The Autonomic Nervous System (ANS)

Introduction to the ANS

The autonomic nervous system (ANS) is a critical component of the peripheral nervous system that operates largely outside conscious awareness. It regulates essential involuntary functions, including body temperature, cardiovascular, respiratory, digestive, excretory, and reproductive systems.

• Regulation: Maintains homeostasis by adjusting internal organ activity.

• Coordination: Integrates multiple organ systems for efficient function.

Comparison of Somatic and Autonomic Nervous Systems

Somatic vs. Autonomic Pathways

The somatic and autonomic nervous systems differ in their targets and pathways:

• Somatic Nervous System: Innervates skeletal muscles; afferent pathways originate in skeletal muscle receptors.

• Autonomic Nervous System: Innervates visceral organs; afferent pathways originate in visceral receptors.

Divisions of the Autonomic Nervous System

Sympathetic and Parasympathetic Subdivisions

The ANS is divided into two major subdivisions:

• Sympathetic Division (Thoracolumbar): Most active during stress, exertion, or emergencies; prepares the body for "fight or flight" responses.

• Parasympathetic Division (Craniosacral): Most active during resting conditions; promotes "rest and digest" activities.

Enteric Nervous System (ENS)

The enteric nervous system is a third, less commonly discussed division. It consists of neurons in the walls of the digestive tract and initiates many visceral reflexes independently of the CNS.

Sympathetic Division

Organization and Anatomy

The sympathetic division consists of preganglionic neurons (T1–L2), ganglia near the vertebral column, and specialized neurons in the suprarenal (adrenal) gland.

• Sympathetic Chain Ganglia: Paravertebral ganglia controlling effectors in the body wall, head, neck, limbs, and thoracic cavity.

• Collateral Ganglia: Prevertebral ganglia innervating abdominopelvic organs.

• Suprarenal Medullae: Modified ganglia releasing hormones (epinephrine and norepinephrine).

Sympathetic Chain Ganglia

Clusters of ganglia parallel to the spinal cord, with specific numbers in each region:

• 3 cervical

• 11–12 thoracic

• 2–5 lumbar

• 4–5 sacral

• 1 coccygeal

Collateral Ganglia and Splanchnic Nerves

Splanchnic nerves are bundles of preganglionic fibers converging on collateral ganglia, which innervate visceral organs in the abdominopelvic cavity.

• Celiac ganglion: Stomach, duodenum, liver, gallbladder, pancreas, spleen, kidney

• Superior mesenteric ganglion: Small intestine, initial large intestine

• Inferior mesenteric ganglion: Terminal large intestine, kidney, bladder, sex organs

Suprarenal Medullae

Preganglionic fibers innervate the adrenal medulla, causing release of epinephrine (adrenaline) and norepinephrine into the bloodstream, affecting metabolic activity throughout the body.

Effects of Sympathetic Stimulation

• Increased alertness and energy

• Enhanced cardiovascular and respiratory activity

• Mobilization of energy reserves

• Dilation of pupils and respiratory tubes

• Stimulation of sweat glands and arrector pili muscles

Neurotransmitter Release and Receptors

Sympathetic ganglion fibers release acetylcholine (ACh) at synapses with ganglionic neurons (cholinergic synapses). Most postganglionic fibers release norepinephrine (adrenergic terminals), but some release ACh.

• Alpha receptors: Respond to epinephrine and norepinephrine

• Beta receptors: Respond to epinephrine

Parasympathetic Division

Organization and Anatomy

Preganglionic neurons are located in the brainstem and sacral segments (S2–S4). Postganglionic neurons are near or within the target organ.

• Cranial Nerves: CN III, VII, IX, X

• Sacral Nerves: Pelvic nerves

General Functions

• Relaxation and energy absorption

• Food processing and nutrient absorption

• Pupil constriction

• Secretion of digestive enzymes and hormones

• Increased smooth muscle activity in the digestive system

• Stimulation of defecation and urination

• Reduced heart rate

• Sexual arousal

Neurotransmitter Release and Receptors

All parasympathetic fibers release ACh. Effects are localized and brief due to rapid breakdown by acetylcholinesterase.

• Nicotinic receptors: Found on ganglionic neurons; always excitatory

• Muscarinic receptors: Found on neuroeffector junctions; can be excitatory or inhibitory

Relationships Between Sympathetic and Parasympathetic Divisions

Dual Innervation

Most vital organs receive input from both divisions, often with opposing (antagonistic) effects. This dual innervation allows fine control of organ function.

• Head: Parasympathetic fibers accompany sympathetic fibers

• Thoracic/Abdominopelvic: Fibers mingle in plexuses (cardiac, pulmonary, esophageal, celiac, mesenteric, hypogastric)

Comparison Table: Sympathetic vs. Parasympathetic Divisions

The following table summarizes key differences:

Visceral Reflexes

Types of Visceral Reflexes

Visceral reflexes provide autonomic motor responses and are polysynaptic. They can be classified as:

• Long Reflexes: Sensory neurons send signals to the CNS, which processes and sends motor commands to visceral organs.

• Short Reflexes: Sensory impulses go directly to ganglionic neurons, bypassing the CNS; motor commands are distributed by postganglionic fibers.

Summary

The autonomic nervous system is essential for maintaining internal balance and responding to environmental changes. Its sympathetic and parasympathetic divisions work together, often antagonistically, to regulate organ function. Understanding the structure, neurotransmitters, and reflex pathways of the ANS is crucial for comprehending human physiology.