Detailed Notes on the Autonomic Nervous System: Parasympathetic and Enteric Systems

Overview of the Autonomic Nervous System (ANS)

• The ANS consists of three main components: the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system.

Parasympathetic Nervous System

• Basic Anatomical Plan:

  • Similar to sympathetic nervous system.

  • Consists of a nerve cell from the central nervous system to a ganglion, then to the target.

• Origin Points:

  • Nerve fibers emerge from the brain (cranial nerves) or the sacral part of the spinal cord (bottom).

  • Preganglionic fibers (from CNS to ganglia) are long, while postganglionic fibers (to the target) are short.

  • Main cranial nerves involved: III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus); especially, the vagus nerve.

Vagus Nerve

• Comprises about 75% of all parasympathetic fibers.

• Innervates many organs, including the heart and GI tract.

Neurotransmitters and Receptors

• Ganglion Level:

  • Both sympathetic and parasympathetic systems use acetylcholine (ACh) at the ganglion.

  • Nicotinic N2 receptors are present here (ligand-gated ion channels).

• Postganglionic Level:

  • The parasympathetic system exclusively releases ACh onto muscarinic receptors (G-protein coupled receptors).

  • Effects:

  • Heart: Decreases heart rate via M2 receptors.

  • Gastrointestinal Tract: Increases motility and secretions via M3 receptors.

  • Eyes: Controls the ciliary muscle for focusing.

Key Differences Between Sympathetic and Parasympathetic Systems

• Preganglionic Fiber Length:

  • Sympathetic: Short

  • Parasympathetic: Long

• Postganglionic Fiber Length:

  • Sympathetic: Long

  • Parasympathetic: Short

• Final Neurotransmitters:

  • Sympathetic: Noradrenaline

  • Parasympathetic: Acetylcholine

Enteric Nervous System

• Independence:

  • Over 100 million neurons; functions independently and is embedded entirely within the gastrointestinal (GI) tract.

  • Controls gastrointestinal motility and secretion; influences sphincters (e.g., lower esophageal, pyloric, ileocecal).

• Neurotransmitters:

  • Uses ACh at ganglia; however, has a broad range of additional neurotransmitters (e.g., substance P, vasoactive intestinal peptide) for diverse functions.

Coordination and Control

• Central Control:

  • The hypothalamus and pituitary gland play significant roles in regulating the autonomic nervous system.

• **Quiz Review:

  • Sympathetic NS originates from thoracic and lumbar regions of spinal cord.

  • Parasympathetic NS has long preganglionic fibers and short postganglionic fibers.

  • Noradrenaline indicates sympathetic activity, while acetylcholine is common in autonomic ganglia.

Diagrams and References

• Detailed diagrams and further reading can be found in recommended texts, such as Silverthorne (8th edition) and Sherwood.

These notes highlight the basic anatomy, functions, and differences of the autonomic nervous system, particularly focusing on the parasympathetic and enteric systems, which should be useful for exam preparation.