Autonomic Nervous System Overview

Autonomic Nervous System

• The Autonomic Nervous System (ANS) is divided into 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.
• The ANS is also called the visceral motor system and controls involuntary bodily functions.

Anatomical Differences between Sympathetic and Parasympathetic Divisions

• Sympathetic preganglionic fibers are short and postganglionic fibers are long.
• Parasympathetic preganglionic fibers are long and postganglionic fibers are short.

Functional Differences between Sympathetic and Parasympathetic Nervous Systems

• The Sympathetic Nervous System mainly releases norepinephrine from postganglionic axons
• The Parasympathetic Nervous System mainly releases acetylcholine from postganglionic axons
• Both Sympathetic and Parasympathetic preganglionic axons release acetylcholine
• Acetylcholine is a naturally occurring neurotransmitter rapidly hydrolyzed by neuronal acetylcholinesterase and plasma butyrylcholinesterase.

Receptors involved in Parasympathetic Nervous System

• There are two types of cholinergic receptors: Nicotinic and Muscarinic
• Nicotinic receptors (N1 or NN) are located on all autonomic ganglia and mediate the release of norepinephrine and epinephrine from the adrenal gland
• Nicotinic receptors (N2 or NM) are located on skeletal muscle and mediate skeletal muscle contraction
• Muscarinic receptors (M) are located on all effector cells stimulated by postganglionic cholinergic fibers
• Muscarinic (M1) receptors are located in the stomach and increase HCl secretion
• Muscarinic (M2) receptors are located in the heart and decrease heart rate, conduction velocity, and force of contraction
• Muscarinic (M3) receptors are located in blood vessels and cause vasodilation due to release of nitric oxide (NO)
• Muscarinic (M3) receptors are also located in the eye and cause miosis, reduction in intraocular pressure (IOP) and accommodation to near vision
• Muscarinic (M3) receptors are located in the respiratory tract and cause bronchoconstriction and increased bronchial secretion
• Muscarinic (M3) receptors are located in the exocrine glands and cause increased secretion of saliva, sweat, and tears
• Muscarinic (M3) receptors are located in the urinary bladder and cause contraction of the bladder wall and relaxation of the ureter and sphincters
• Muscarinic (M3) receptors are located in the intestine and cause increased motility and secretion and relaxation of sphincters

Parasympathetic Nervous system Key Roles

• Parasympathetic postganglionic neurons release acetylcholine. They are located in the autonomic ganglia, adrenal medulla, and skeletal muscles.
• They play a role in regulating heart rate, blood pressure, digestion, and other bodily functions.

Key Parasympathetic Functions

• Decrease heart rate
• Slow heart conduction
• Decrease atrial contraction
• Vasodilation
• Miosis (pupil constriction).
• Reduction in Intraocular Pressure (IOP).
• Accommodation to near vision.
• Bronchoconstriction.
• Increase bronchial secretion.
• Increase salivary, lacrimal (tear) and sweat gland secretion..
• Increase gastrointestinal motility and secretion..
• Increase bladder contraction.
• Relaxation of urethral sphincter.
• Erection of penis and clitoris .