Sympathetic nervous system

The (ortho-) sympathetic nervous system (SNS) is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response. It is, however, constantly active at a basal level to maintain homeostasis.

Overview
Alongside the other two components of the autonomic nervous system, the sympathetic nervous system aids in the control of most of the body's internal organs. Stress—as in the flight-or-fight response—is thought to counteract the parasympathetic system, which generally works to promote maintenance of the body at rest. In truth, the functions of both the parasympathetic and sympathetic nervous systems are not so straightforward, but this is a useful rule of thumb.

There are two kinds of neurons involved in the transmission of any signal through the sympathetic system; pre- and post- ganglionic. The shorter preganglionic neurons originate from the thoracolumbar region of the spinal cord (levels T1 - L2, specifically) and travel to a ganglion, often one of the paravertebral ganglia, where they synapse with a postganglionic neuron. From there, the long postganglionic neurons extend across most of the body.

At the synapses within the ganglia, 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 postganglionic neurons innervating sweat glands&mdash;which release acetylcholine for the activation of muscarinic receptors - and the adrenal medulla. The adrenal medulla develops in tandem with the sympathetic nervous system, and acts as a modified sympathetic ganglion: synapses occur between pre- and post- ganglionic neurons within it, but the post ganglionic neurons do not leave the medulla; instead they directly release norepinephrine and epinephrine into the blood.

Function
The sympathetic nervous system is responsible for up- and down-regulating many homeostatic mechanisms in living organisms. Fibers from the SNS innervate tissues in almost every organ system, providing at least some regulatory function to things as diverse as pupil diameter, gut motility, and urinary output. It is perhaps best known for mediating the neuronal and hormonal stress response commonly known as the fight-or-flight response. This response is also known as sympatho-adrenal response of the body, as the preganglionic sympathetic fibers that end in the adrenal medulla (but also all other sympathetic fibers) secrete acetylcholine, which activates the great secretion of adrenaline (epinephrine) and to a lesser extent noradrenaline (norepinephrine) from it. Therefore, this response that acts primarily on the cardiovascular system is mediated directly via impulses transmitted through the sympathetic nervous system and indirectly via catecholamines secreted from the adrenal medulla.

Some evolutionary theorists suggest that the sympathetic nervous system operated in early organisms to maintain survival as the sympathetic nervous system is responsible for priming the body for action. One example of this priming is in the moments before waking, in which sympathetic outflow spontaneously increases in preparation for action.

Organization
Sympathetic nerves originate inside the vertebral column, toward the middle of the spinal cord in the intermediolateral cell column (or lateral horn), beginning at the first thoracic segment of the spinal cord and are thought to extend to the second or third lumbar segments. Because its cells begin in the thoracic and lumbar regions of the spinal cord, the SNS is said to have a thoracolumbar outflow. Axons of these nerves leave the spinal cord through the anterior rootlet/root. They pass near the spinal (sensory) ganglion, where they enter the anterior rami of the spinal nerves. However, unlike somatic innervation, they quickly separate out through white rami connectors (so called from the shiny white sheaths of myelin around each axon) that connect to either the paravertebral (which lie near the vertebral column) or prevertebral (which lie near the aortic bifurcation) ganglia extending alongside the spinal column.

To reach target organs and glands, the axons must travel long distances in the body, and, to accomplish this, many axons relay their message to a second cell through synaptic transmission. The ends of the axons link across a space, the synapse, to the dendrites of the second cell. The first cell (the presynaptic cell) sends a neurotransmitter across the synaptic cleft where it activates the second cell (the postsynaptic cell). The message is then carried to the final destination.

Presynaptic nerves' axons terminate in either the paravertebral ganglia or prevertebral ganglia. This can occur through one of four methods:

1. The nerve enters the paravertebral ganglion at the level of its originating spinal nerve, and then ascends to a more superior paravertebral ganglion, where it synapses with the postsynaptic cell.

2. The nerve enters the paravertebral ganglion at the level of its originating spinal nerve and synapses with the postsynaptic cell at that level.

3. The nerve enters the paravertebral ganglion at the level of its originating spinal nerve, and then descends to a more inferior paravertebral ganglion, where it synapses with the postsynaptic cell.

4. The nerve enters the paravertebral ganglion at the level of its originating spinal nerve and then descends to a prevertebral ganglion, where it synapses 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.

A notable exception to the routes mentioned above is the sympathetic innervation of the suprarenal (adrenal) glands. In this case, presynaptic neurons pass through paraverterbral ganglia, on through prevertebral ganglia and then synapse directly with suprarenal tissue. This tissue consists of cells that have pseudo-neuron like qualities in that when activated by the presynaptic neuron, they will release their neurotransmitter (epinephrine) directly into the blood stream.

In the SNS and other components of the peripheral nervous system, these synapses are made at sites called ganglia. The cell that sends its fiber is called a preganglionic cell, while the cell whose fiber leaves the ganglion is called a postganglionic cell. As mentioned previously, the preganglionic cells of the SNS are located between the first thoracic segment and third lumbar segments of the spinal cord. Postganglionic cells have their cell bodies in the ganglia and send their axons to target organs or glands.

The ganglia include not just the sympathetic trunks but also the cervical ganglia (superior, middle and inferior), which sends sympathetic nerve fibers to the head and thorax organs, and the celiac and mesenteric ganglia (which send sympathetic fibers to the gut).

Information transmission
Messages travel through the SNS in a bidirectional flow. Efferent messages can trigger changes in different parts of the body simultaneously. For example, the sympathetic nervous system can accelerate heart rate; widen bronchial passages; decrease motility (movement) of the large intestine; constrict blood vessels; increase peristalsis in the esophagus; cause pupillary dilation, piloerection (goose bumps) and perspiration (sweating); and raise blood pressure. Afferent messages carry sensations such as heat, cold, or pain.

The first synapse (in the sympathetic chain) is mediated by nicotinic receptors physiologically activated by acetylcholine, and the target synapse is mediated by adrenergic receptors physiologically activated by either noradrenaline (norepinephrine) or adrenaline (epinephrine). An exception is with sweat glands, which receive sympathetic innervation but have muscarinic acetylcholine receptors, which are normally characteristic of Parasympathetic nervous system. Another exception is with certain deep muscle blood vessels, which dilate (rather than constrict) with an increase in sympathetic tone. This is because of the presence of more beta2 receptors (rather than alpha1, which are frequently found on other vessels).

Sympathicotonia
Sympathicotonia is a stimulated condition of the sympathetic nervous system, marked by vascular spasm, elevated blood pressure, and goose bumps.