Autonomic Nervous System (ANS)

Introduction

The Autonomic Nervous System (ANS) is a division of the peripheral nervous system responsible for involuntary physiological functions. It operates outside conscious control, regulating processes such as heart rate, digestion, respiratory rate, and glandular activity. The ANS is essential for maintaining homeostasis and allows survival even in unconscious states, such as coma.

• Key Functions: Regulation of body temperature, cardiovascular, respiratory, digestive, excretory, and reproductive systems.

• Effectors: Smooth muscle, cardiac muscle, glands, and adipose tissue.

• Routine Adjustments: The ANS continuously adjusts internal conditions to meet the needs of the body.

Organization of the Nervous System

CNS and PNS Overview

The nervous system is divided into the Central Nervous System (CNS) and Peripheral Nervous System (PNS). The PNS includes the Somatic Nervous System (SNS) and the Autonomic Nervous System (ANS). The ANS itself is subdivided into the Sympathetic and Parasympathetic divisions, which often have opposing effects on target organs.

• SNS: Controls voluntary movements via skeletal muscles.

• ANS: Controls involuntary functions via visceral organs.

• Motor Pathways: SNS uses a single motor neuron; ANS uses two (preganglionic and postganglionic).

Sympathetic Division (Thoracolumbar Division)

Structure and Pathways

The Sympathetic Division prepares the body for 'fight or flight' responses during stress or emergency. Preganglionic neurons originate from spinal segments T1–L2, with cell bodies in the lateral gray horns. Their axons pass through ventral roots to ganglia near the vertebral column.

• Sympathetic Chain Ganglia: Paired structures alongside the vertebral column; innervate body wall, limbs, and thoracic cavity.

• Collateral Ganglia: Unpaired structures anterior to the vertebral column; innervate abdominopelvic organs.

• Adrenal Medulla: Specialized neurons release epinephrine and norepinephrine into the bloodstream.

Sympathetic Chain Ganglia – Pathways

Preganglionic fibers enter the sympathetic chain ganglia via the white ramus communicans. They may synapse at the same level, ascend or descend before synapsing, or pass through to collateral ganglia.

• White Ramus: Carries myelinated preganglionic fibers.

• Gray Ramus: Carries unmyelinated postganglionic fibers back to the spinal nerve.

• Motor Commands: Postganglionic fibers travel to effectors via spinal nerves or sympathetic nerves.

Collateral Ganglia

Preganglionic fibers destined for abdominopelvic viscera pass through the sympathetic chain without synapsing and reach collateral ganglia. Here, they synapse with postganglionic neurons, which innervate organs such as the digestive tract, kidneys, bladder, and reproductive organs.

• Celiac Ganglion: Greater splanchnic nerve; innervates much of the digestive system and kidneys.

• Superior Mesenteric Ganglion: Lesser splanchnic nerve; innervates intestines.

• Inferior Mesenteric Ganglion: Lumbar splanchnic nerve; innervates lower intestines, bladder, and sex organs.

Adrenal Medulla

Some preganglionic fibers bypass both sympathetic chain and collateral ganglia to synapse on modified neurons in the adrenal medulla. These cells release epinephrine (E) and norepinephrine (NE) directly into the bloodstream, acting as hormones to amplify sympathetic effects throughout the body.

• Hormonal Effects: Increased alertness, cardiovascular and respiratory activity, energy mobilization.

Parasympathetic Division (Craniosacral Division)

Structure and Pathways

The Parasympathetic Division promotes 'rest and digest' activities, supporting relaxation, food processing, and energy absorption. Preganglionic neurons originate in the brainstem (cranial nerves III, VII, IX, X) and sacral spinal segments (S2–S4). Ganglionic neurons are located close to or within target organs.

• Cranial Nerves: N III, VII, IX innervate head structures; N X (Vagus) innervates thoracic and abdominopelvic organs.

• Pelvic Nerves: S2–S4 fibers innervate lower abdominopelvic organs.

• Effects: More specific and localized than sympathetic division.

Major Effects of Parasympathetic Stimulation

• Pupil constriction

• Secretion of digestive enzymes and hormones

• Increased smooth muscle activity in the digestive system

• Stimulation and coordination of defecation

• Contraction of the urinary bladder

• Constriction of respiratory passages

• Reduced heart rate and force of contraction

• Sexual arousal

Synapses and Neurotransmitter Release

Sympathetic Division

Sympathetic preganglionic neurons release acetylcholine (ACh) at synapses in ganglia. Postganglionic neurons typically release norepinephrine (NE) at target organs, except for sweat glands, which use ACh. The adrenal medulla releases epinephrine (E) and norepinephrine (NE) as hormones.

• Preganglionic fibers: Short, myelinated, release ACh.

• Postganglionic fibers: Long, unmyelinated, release NE (or ACh for sweat glands).

Parasympathetic Division

Both preganglionic and postganglionic neurons release acetylcholine (ACh). Preganglionic fibers are long and myelinated, while postganglionic fibers are short and unmyelinated.

• Preganglionic fibers: Long, myelinated, release ACh.

• Postganglionic fibers: Short, unmyelinated, release ACh.

Comparison of Sympathetic and Parasympathetic Divisions

Clinical Anatomy: Raynaud’s Disease

Overview

Raynaud’s Disease is a condition in which the sympathetic nervous system constricts small arteries in the fingers and toes, often in response to cold or stress. This reduces blood flow, causing affected areas to turn white or blue, lose sensation, and sometimes become painful. It increases the risk of frostbite.

• Symptoms: Color changes, numbness, pain.

• Mechanism: Excessive sympathetic stimulation leads to vasoconstriction.

Summary Table: ANS Divisions and Pathways

Additional info: The Enteric Nervous System (ENS) is sometimes considered a third division of the ANS, controlling gastrointestinal function independently.