Human Body Function 102: Parasympathetic Nervous System

Faculty of Medicine Details

• Academic Year: 2024-2025
• Year: 1
• Module: Human Body Function (HBF) 102

Parasympathetic Nervous System II

• Presented by: Dr. Mohamed abo El Hassan and Dr. Ramadan Saad
• Department: Clinical Physiology

Objectives

• Describe the functions of the parasympathetic cranial nerves (VII Facial, IX Glossopharyngeal, and X Vagus).
• Describe the functions of the sacral part of the parasympathetic nervous system.
• Illustrate the relationship between the sympathetic and parasympathetic nervous systems.
• Compare the functions of sympathetic and parasympathetic systems.

Origin of Parasympathetic Nervous System

• Cranial outflow: Nuclei of the 3rd, 7th, 9th, and 10th cranial nerves in the brainstem.
• Sacral outflow: Lateral gray horn of S2-S4 segments of the spinal cord.

Cranial Outflow

• Preganglionic fibers from cranial outflow are carried by the 3rd, 7th, 9th, and 10th cranial nerves.
• These fibers terminate in ciliary, pterygopalatine, submandibular, otic, and peripheral ganglia.
• Postganglionic fibers innervate organs of the head, neck, thorax, and abdomen.

VII - Facial Nerve

• Origin & Relay: Superior salivary nucleus in pons, relays in the sphenopalatine ganglion via the superficial petrosal nerve.
• Functions:
  • Secretion of tears to lacrimal glands
  • Increase mucous secretion to mucous glands in nose, mouth, and pharynx
  • Vasodilation (VD) of blood vessels.

Chorda Tympani Nerve (Branch of Facial Nerve)

• Relay: Submandibular ganglion
• Postganglionic fiber functions:
  • Stimulation of submandibular and sublingual salivary glands, resulting in watery saliva with lower organic substance content.
  • Vasodilation of blood vessels in the anterior 2/3 of the tongue.

IX - Glossopharyngeal Nerve

• Origin & Relay: Inferior salivary nucleus in medulla oblongata, relays in the otic ganglion.
• Functions:
  • Stimulate secretion in the parotid salivary gland.
  • Vasodilation (VD) of blood vessels in the posterior 1/3 of the tongue

X - Vagus Nerve

• Origin & Relay: Dorsal nucleus in medulla oblongata, relays in terminal ganglia in thoracic and abdominal viscera.
• Functions:
  • Represents 75% of the parasympathetic supply.

Parasympathetic to thorax & abdomen

• In the Heart: Inhibits cardiac activity (decreases conductivity, contractility, excitability, and rhythmicity), constricts coronary blood vessels (decreasing blood flow).
• In the Lungs: Causes bronchoconstriction (contraction of bronchi and bronchioles), increases mucous secretion, and dilates pulmonary blood vessels.
• In the Gastrointestinal Tract: Contracts smooth muscles of the esophagus, stomach, small intestine, and proximal large intestine, relaxes sphincters, promotes swallowing and evacuation of food from the GIT, increases HCL and mucous secretion from gastric glands, and dilates blood vessels.
• In the Liver: Stimulates bile secretion, and contracts gall bladder wall and relaxes the sphincter of Oddi for bile evacuation.
• In the Pancreas: Stimulates pancreatic juice secretion rich in enzymes and insulin secretion by B-cells.

Why It's Called Vagus?

• Afferent and efferent functions
• Stimulatory and inhibitory effects
• Widespread distribution

Sacral Outflow

• Preganglionic fibers arise from the L2,3,4 sacral segments.
• Carried by pelvic splanchnic nerves to terminal ganglia in the pelvis where they synapse.
• Postganglionic fibers innervate organs of the pelvis and lower abdomen.

Sacral Division of Parasympathetic

• Origin: Sacral nerve roots S2, S3, and S4
• Relay ganglia: Hypogastric ganglia

Parasympathetic to the Pelvis (Pelvic nerve)

• Urinary bladder: Wall contraction and internal urethral sphincter relaxation, enabling micturition.
• Rectum & anal canal: Wall contraction and internal anal sphincter relaxation, enabling defecation.
• Sex Organs: Vasodilation of blood vessels in the genitals, contributing to erection in males and clitoris in females.

Micturition

• Pelvic nerve contracts the urinary bladder wall and relaxes the internal urethral sphincter to allow urine passage.

Defecation

• Pelvic nerve contracts the rectum wall and relaxes the internal anal sphincter to facilitate stool passage. Important Note: External urethral/anal sphincters are under somatic control (pudendal nerve), not autonomic.

Summary of Parasympathetic Activity

• "Rest and digest" responses including:
  • Decreased metabolic rate
  • Decreased heart rate and blood pressure
  • Increased secretion from salivary and digestive glands
  • Increased digestive tract motility and blood flow
  • Stimulation of urination and defecation.
• Also known as the anabolic system, which increases nutrients in the blood for growth and development and storing as lipids or glycogen for energy.

Relation between Sympathetic and Parasympathetic

• Commonly antagonistic (opposed) action in the heart, lungs, and pelvis
• Sometimes synergistic (collaborative) in, for example, salivary secretion; when one system provides trophic/nutritious support, the other provides the active secretion.
• Complementary actions in sexual intercourse, where parasympathetic nerves regulate erection and sympathetic nerves regulate ejaculation in males and orgasm in females.
• Generally, these actions are balanced to maintain homeostasis.

Contrast between Sympathetic and Parasympathetic

• Sympathetic (GAS PEDAL): Fight or flight, protection and survival, stress response, adrenal activation.
• Parasympathetic (BRAKE PEDAL): Rest, digest, relax, growth, development.

Interactive Questions

• General vagus nerve, except for absence of direct effect on ventricular contraction
• Pelvic nerve stimulation, with regards to specific actions or regions of the body.
• Sympathetic tone initiation, factors affecting control, and effects of blocking drugs.
• Effects of vagal tone on the heart.