Corpus: Sympathetic nervous system

The sympathetic nervous system is a part of the autonomic nervous system. It primarily controls bodily functions that prepare the body for action, resulting in the depletion of energy reserves. This effect is also known as ergotropic. The sympathetic nervous system is predominantly antagonistic to the parasympathetic nervous system.

The core areas of the sympathetic nervous system are located in the intermediomedial and intermediolateral nuclei of the lateral horn in the thoracic spinal cord. These nerve fibers emerge from the intervertebral foramina of segments T1 to L2, forming a series of ganglia on both sides of the spinal column, which are connected via fiber systems to form the sympathetic trunk:

• In the cervical region, there are three cervical ganglia, with the inferior cervical ganglion often merging with the first thoracic ganglion to form the stellate ganglion.
• In the thoracic region, there are twelve ganglia on each side of the spinal column (paravertebral) in the sympathetic trunk.
• The lumbar region typically has four paravertebral ganglia.
• In the sacral region, there is a single, unpaired ganglion, the ganglion impar, formed by the fusion of the last fibers of the sympathetic trunk.

In the thoracic region, three sympathetic splanchnic nerves emerge from the sympathetic trunk and pass through the diaphragm: the greater splanchnic nerve, the lesser splanchnic nerve, and the least splanchnic nerve. Fibers from these nerves form the following nerve plexuses at the unpaired vascular outlets of the abdominal aorta (celiac trunk, superior mesenteric artery, inferior mesenteric artery):

• Celiac plexus
• Superior mesenteric plexus
• Inferior mesenteric plexus

In these plexuses, some sympathetic nerve fibers switch from the preganglionic to the postganglionic neuron. These fibers, along with unswitched fibers, travel to the plexuses of the internal organs. Sympathetic fibers that tone the blood vessels of the brain, reach the pineal gland, or innervate the eye also emerge from the thoracic spinal cord.

3.1. Transmitter-receptor interaction

The sympathetic nervous system uses acetylcholine as the preganglionic neurotransmitter and norepinephrine (not epinephrine) as the postganglionic neurotransmitter. The effect of norepinephrine is mediated via adrenoceptors, which are divided into α- and β-receptors. Norepinephrine has a higher affinity for α-receptors.

An exception is the innervation of sweat glands, which occurs both pre- and postsynaptically via acetylcholine. Consequently, sweat glands have cholinergic receptors instead of adrenergic receptors, explaining why acetylcholinesterase inhibitors such as E605 cause increased sweat production despite an otherwise parasympathetic effect.

The neuroendocrine cells of the adrenal medulla are also unique in the sympathetic nervous system. They are innervated by preganglionic (cholinergic) neurons and release epinephrine and norepinephrine directly into the blood. As the adrenal medulla serves as an interface between the autonomic and endocrine systems, it is known as a paraganglion. The humoral effect of epinephrine is also mediated via adrenoceptors. In contrast to norepinephrine, epinephrine has a higher affinity for β-receptors.

In addition to norepinephrine and acetylcholine, co-transmitters such as adenosine triphosphate (ATP) and neuropeptide Y play roles in the sympathetic nervous system's transmission of signals.

3.2. Organotropic effects of the sympathetic nervous system

The organotropic effects of the sympathetic nervous system can be simplified and visualized as a fight-or-flight response, which quickly adapts the body to dangerous situations. Key effects include:

• Heart
  • Positive chronotropic effect (increased heart rate)
  • Positive dromotropic effect (increased conduction velocity)
  • Positive inotropic effect (increased contractility)
  • Positive bathmotropic effect (increased excitability)
  • Positive lusitropic effect (improved relaxation)
• Blood vessels
  • Vasoconstriction
  • Increased vascular tone
• Bronchial system
  • Bronchodilation
  • Reduced liquid mucus secretion
• Gastrointestinal tract
  • reduced gland secretion
  • reduced peristalsis
• Urinary bladder
  • Contraction of the urethral sphincter muscle
  • Relaxation of the detrusor muscle
• Eye
  • Contraction of the dilator pupillae muscle, leading to mydriasis (pupil dilation)
• Sweat glands
  • Increased sweat secretion

A sympathectomy, or disruption of sympathetic nervous system activity, can lead to disturbances in sweat secretion. The affected skin areas may become brittle, cracked, and dry, requiring moisturization.

The general state of excitation of the sympathetic nervous system is referred to as sympathetic tone.

• Drugs that strengthen the sympathetic nervous system are called sympathomimetics. These include direct sympathomimetics (e.g., epinephrine) that act on adrenoceptors and indirect sympathomimetics that inhibit the breakdown of catecholamines.
• Drugs that inhibit the sympathetic nervous system are called sympatholytics.

See also:

• Parasympathetic nervous system
• Autonomic nervous system

• Schünke, Schüll, Schumacher,Voll, Wesker, "Prometheus, Head, Neck and Neuronanatomy", Chap. 3.1