Key Functions and Components of the 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 on Sympathetic Nervous System Explained
1. What is the sympathetic nervous system (SNS)?
The sympathetic nervous system (SNS) is a major division of the autonomic nervous system (ANS). Its primary role is to mobilise the body's resources under stress, triggering the rapid, involuntary responses associated with the "fight-or-flight" state. It prepares the body for immediate physical action in response to a perceived danger or threat.
2. What are the main functions of the sympathetic nervous system?
The sympathetic nervous system orchestrates several simultaneous responses to prepare the body for intense activity. Key functions include:
- Increasing heart rate and the force of heart contractions to improve blood flow.
- Dilating the pupils (mydriasis) to enhance vision.
- Redirecting blood flow away from non-essential organs, like the digestive system, towards skeletal muscles, the brain, and the heart.
- Increasing respiration rate and dilating the bronchioles to maximise oxygen intake.
- Stimulating the adrenal glands to release epinephrine (adrenaline) and norepinephrine (noradrenaline), which further amplify the stress response.
- Promoting the breakdown of glycogen to glucose in the liver for a quick energy supply.
3. How does the sympathetic nervous system differ from the parasympathetic nervous system?
The sympathetic and parasympathetic nervous systems are both divisions of the autonomic nervous system but have largely antagonistic (opposing) effects. The main difference lies in their function: the sympathetic system prepares the body for stressful situations ("fight-or-flight"), while the parasympathetic system promotes "rest-and-digest" activities, conserving energy and overseeing routine operations like digestion and urination. Their neurons also originate from different parts of the central nervous system.
4. Why is the sympathetic nervous system often called the "fight-or-flight" system?
This term is a functional description of the SNS's role in survival. When faced with a threat, the body must quickly decide whether to fight or flee. The SNS coordinates all the physiological changes needed for either action: it increases heart rate for circulation, boosts muscle blood flow for power, heightens senses like vision, and provides a rapid energy source. Because these changes are essential for confronting or escaping danger, the "fight-or-flight" label accurately summarises its primary, life-preserving function.
5. What are the key neurotransmitters involved in the sympathetic nervous system response?
The primary neurotransmitters of the sympathetic nervous system are acetylcholine (ACh) and norepinephrine (noradrenaline). Acetylcholine is released by preganglionic neurons to activate postganglionic neurons. The postganglionic neurons then release norepinephrine onto the target organs to trigger the "fight-or-flight" effects. Additionally, the SNS stimulates the adrenal medulla to release both epinephrine (adrenaline) and norepinephrine into the bloodstream, which act as hormones to sustain and enhance the sympathetic response throughout the body.
6. Where are the sympathetic ganglia located and what is their role?
The sympathetic ganglia are clusters of nerve cell bodies located just outside the spinal cord in the thoracic and lumbar regions. They form two chains that run parallel to the spinal column, known as the sympathetic trunks or chains. This arrangement is significant because it allows a single preganglionic neuron to synapse with multiple postganglionic neurons. This divergence enables the sympathetic nervous system to activate many different organs simultaneously, creating a widespread, coordinated, and rapid response crucial for the "fight-or-flight" mechanism.
7. Can overactivation of the sympathetic nervous system be harmful?
Yes, while essential for short-term survival, chronic or prolonged activation of the sympathetic nervous system can be harmful. Constant stress keeps the body in a "fight-or-flight" state, leading to conditions like chronic high blood pressure (hypertension), increased risk of heart disease, anxiety disorders, and a weakened immune system. This highlights the importance of the parasympathetic system in counterbalancing the SNS to maintain long-term health, a state known as homeostasis.
