Sympathetic nervous system

In this note, we are going to continue studying the anatomy and function of the autonomic nervous system.

Before watching this video, we recommend familiarizing yourself with the content of the topic: “Autonomic Nervous System. Vegetative Reflex Arc,” which covers the general principles of the anatomy and function of the ANS.

Let’s move on to a detailed examination of the sympathetic division of the ANS.

In general, the sympathetic division has an activating effect on the body, providing “fight or flight” responses which are designed to mobilize energy in stressful situations.

The central part of the sympathetic nervous system is located in the lateral horn of the thoracolumbar region of the spinal cord (from T1 to L2).

From there, the impulse travels along the preganglionic fibers to the autonomic ganglia:

Paravertebral ganglia: these form the sympathetic trunk

Prevertebral or preaortic ganglia: these are located anterior to the aorta

In the sympathetic arc, the preganglionic fibers are short, while the postganglionic fibers are long.

The neurotransmitter for preganglionic components is acetylcholine, while for postganglionic transmission it is norepinephrine (with some exceptions).

It is worth noting that the sympathetic centers of the spinal cord are controlled by the hypothalamus, nuclei of the limbic system, and the cerebral cortex.

Let’s start with the cervical ganglia of the sympathetic trunk.

The superior cervical ganglion receives impulses from the T1, T2, and according to some sources, T3 segments.

Postganglionic fibers from this ganglion form the internal carotid plexus

and the external carotid plexus.

Nerves emerging from these plexuses innervate several structures:

The dilator pupillae muscle. When the sympathetic system is activated, this muscle dilates the pupil.

The ciliary muscle. When relaxed (which is the sympathetic effect on this muscle), it flattens the lens, providing “far point” accommodation for better vision of objects, located far away.

Salivary glands (parotid, submandibular, and sublingual) – stimulation either directly or via associated blood vessels increases the secretion of thick, mucin-rich saliva.

The lacrimal gland. Stimulation decreases tear production.

The middle cervical ganglion and the inferior cervical ganglion.

The latter sometimes merges with the first thoracic ganglion, forming the cervicothoracic (stellate) ganglion.

They receive impulses from the T1 to T4-T5 segments.

Both of these cervical ganglia (as well as part of the fibers from the superior ganglion) give off postganglionic fibers that innervate the following structures:

Cells of the cardiac conduction system. Stimulation increases the heart rate.
Myocardium. Sympathetic stimulation increases its contractility and, consequently, increases cardiac output.

It’s worth noting that the fibers running from the cervical portion of the sympathetic trunk to the heart form three nerves: the superior, middle, and inferior cervical cardiac nerves.

These nerves, together with nerves from the thoracic ganglia, form the cardiac plexus.

Esophagus. The sympathetic effect of stimulation is an inhibition of peristalsis. Nerves that innervate the esophagus are part of the esophageal plexus.
Bronchi. The sympathetic effect is bronchodilation, which is the widening of the bronchial lumen. Nerves that innervate the bronchi are part of the pulmonary plexus.

Additionally, some fibers provide innervation to the bronchial arteries, causing vasoconstriction.

Let’s move on to the thoracic ganglia.

From the upper thoracic ganglia arise the thoracic cardiac nerves, which participate in the formation of the previously mentioned cardiac plexus.

The remaining thoracic ganglia give rise to nerves that are part of the esophageal, pulmonary, and aortic plexuses, causing all the mentioned sympathetic effects in these organs.

Additionally, some fibers from the T5 to T9 segments form the greater splanchnic nerve.

These fibers do not synapse in the paravertebral ganglia but instead proceed to the celiac ganglia, forming the celiac (or solar) prevertebral plexus.

Branches of the celiac plexus innervate several organs:

Stomach and the initial part of the duodenum. The effect of sympathetic stimulation includes reduced peristalsis, absorption, and gastric secretion, as well as vasoconstriction and contraction of the pyloric sphincter.
Liver. Sympathetic stimulation activates glycogenolysis, which is the breakdown of glycogen into glucose for energy mobilization.
Gallbladder and biliary tract. Stimulation causes reduced contractility, leading to decreased bile secretion.
Pancreas. Sympathetic activation reduces insulin secretion and increases glucagon secretion.
Spleen. Stimulation causes contraction of the spleen’s capsule.

The greater splanchnic nerve also gives off a branch to the chromaffin cells of the adrenal medulla. Note that in this case, there is no synapse in a peripheral ganglion. The chromaffin cells themselves act as a functional equivalent of a ganglion. Upon stimulation, these cells increase the secretion and release of epinephrine and norepinephrine.

In some individuals, the greater splanchnic nerve also sends branches to the superior mesenteric ganglion, which innervates the cecum, ascending colon, part of the transverse colon, duodenum, and jejunum.

The sympathetic effect is the inhibition of all processes related to digestion: peristalsis, absorption, and secretion.

Fibers from the T10 and T11 segments form the lesser splanchnic nerve,

whose branches synapse in the so-called aorticorenal ganglia.

These ganglia innervate the kidneys and the initial part of the ureter via the renal plexus.

Upon stimulation, urine production decreases and the renin-angiotensin-aldosterone system is activated, the effects of which are aimed at increasing blood pressure.

The T12 segment gives off the so-called least thoracic splanchnic nerve to the renal plexus. It primarily innervates the ureter, reducing the contraction of its muscular wall.

Let’s move on to the lumbar region.

The L1, L2, and sometimes L3 segments form the lumbar splanchnic nerves.

They proceed to the inferior mesenteric ganglion.

Branches of this ganglion innervate the remaining part of the transverse colon, descending colon, sigmoid colon, and the upper part of the rectum. The sympathetic effect is the inhibition of all processes related to digestion: peristalsis, absorption, and secretion.

The L1, L2, and sometimes also T11 and T12 segments give off lumbar splanchnic nerves to two more groups of prevertebral ganglia, forming plexuses. These are the superior hypogastric plexus

and the intermesenteric plexus.

The superior hypogastric plexus innervates the internal urethral sphincter and the detrusor muscle of the bladder.

Upon sympathetic stimulation, the contractility of the detrusor muscle decreases, while the contractility of the sphincter increases.

The intermesenteric plexus innervates various sections of the intestine as well as, partially, the pelvic organs.

Fibers from the T10 to L2 segments pass through the sacral ganglia of the sympathetic trunk, which subsequently form the sacral splanchnic nerves.

These nerves innervate the male and female reproductive organs, forming part of the inferior hypogastric plexus.

However, the main part of this plexus is formed by branches from the superior hypogastric plexus located above.

Overall, the sympathetic influence on the reproductive organs is directed toward the initiation of ejaculation in men and the contraction of the non-pregnant uterus in women.

In addition, it is important to remember that some fibers from the ganglia of the sympathetic trunk pass through the gray rami communicantes and, as part of the spinal nerves, innervate the following structures:

Blood vessels (vasomotor fibers)
Skin glands (sudomotor fibers)
Arrector pili muscle (pilomotor fibers)

The neurotransmitter of the postganglionic fibers innervating sweat glands is acetylcholine, except for the glands in the areas of the feet, palms, and partially the face, where the neurotransmitter is norepinephrine.

It should be noted that the sympathetic nervous system has high anatomical variability, so the branching of all the mentioned nerves, as well as the segments innervating them, can differ significantly among individuals.

Golosary