Sympathetic Nervous System

What is a Sympathetic Nervous System?

The sympathetic nervous system is a division of the nervous system that is responsible for forming localised adjustments (such as sweating in response to a rise in temperature) and reflexing cardiovascular system adjustments. 

Under stressful circumstances, the entire sympathetic nervous system can be activated, resulting in an immediate widespread reaction, which is called the fight-or-flight response. This particular response is characterized by the release of large epinephrine quantities from the adrenal gland, which is an increase in the heart rate and in cardiac output and piloerection.


Actions of Sympathetic Nervous System

The actions of the sympathetic nervous system take place in accordance with other hormonal or neural responses to stress, including an increase in cortisol and corticotropin secretion. In the case of humans, chronic stress results in a long-term stimulation of fight-or-flight response that leads to the constant secretion and production of catecholamines (for example, epinephrine), including the hormones such as cortisol.

A long-term stress-induced secretion of these specific substances is associated with various physiological consequences, including hyperglycemia (which is the high blood glucose levels), which can lead to diabetes mellitus of type 2, and hypertension (at high blood pressure), which can lead to the cardiovascular diseases.

Anatomically, the sympathetic preganglionic neuron, the cell bodies, the ones which are located within the central nervous system that originates in the lateral horns of the 12 thoracics and either the spinal cord's first 2 or 3 lumbar segments. (The sympathetic system, also known as the thoracolumbar outflow, is responsible for this.) These neurons' axons exit the spinal cord in the ventral roots, where they synapse with either specialised cells or sympathetic ganglion cells in the adrenal gland, which are known as chromaffin cells.

The sympathetic nervous system is one of two antagonistic sets of nerves in the autonomic nervous system, whereas the parasympathetic nervous system is the other.


Structure

There exist two kinds of neurons, which are involved in the transmission of any signal with the help of the sympathetic system: preganglionic and postganglionic. The shorter preganglionic neurons get originated in the thoracolumbar division of the spinal cord, particularly at T1 to L2~L3, and travel to the ganglion, often one of the paravertebral ganglia, where they synapse with the postganglionic neuron. From that point, the long postganglionic neurons extend to most of the body.

At the synapses within ganglia, the preganglionic neurons release acetylcholine, which is a neurotransmitter that activates the nicotinic acetylcholine receptors on postganglionic neurons. The postganglionic neurons release norepinephrine in response to this stimulation, which activates the adrenergic receptors on the peripheral target tissues. The target tissue receptor's activation causes the effects associated with the sympathetic system.


Organization of the Sympathetic Nervous System

Sympathetic nerves emerge from the intermediolateral nucleus of the lateral grey column in the middle of the spinal cord, starting at the first thoracic vertebra and extending to the second or third lumbar vertebra. Due to this reason, its cells begin in the thoracolumbar division, the lumbar and thoracic regions of the spinal cord - the sympathetic nervous system is explained to have a thoracolumbar outflow.

These nerves' axons leave the spinal cord via the anterior root. They pass near the spinal (otherwise sensory) ganglion, where they enter the anterior rami of spinal nerves. However, unlike the somatic innervation, they separate out through the white rami connectors quickly (which are so-called from the shiny white sheaths of myelin near every axon) that connect to either the paravertebral (that lie near vertebral column) or the prevertebral (that lie near aortic bifurcation) ganglia extending the spinal column alongside.

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


Information Transformation in Sympathetic Nervous System

Messages travel via sympathetic nervous systems in a bi-directional flow. Different messages may trigger changes in various body parts simultaneously. For example, sympathetic nervous systems can widen bronchial passages; accelerate the heart rate; constrict blood vessels; decrease motility (or movement) of the large intestine; increase the peristalsis in the oesophagus; cause goosebumps, sweating, and raise blood pressure. An exception is with certain blood vessels like those present in the coronary and cerebral arteries that dilate (rather than constrict) with an increase in the sympathetic tone.


Functions of Sympathetic Nervous System

In living organisms, the sympathetic nervous system is entirely responsible for both up-and down-regulating several homeostatic mechanisms. Fibres from SNS innervate tissues in almost each of the organ systems by providing at least a few regulations of functions as diverse as pupil diameter, urinary system function and output, and gut motility. Perhaps, it is best known for mediating the hormonal and neuronal stress response, commonly called the fight-or-flight response.

FAQs (Frequently Asked Questions)

1. How is Sensory Information Passed in a Sympathetic Nervous System?

Answer: The autonomic nervous system's afferent fibres, which transmit the sensory information from internal body organs back to the central nervous system (otherwise CNS), are not divided into the sympathetic and parasympathetic fibres as the efferent fibres are. Instead, autonomic sensory information can be conducted by the general visceral afferent fibres.

2. Explain the Relationship Between the Sympathetic and the Parasympathetic Nervous System.

Answer: Including the other autonomic nervous system's components, the sympathetic nervous system as well the parasympathetic nervous system aids in the control of most of the internal body organs. Reaction to the stress, as in the flight-or-fight response, is thought to counteract the parasympathetic system that generally works to promote body maintenance at rest. The comprehensive functions of both the sympathetic and parasympathetic nervous systems are not straightforward but is a useful rule of the thumb.

3. What are the Disorders of the Sympathetic Nervous System?

Answer: In heart failure, the sympathetic nervous system increases the activity, leading to an increased force of muscular contractions that in turn increases the volume of the stroke and the peripheral vasoconstriction to maintain the blood pressure. But, these effects accelerate the disease progression, eventually increasing mortality in the failure of the heart.

4. Explain the Etymology of the Sympathetic Nervous System?

Answer: The name of this specific system is traced to the sympathy concept, in the sense of "connection between the parts", which is first used medically by Galen.