The Parasympathetic Nervous System: 'Rest and Digest'

The Autonomic Nervous System (ANS)

  • The Autonomic Nervous System (ANS) is a vital part of the peripheral nervous system, primarily responsible for regulating involuntary bodily functions to maintain homeostasis.

  • It operates largely outside of conscious awareness, controlling functions such as heart rate, digestion, respiration, pupil dilation, and sexual arousal.

  • The ANS is typically divided into two main branches that often have opposing effects: the Sympathetic Nervous System (SNS) and the Parasympathetic Nervous System (PNS).

The Parasympathetic Nervous System (PNS): 'Rest and Digest'

  • Core Function: As highlighted, the parasympathetic nervous system is primarily responsible for the body's 'rest and digest' or 'feed and breed' activities. This means it becomes more active during periods of relaxation, calm, and when the body is conserving energy.

  • Physiological Effects during 'Rest and Digest': When the PNS is active, it promotes a range of bodily functions aimed at recovery and maintenance:

    • Heart Rate: Decreases heart rate and force of contraction to conserve energy.

    • Respiration: Constricts bronchi and slows breathing rate.

    • Digestion: Increases gastrointestinal motility and secretions (e.g., saliva, digestive enzymes) to facilitate the digestion and absorption of nutrients.

    • Pupils: Constricts pupils (miosis) to reduce light intake and improve near vision, suitable for close-up tasks like reading or eating.

    • Urinary System: Promotes bladder contraction for urination.

    • Sexual Arousal: Facilitates vasodilation leading to erection (in males) and engorgement (in females).

    • Glandular Secretions: Stimulates activity of salivary glands and lacrimal (tear) glands.

  • Neurotransmitters: The primary neurotransmitter used by the PNS at both preganglionic and postganglionic synapses is acetylcholine (ACh). These postganglionic neurons typically release ACh onto muscarinic receptors on target organs.

  • Anatomical Origin: The parasympathetic nerves originate from the craniosacral regions of the spinal cord (from cranial nerves III, VII, IX, X and sacral spinal nerves S22-S44).

    • The Vagus nerve (Cranial nerve X) is particularly significant, innervating many visceral organs in the thorax and abdomen, playing a massive role in regulating heart rate, breathing, and digestion.

Contrast with the Sympathetic Nervous System (SNS): 'Fight or Flight'

  • While the PNS promotes 'rest and digest', the Sympathetic Nervous System (SNS) is responsible for the body's 'fight or flight' response, preparing the body for stressful or emergency situations.

  • Opposing Effects: The SNS would typically:

    • Increase heart rate and blood pressure.

    • Dilate bronchi for increased oxygen intake.

    • Inhibit digestion.

    • Dilate pupils (mydriasis).

    • Divert blood flow to skeletal muscles.

  • Balance: The continuous interplay and balance between the PNS and SNS ensure the body can adapt to varying internal and external conditions, maintaining internal stability or homeostasis. Dysfunction in this balance can lead to various health issues.