Detailed Study Notes on the Parasympathetic Nervous System

Parasympathetic Nervous System Overview

  • Definition: The parasympathetic nervous system (PNS) is one of the two main divisions of the autonomic nervous system, primarily responsible for regulating the body's "rest and digest" activities.

Function of the Parasympathetic Nervous System

  • The primary controller of most autonomic functions during the majority of the time.

  • Key roles:

    • Bronchi: Constriction of the bronchi and secretion of mucus.

    • Eyes:

    • Accommodation: Adjustment of the lens for near vision.

    • Miosis: Constriction of the pupils.

    • Saliva: Production of copious, watery saliva.

    • Gastrointestinal (GI) Tract:

    • Enhanced secretion of digestive juices.

    • Increased peristalsis (wave-like muscle contractions).

    • Increased sphincter tone.

Responses to Parasympathetic Activation

  • Heart:

    • Decreases heart rate and lowers blood pressure.

  • Bladder:

    • Manages sphincter tone and promotes contraction of the detrusor muscle, aiding in urination.

Effects of Parasympathetic Stimulation

  • Often referred to as the “Rest and Digest” system, it promotes the following physiological processes known as the 3Ds:

    • Digestion

    • Defecation

    • Diuresis (increased urination)

  • Additional effects include Lacrimation (tearing).

  • SLUDD response: Salivation, Lacrimation, Urination, Digestion, Defecation.

Parasympathetic Nervous System Structure

Preganglionic Neurons

  • Preganglionic sympathetic cell bodies are located in the:

    • Brainstem.

    • Lateral gray columns of sacral spinal segments S2, S3, and S4.

  • This system is referred to as the craniosacral division.

  • Axons are carried via:

    • Cranial Nerves III (Oculomotor), VII (Facial), IX (Glossopharyngeal), and X (Vagus).

    • Note: CN X (Vagus) accounts for approximately 75% of total parasympathetic outflow.

    • Pelvic splanchnic nerves from sacral segments; these do not emerge from spinal nerves.

  • Preganglionic axons terminate at terminal ganglia, located very close to or in the walls of the organs they innervate.

Terminal Ganglia and Postganglionic Neurons

  • Preganglionic neurons synapse with one postganglionic neuron at a terminal ganglion, leading to:

    • Finite Control: The action results in specific and precise responses.

  • Components of a parasympathetic ganglion (terminal ganglion):

    • Axons of preganglionic neurons.

    • Dendrites, cell bodies, and axons of postganglionic neurons.

  • Function of postganglionic axons:

    • Direct innervation of the target organ.

    • No gray or white rami communicantes connected.

Parasympathetic Outflow

Cranial Outflow via Cranial Nerves

  • Cranial Nerves and their Functions:

    • Ciliary ganglion (CN III): Innervates smooth muscles of the eye for miosis and accommodation.

    • Pterygopalatine ganglion (CN VII): Innervates the lacrimal gland to promote lacrimation.

    • Submandibular ganglion (CN VII): Innervates submandibular and sublingual salivary glands for salivation.

    • Otic ganglion (CN IX): Innervates the parotid salivary glands for salivation.

    • Vagus nerve (CN X): This nerve innervates the heart, lungs, and most visceral organs, with influences spread throughout the thoracic and abdominal viscera and terminating at specific target organs.

Sacral Outflow via Pelvic Splanchnic Nerves

  • Originate from spinal levels S2-S4 and involve innervation to:

    • Large intestine.

    • Urinary bladder.

    • Ureters.

    • Reproductive organs.

Peripheral Ganglia - Terminal Ganglia Locations

  • Specific Terminal Ganglia:

    • Ciliary ganglion: Connected to CN III for eye muscle innervation.

    • Pterygopalatine ganglion: Connected to CN VII for lacrimal gland function.

    • Submandibular ganglion: Connected to CN VII for salivation of submandibular glands.

    • Otic ganglion: Connected to CN IX for parotid gland innervation.

Nuclei and Connection Points in Central Nervous System

  • Key nuclei associated with the parasympathetic nervous system include:

    • Edinger-Westphal nucleus (CN III): Responsible for controlling smooth muscles of the eye, mediating pupillary constriction and accommodation.

    • Dorsal motor nucleus of the vagus (CN X): Supplies parasympathetic fibers to thoracic and abdominal viscera.

    • Inferior salivatory nucleus (CN IX): Involved in saliva production from the parotid gland.

    • Superior salivatory nucleus (CN VII): Involved in salivation processes targeted by facial nerve.

Cortical and Peripheral Nerve Interactions

  • Understanding interactions between the vagus nerve and various organs:

    • Vagus Nerve Pathways:

    • Innervates the heart, liver, colon, stomach, lungs, spleen, kidney, and small intestine, indicating a broad functional role across diverse organ systems.

  • Vagal Branch Connections:

    • Celiac plexus: Supplies nerve fibers to organs such as the stomach, liver, pancreas, and intestines.

    • Pulmonary and cardiac plexuses: Provide regulation of pulmonary and cardiac functions.

Summary of Parasympathetic Nervous System Effects

  • Promotes maintenance of homeostasis within the body through rest and digestive functions.

  • Broad influence across various organ systems, ensuring balance in metabolic processes and stimulation of restorative activities post-exercise or after stress.