Anatomy and physiology of sympathetic nervous system

The sympathetic nervous system (SNS) is one of the two main divisions of the autonomic nervous system, the other being the parasympathetic nervous system. The sympathetic system plays a crucial role in the body’s “fight or flight” response, preparing the organism to respond to stressful situations. Let’s explore the anatomy and physiology of the sympathetic nervous system in detail:

Anatomy

1. Location of Sympathetic Ganglia:

• Sympathetic ganglia are located near the spinal cord.
• The sympathetic chain, or sympathetic trunk, runs along both sides of the vertebral column.

1. Preganglionic and Postganglionic Neurons:

• Preganglionic neurons originate in the thoracic and lumbar regions of the spinal cord (thoracolumbar outflow).
• Preganglionic fibers synapse with postganglionic neurons in sympathetic ganglia.

1. Sympathetic Chain Ganglia:

• Ganglia in the sympathetic chain are interconnected and extend from the base of the skull to the coccyx.
• These ganglia house the cell bodies of postganglionic neurons.

1. Prevertebral Ganglia:

• Some preganglionic fibers pass through the sympathetic chain and synapse in prevertebral ganglia located anterior to the vertebral column.
• Prevertebral ganglia include the celiac ganglion, superior mesenteric ganglion, and inferior mesenteric ganglion.

Physiology

1. Preganglionic Neuron Activation:

• In response to stress or stimulation, preganglionic neurons release the neurotransmitter acetylcholine (ACh).
• ACh binds to nicotinic receptors on postganglionic neurons.

1. Postganglionic Neuron Activation:

• Upon activation, postganglionic neurons release the neurotransmitter norepinephrine (NE).
• NE binds to adrenergic receptors on target organs.

1. Adrenal Medulla:

• Preganglionic neurons also innervate the adrenal medulla (inner part of the adrenal gland).
• Adrenal medulla cells release epinephrine (adrenaline) and norepinephrine directly into the bloodstream.
• This enhances the overall “fight or flight” response.

1. Target Organs and Responses:

• Sympathetic activation prepares the body for physical activity and stress.
• Effects include increased heart rate, dilation of the airways, increased blood pressure, and mobilization of energy reserves.

1. Dilation of Pupils (Mydriasis):

• Sympathetic stimulation causes the pupils to dilate, enhancing vision for heightened alertness.

1. Inhibition of Digestive Functions:

• Sympathetic activation inhibits digestive processes, redirecting blood flow to essential organs involved in the stress response.

1. Vasoconstriction:

• Blood vessels constrict in non-essential areas, directing blood toward vital organs like the heart and muscles.

1. Glycogenolysis and Gluconeogenesis:

• The liver releases glucose into the bloodstream through the breakdown of glycogen (glycogenolysis) and the production of new glucose molecules (gluconeogenesis).

1. Sweat Gland Activation:

• Sympathetic stimulation activates sweat glands to cool the body during physical activity.

1. Pain Inhibition:
   • Sympathetic activation may inhibit pain perception in certain situations.

Understanding the anatomy and physiology of the sympathetic nervous system is crucial for comprehending the body’s response to stress and its role in maintaining homeostasis during challenging situations. The sympathetic and parasympathetic systems work in balance to regulate various bodily functions and ensure adaptability to changing environmental conditions.