Sympathetic and Parasympathetic

The Autonomic nervous system (ANS) is one of the components of the nervous system. It innervates all the musculature except the skeletal muscles. It influences the activity of sweat glands, adrenal glands, heart, smooth muscles and other glands. The basic objective of ANS is to maintain homeostasis i.e. regulation of the internal environment of the body. Thereby we can infer that it controls the bodily functions which are carried on without conscious control. The autonomic nervous system can be classified into two divisions namely,

• Parasympathetic nervous system (PNS)- the colloquial term is rest and digest
• Sympathetic nervous system (SNS) – the colloquial term is fight or flight

Chief Differences between Sympathetic and Parasympathetic Nervous System

Following are the basis on which we draw differences between the two divisions of the autonomic nervous system.

1. Site of Origin of Fibres

• Sympathetic fibres originate in thoracic (T1) and lumbar segments (L2/L3) of the spinal cord, hence also called ‘thoracolumbar division of ANS’
• Parasympathetic fibres originate in the sacral part of the spinal cord (sacral outflow) and cranial nerves (cranial outflow) i.e. oculomotor nerve/ third cranial nerve, facial nerve/ seventh cranial nerve, glossopharyngeal/ ninth cranial nerve and vagus nerve or tenth cranial nerve
• The vagus nerve has the maximum concentration of parasympathetic nerve fibres (75%)

2. Preganglionic Fibres

• The sympathetic fibres are short and have myelin sheath encasing
• They exit the spinal cord via the ventral root of the spinal nerves
• The parasympathetic preganglionic fibres are long and myelinated

3. Postganglionic Fibres

• Long and absent myelin sheath in postganglionic fibres of the sympathetic component
• These fibres pass to the viscera of sympathetic nerve division
• Short and non-myelinated nerves in the parasympathetic component

4. Location of Ganglia

• In PNS, the ganglia are embedded in the target tissue
• In the sympathetic nervous system, the site of ganglia is the paravertebral ganglia chain

5. Spread of activity of both the divisions

• The effect of the activity of sympathetic segments is over many parts, whereas the parasympathetic fibres have a localised effect, that is it has an impact on a single target organ

6. Chief Neurotransmitter

• In the case of the sympathetic component, the main neurotransmitter that is produced between pre and post ganglionic endings is acetylcholine (A-ch) and nor-epinephrine at postganglionic endings
• Acetylcholine is the primary neurotransmitter released at both pre and postganglionic endings

7. Conditions of Predomination

• The sympathetic component helps the individual to cope with situations of ‘flight or fight reactions whereas the parasympathetic component is concerned mainly with day-to-day living tasks

Influence of Sympathetic and Parasympathetic Segments of Various Organs

The two divisions of the Autonomic nervous system have different impacts on various organs of the body. The following table is a brief outline of the impact of sympathetic and parasympathetic segments on the organs.

Conclusion

The autonomic nervous system is divided into its two components i.e. sympathetic and parasympathetic components. They bring homeostasis by acting antagonistically to each other but act in a synergetic pattern to meet specific demands in imposed conditions. ANS is also known as the involuntary nervous system as it is concerned with vegetative aspects of daily life. The two divisions of ANS are responsible to upregulate and downregulate numerous functions of organs. Stressful events like emergencies and threats enhance sympathetic activity whereas relaxed events are much of concern with PNS.