Introduction to the Autonomic Nervous System

Introduction to the Autonomic Nervous System

• The autonomic nervous system (ANS) regulates visceral functions of the body, also called the involuntary nervous system
• It controls various bodily processes
• The ANS is divided into sympathetic and parasympathetic nervous systems

Nervous System Organization

• The nervous system is comprised of the central nervous system (CNS) and peripheral nervous system (PNS)
• The CNS consists of the brain and spinal cord
• The PNS connects the CNS to the rest of the body and is divided into somatic and autonomic
• The somatic nervous system controls voluntary movements
• The autonomic nervous system controls involuntary functions (visceral functions)

Somatic vs. Autonomic Nervous Systems

• The somatic nervous system has a single neuron connecting the CNS to the effector organ (skeletal muscle)
• It only leads to muscle excitation
• The autonomic nervous system has two neurons connecting the CNS to the effector organ (cardiac muscles, smooth muscles, and glands)
• It can be excitatory or inhibitory

Autonomic Nervous System (ANS) Divisions

• The ANS is divided into the sympathetic and parasympathetic nervous systems
• These systems have opposing effects to maintain homeostasis

Anatomy of the Sympathetic Nervous System

• Originates from the thoracic and lumbar regions of the spinal cord (thoracolumbar)
• Pre and post ganglionic neurons
• The preganglionic neuron is short, while the postganglionic neuron is long
• Sympathetic ganglia are located near the spinal cord

Anatomy of the Parasympathetic Nervous System

• Originates from the cranial nerves (III, VII, IX, X) and sacral regions of the spinal cord (craniosacral)
• Preganglionic neurons are long, postganglionic neurons are short
• Ganglia are located near or within the organs they innervate

Sympathetic Nervous System Functions

• I - Cervical Division (Eye): Pupil dilation, eyelid widening, muscle contraction, and blood vessels constriction
• II - Cardio-Pulmonary Division (Heart): Increased heart rate, increased cardiac efficiency, increased conductivity, widening of coronary vessels
• III - Splanchnic Division (Stomach & Intestine): Relaxation of stomach and intestinal walls, reduced intestinal motility, secretion inhibition, and decreased excretion
• IV - Pelvic Division (Bladder & Rectum): Inhibits bladder wall and contracts urethral sphincter for urine retention, inhibits rectum wall and contracts anal sphincter for feces retention

Parasympathetic Nervous System Functions

• Cranial Outflow (Eye): Pupil constriction, lens convexity increase
• Cranial Outflow (Salivary Glands): Increased secretion of saliva, increasing blood vessels
• Cranial Outflow (Parotid Glands): Increased secretion of saliva and vasodilation of the vessel
• Cranial Outflow (Tongue Glands): Increased secretion in the glands of the posterior tongue
• Cranial Outflow (Heart): Decreased heart rate and contractility
• Vagus Nerve (Lungs): Decreased heart rate, reduced contractility of cardiac muscles, bronchoconstriction, and vasodilatation
• Vagus Nerve (GIT): Contraction of intestinal wall and peristalsis, increasing gastric motility, relaxation of gall bladder, stimulating excretion of digestive enzymes to stomach, pancreas, liver
• Cranial Outflow (Bladder & Rectum): Stimulates bladder wall for urination, inhibiting urethral sphincter contraction, and contracting rectum wall with inhibition contraction of anal sphincter for defecation

Sympathetic vs. Parasympathetic Overview

• Sympathetic: "Fight or flight" response
• Parasympathetic: "Rest and digest" response

Key Structures/Anatomical Regions

• Brain, Spinal Cord, Cranial Nerves, Thoracic/Lumbar regions, Preganglionic and Postganglionic neurons, Spinal nerves, ganglia