Parasympathetic Nervous System - Autonomic Nervous system

The parasympathetic nervous system, often referred to as the "rest and digest" system, is a crucial component of the autonomic nervous system, responsible for conserving energy and maintaining bodily functions during periods of relaxation.

It is characterized by craniosacral outflow, indicating that its preganglionic neurons originate from the brainstem and sacral spinal cord.

Craniosacral Outflow

The parasympathetic nervous system originates from specific cranial nerves and the sacral division of the spinal cord.

  • Cranial Nerve 3: Oculomotor nerve

    • Fibers travel with the oculomotor nerve to the ciliary ganglion.

  • Cranial Nerve 7: Facial nerve

    • Arises from the superior salivatory nucleus in the pons.

    • Its fibers travel via the greater petrosal nerve and chorda tympani to reach the pterygopalatine and submandibular ganglia, respectively.

  • Cranial Nerve 9: Glossopharyngeal nerve

    • Originates from the inferior salivatory nucleus in the medulla.

    • Fibers travel via the lesser petrosal nerve to the otic ganglion.

  • Cranial Nerve 10: Vagus nerve

    • The vagus nerve is the primary nerve of the parasympathetic nervous system, innervating various organs in the thorax and abdomen.

  • Sacral Division: S2 to S4

  • Preganglionic fibers from the sacral spinal cord form the pelvic splanchnic nerves.

  • These nerves innervate the pelvic organs, including the bladder, rectum, and reproductive organs.

Hypothalamus

  • The hypothalamus serves as the control center for the activity of the parasympathetic nervous system, integrating sensory information and coordinating appropriate autonomic responses.

  • It is connected to the pituitary gland via the infundibulum, allowing for hormonal regulation of parasympathetic functions.

  • The parasympathetic portion of the hypothalamus sends presynaptic fibers downwards to stimulate the nuclei of:

    • Cranial nerve 3: Edinger-Westphal nucleus

    • Cranial nerve 7: Superior salivatory nucleus

    • Cranial nerve 9: Inferior salivatory nucleus

    • Cranial nerve 10: Dorsal vagus nucleus

  • This initiates parasympathetic responses throughout the body.

Cranial Nerve 3: Oculomotor Nerve

  • Originates from the Edinger-Westphal nucleus, located in the midbrain.

  • The oculomotor nerve contains both parasympathetic and somatic motor fibers, enabling control of eye movements and pupillary constriction.

  • The Edinger-Westphal nucleus sends out long preganglionic parasympathetic motor neurons that travel with the oculomotor nerve.

  • These neurons synapse on the ciliary ganglion, a peripheral ganglion located near the eye.

  • The ciliary ganglion is an example of an intramural or terminal ganglion.

  • Postganglionic fibers from the ciliary ganglion innervate:

    • Ciliary muscle - Contraction of the ciliary muscle relaxes the zonules (suspensory ligaments). This allows the lens to become more globular, aiding in near vision or accommodation, enabling the eye to focus on close objects.

    • Sphincter pupillae - Contraction of the sphincter pupillae causes pupil constriction (miosis). This reduces the amount of light entering the eye and assists in near vision, improving focus and depth of field.

Cranial Nerve 7: Facial Nerve

  • Associated with the superior salivatory nucleus, located around the pons-medulla junction.

  • The facial nerve sends out long preganglionic axons via the greater petrosal nerve and chorda tympani, facilitating parasympathetic innervation of various glands.

  • These axons go to:

    • Pterygopalatine ganglion (via the greater petrosal nerve), innervating the lacrimal glands, nasal glands, and palatine glands.

    • Stimulates watery secretions, such as tears from the lacrimal glands and nasal secretions, through acetylcholine release, aiding in lubrication and protection of the eyes and nasal passages.

    • Submandibular ganglion (via the chorda tympani), innervating the submandibular and sublingual salivary glands.

    • Increases the production of enzyme-rich saliva (containing salivary amylase) for breaking down carbohydrates, facilitating digestion.

Cranial Nerve 9: Glossopharyngeal Nerve

  • Associated with the inferior salivatory nucleus in the medulla.

  • The glossopharyngeal nerve sends fibers via the lesser petrosal nerve to the otic ganglion.

  • The otic ganglion innervates the parotid salivary gland, responsible for producing saliva.

  • Stimulates the release of acetylcholine onto muscarinic receptors of the parotid gland, increasing salivary secretions and salivary amylase, aiding in the digestion of carbohydrates.

Cranial Nerve 10: Vagus Nerve

  • The dorsal nucleus of the vagus nerve, located in the medulla, serves as the origin of the vagus nerve.

  • Controls approximately 90% of parasympathetic responses, making it the primary regulator of internal organ functions.

  • About 70% of the afferent neurons within the abdominal visceral area are vagus nerve, conveying sensory information from the internal organs to the brain.

  • Various plexuses:

    • Pulmonary Plexus - Goes to the lungs and bronchi. Causes bronchoconstriction due to smooth muscle contraction, reducing airflow into the lungs, which leads to increase in mucus production, which can help trap pathogens but may also contribute to respiratory congestion.

    • Cardiac Plexus - Goes to the SA and AV node in the heart. More specifically, the left vagus nerve innervates the AV node and the left vagus nerve innervates the SA node. The nerve impulses decreases the action potentials of the SA and AV nodes, slowing down the heart rate (negative chronotropic effect), which leads to a decrease in blood pressure and reduced cardiac output.

    • Esophageal Plexus - Nerves go to the esophagus. Increases peristalsis when the nerve is stimulated, facilitating the movement of food down the esophagus. The esophageal plexus also has Gastric branches innervating the anterior and posterior gastric branches of the stomach. This increases motility via peristalsis, promoting the mixing and movement of stomach contents.

    • Celiac Plexus (Right Vagus Nerve) - Supplies the kidney (increase urine formation), ureter (increase ureter constriction), ascending colon, transverse colon (proximal two-thirds), duodenum, jejunum, ileum, and pancreas. When stimulated, it increases motility in the colon and small intestine, promoting the movement of intestinal contents and stimulates insulin release from pancreatic beta cells, regulating blood glucose levels. The celiac plexus also innervates the spleen.

    • Hepatic Plexus (Left Vagus Nerve) - Goes to the liver and gallbladder. Stimulates glycogenesis and gallbladder contractions, aiding in bile release. It also relaxes the sphincter of Oddi, allowing bile and pancreatic enzymes to enter the duodenum for digestion.

Sacral Division: S2-S4

  • Presynaptic fibers from S2, S3, and S4 join together to form the ‘PELVIC‘ splanchnic nerve. It stimulates nuclei in the lateral gray horn of the spinal cord.

  • Innervates structures not supplied by the vagus nerve, including the gonads, bladder, internal urethral sphincter, internal anal sphincter, rectum, sigmoid colon, and the distal one-third of the transverse colon.

  • Effects:

    • Increases motility in the sigmoid colon, distal transverse colon, and descending colon via peristalsis, promoting the movement of fecal matter.

    • Causes explosive contractions in the rectum and relaxes the internal anal sphincter, facilitating defecation.

    • Stimulates bladder contractions and relaxes the internal urethral sphincter, allowing urination.

    • In males, increases blood flow to the penis, aiding in erection through nitric oxide release.

    • In females, increases blood flow to the clitoris, causing engorgement.

  • The plexus associated with the Inferior Hypogastric plexus.