Autonomic Nervous System Overview

Autonomic Nervous System

• The autonomic nervous system (ANS) has two divisions: sympathetic and parasympathetic.
• The sympathetic nervous system is also called the thoracolumbar division.
• The parasympathetic nervous system is also called the craniosacral division.

Anatomical Differences between the Sympathetic and Parasympathetic Divisions

• The preganglionic fibers of the sympathetic nervous system are short.
• The preganglionic fibers of the parasympathetic nervous system are long.
• The postganglionic fibers of the sympathetic nervous system are long.
• The postganglionic fibers of the parasympathetic nervous system are short.

Functional differences between the Sympathetic and Parasympathetic Nervous Systems

• The sympathetic nervous system is responsible for the "fight or flight" response.
• The parasympathetic nervous system is responsible for the "rest and digest" response.

### Neurotransmitter release

• The preganglionic axons of both branches release acetylcholine.
• The postganglionic axons of the sympathetic nervous system mainly release norepinephrine.
• The postganglionic axons of the parasympathetic nervous system release acetylcholine.

Receptors involved in parasympathetic nervous system

• Nicotinic Receptors:
  • Stimulate all autonomic ganglia.
  • Secretion of norepinephrine and epinephrine from the suprarenal gland.
  • Skeletal muscle contraction.
• Muscarinic Receptors:
  • Increase HCl secretion in the stomach.
  • Decrease heart rate (negative chronotropy).
  • Slow heart conductivity (negative dromotropy).
  • Decrease atrial contraction (negative inotropic effect).
  • Bronchoconstriction and increase bronchial secretion in the respiratory tract.
  • Increase secretion of glands (salivation, sweating, lacrimation, etc.).
  • Contraction of the urinary bladder wall and relaxation of the ureter and sphincters of the urinary tract.
  • Increase intestinal motility and secretion, and relax sphincters.
  • Accommodation to near vision.

### Acetylcholine

• Acetylcholine is a neurotransmitter that is rapidly hydrolyzed by neuronal acetylcholinesterase and plasma butyrylcholinesterase.
• Acetylcholine is the primary transmitter in postganglionic sympathetic and parasympathetic nerves, thermoregulatory sweat glands, and neuromuscular junctions.

Explore the crucial functions and anatomical differences between the sympathetic and parasympathetic divisions of the autonomic nervous system. Understand their roles in responses such as 'fight or flight' and 'rest and digest'. Test your knowledge on neurotransmitter release and other key concepts related to the ANS.