What Is the Parasympathetic Nervous System?

The parasympathetic nervous system, abbreviated as PANS or PSNS, is a division of the autonomic nervous system, the other two being sympathetic and enteric nervous systems. The PANS is associated with rest and digest or feed and breed functioning of the body, as opposed to the sympathetic nervous system that deals with the fight or flight response.

The parasympathetic nerves arise from the central nervous system. The specific parasympathetic nerves include the vagus nerve, cranial nerves such as the facial nerve and oculomotor nerve, and pelvic splanchnic nerves.

Structure of the PSNS

The parasympathetic nerves primarily supply the given three areas:

Cranial Nerves: The cranial nerves of the cranium arise from the central nervous system and tend to synapse at otic, submandibular, ciliary and pterygopalatine parasympathetic ganglia. The ganglions then supply the effector organs via the trigeminal branches such as the mandibular nerve, ophthalmic nerve and maxillary nerve.
Vagus Nerves: The parasympathetic fibres of the vagus nerve have a broad reach near the thoracic viscera such as trachea, heart, oesophagus and abdominal viscera such as kidney, liver, pancreas and intestine.
Pelvic Splanchnic Nerves: The preganglionic nerve cell bodies of the pelvic splanchnic nerves arise from the T-12 or L-1 level of vertebra and terminate at the S2 or S4 of the vertebra. Their axons run away from the CNS to synapse at innervations near organs.

Parasympathetic Nervous System Function

Sensation: The afferent fibres of the autonomic nervous system (ANS) transfers signals from the internal organs of the body to the central nervous system (CNS). These afferent fibres are not divided into sympathetic and parasympathetic, instead this information is transmitted by general visceral afferent fibres.
Vascular Effects: The heart rate is controlled by activities of the pacemaker. The pacemaker, also called the sinoatrial node (SAN) has the ability to generate electrical impulses that are transmitted throughout the heart, thus maintaining the heart rate.

Along with the sinoatrial node, the two branches of the autonomic nervous system (sympathetic and parasympathetic system) work together to modulate the heart rate. The vagus nerve in particular acts on the SAN to slow the conduction of electrical impulse and thus modulate the vagal tone. This modulation is achieved by release of acetylcholine and changes in the ionic current of the heart cells.

The vagus nerve plays a crucial role in the modulation of heart rate. In simple words, increased vagal tone is associated with reduced and variable heart rate.

Sexual Activity: In males, the cavernous nerves stimulates the arteries of the penis to relax and fills blood in the penis.

In females, the pudendal nerves stimulate release of certain secretions that reduce friction. Another function of the parasympathetics is to innervate the fallopian tubes that causes peristaltic contraction and helps in the movement of oocytes to the uterus for implantation.

Receptors: The parasympathetic nervous system uses neurotransmitters such as acetylcholine to induce muscarinic and nicotinic receptors. Upon stimulation, the preganglionic nerves release acetylcholine at the ganglion which induces the postganglionic neurons of the nicotinic receptors. The postganglionic neuron in turn, stimulates the muscarinic receptor of the target organ.

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Frequently Asked Questions

What is the difference between sympathetic and parasympathetic nervous systems?

The sympathetic nervous system gets activated at the time of a quick response called a fight or flight response, whereas the parasympathetic nervous system restores the body after the effect of the sympathetic nervous system. It is called rest and digest and feed and breed response.

What does stress do to our body?

Stress activates the autonomic nervous system and in turn the sympathetic nervous system which gives us fight or flight response against stress.