are usually neuropeptides that inhibit intestinal motility.

c Sympathetic fibers are not abundant in the muscular layers of the bowel wall. Postsynaptic adrenergic fibers inhibit the motor and secretory neurons in the plexuses.

The clinical importance of autonomic bowel innervation is illustrated below:

 During shock, the vessels in the bowel are constricted and the intestinal mucosa is accordingly deprived of oxygen. This results in disruption of the epithelial barrier, which may then be penetrated by microorganisms from the bowel lumen. This is an important mechanism contributing to multisystem failure in shock.

 There may be a cessation of intestinal motility (atonic bowel) after intestinal operations involving surgical manipulation of the digestive tube.

 Medications (especially opiates) may suppress the motility of the enteric nervous system, causing constipation.

C Functional interactions of the sympathetic and parasympathetic nervous systems at the target organ

The transmitters of the sympathetic and parasympathetic nervous systems (norepinephrine and acetylcholine, respectively) act upon both the target organ and the (para)sympathetic nerve endings at the synapse. Noradrenergic receptors on the target tissue (pi, shown in blue) and nerve endings themselves (cc2, shown in pink) modulate target cell responses on two levels: norepinephrine binding to the pi receptor directly promotes a cellular response in heart tissue, while similar binding to the a2 receptors on the postsynaptic nerve endings allows for regulation of subsequent neurotransmitter release, through positive and negative feedback loops. The muscarinergic receptors (m, shown in green) mediate a similar process upon binding of acetylcholine. The neurotransmitters of the autonomic nervous system can therefore self- and cross-regulate in a multifaceted control mechanism.

D Sympathetic effects on arteries

An important function of the sympathetic nervous system is to regulate the caliber of the arterioles (blood pressure regulation). When sympathetic fibers release norepinephrine into the media of the arterioles, the a1 receptor mediates contraction of the vascular smooth muscle, and the blood pressure rises. Meanwhile, epinephrine from the blood acts on the p2 receptors in the sarcolemma of the same vascular smooth muscle cells, inducing vasodilation and a corresponding drop in blood pressure.

Note: Parasympathetic fibers do not terminate on blood vessels.

E Autonomic innervation of the trachea and bronchi

Parasympathetic stimulation of the local ganglia promotes secretion by the bronchial glands and narrowing of the bronchial passages. For this reason, the preparations for bronchoscopy include the administration of a drug (atropine) which blocks parasympathetic innervation, ensuring that mucous secretions will not obscure the bronchial mucosa. A similar reduction in bronchial secretions can be achieved through sympathetic stimulation. Epinephrine from the bloodstream acts on adrenergic β2 receptors to induce bronchodilation. This effect is used to treat severe asthma attacks.