In-Depth Notes on the Autonomic Nervous System

The Autonomic Nervous System Overview

• The autonomic nervous system (ANS) innervates smooth and cardiac muscle and glands, controlling visceral activity.
• The ANS operates without conscious awareness and has two main divisions: the sympathetic and parasympathetic.

General Structure of the ANS

• Neurons:
  • Preganglionic neurons: Reside in CNS (spinal cord or brainstem), have lightly myelinated axons, and always release acetylcholine (ACh).
  • Postganglionic neurons: Cell bodies are in autonomic ganglia, have unmyelinated axons, and can release either norepinephrine (NE) or acetylcholine depending on the division.

Neurotransmitters

• Main Neurotransmitters:
  • Norepinephrine (NE): Predominantly used by sympathetic fibers (99.9%).
  • Acetylcholine (ACh): Exclusively used in parasympathetic fibers (100%).
• Neurotransmitters act on specific receptors influencing target organ responses:
  • Cholinergic receptors: Nicotinic and muscarinic.
  • Adrenergic receptors: Alpha and beta subtypes.

Sympathetic Division (Fight or Flight)

• Origin: Thoracolumbar region (T1-L2).
• Preganglionic fibers: Short with long postganglionic fibers.
• Function: Mobilizes resources during activity, increases heart rate, dilates pupils, and decreases gastrointestinal activity.
  • Example responses include rapid heart rate, increased respiratory rate, and energy mobilization (glucose release from liver).
• Pathway: Can synapse at the same level, at a higher level, or at collateral ganglia (forming splanchnic nerves).

Parasympathetic Division (Rest and Digest)

• Origin: Craniosacral (Cranial nerves III, VII, IX, X & S2-S4).
• Preganglionic fibers: Long and extend to nearby ganglia near effector organs, leading to short postganglionic fibers.
• Function: Conserves energy and promotes maintenance functions, such as slowing heart rate, stimulating digestive activities, and constricting pupils.
  • Responsible for “rest and digest” responses such as increased gastrointestinal motility and salivation.

Dual Innervation

• Many organs receive input from both divisions:
  • The sympathetic division generally stimulates (increases heart and respiratory rates), while the parasympathetic division inhibits these activities (decreases heart rate, promotes digestion).
• Cooperative Effects: In functions like sexual arousal where the parasympathetic promotes erection, and sympathetic promotes ejaculation.

Anatomy of ANS

• Sympathetic ganglia: Located close to the spinal column, include the sympathetic trunk.
• Parasympathetic ganglia: Located near or in the target organ (terminal ganglia).
• Gray and White Communicants:
  • Gray rami (unmyelinated) carry postganglionic fibers.
  • White rami (myelinated) carry preganglionic fibers.

Disorders of the ANS

• Hypertension: High blood pressure linked to sympathetic overactivity.
• Raynaud's Disease: A condition that causes decreased blood flow to extremities in response to cold or stress.
• Autonomic Dysreflexia: A potentially life-threatening condition occurring with spinal cord injuries above T6, causing extreme hypertension.

Summary of Responses by the Divisions

• Sympathetic Effects:
  • Dilates pupils, increases heart rate, bronchodilation, glucose mobilization, and inhibits digestive functions.
• Parasympathetic Effects:
  • Constricts pupils, decreases heart rate, stimulates digestive functions, and promotes urination.

Conclusion

• Understanding the ANS's structure, functions, and balance between its sympathetic and parasympathetic divisions is critical for grasping how body functions are regulated under varying physiological conditions.