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Autonomic Nervous System (ANS)

Overview of the ANS

The Autonomic Nervous System (ANS) is a division of the peripheral nervous system responsible for regulating involuntary physiological functions, including heart rate, blood pressure, digestion, and respiratory rate. It operates through two main subdivisions: the sympathetic and parasympathetic divisions, which often have opposing effects on target organs.

• Sympathetic Division: Prepares the body for 'fight or flight' responses.

• Parasympathetic Division: Promotes 'rest and digest' activities.

• Dual Innervation: Most organs receive input from both divisions, allowing precise regulation.

Sympathetic Division

Structural Organization

The sympathetic division is also known as the thoracolumbar system because its preganglionic neurons originate in the lateral horns of the thoracic and lumbar spinal cord segments.

• Preganglionic Neurons: Located in the CNS; their axons (preganglionic fibers) are short and synapse in ganglia near the spinal cord.

• Postganglionic Neurons: Located in peripheral ganglia; their axons (postganglionic fibers) are long and innervate target organs.

• Ganglia Types:

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

  • Collateral Ganglia: Prevertebral, anterior to vertebral column, innervate abdominopelvic viscera.

  • Adrenal Medullae: Modified ganglia, release epinephrine and norepinephrine into bloodstream.

Major Ganglia and Splanchnic Nerves

• Celiac Ganglion: Upper abdomen organs (stomach, liver, spleen).

• Superior Mesenteric Ganglion: Large intestine.

• Inferior Mesenteric Ganglion: Urinary and reproductive systems.

• Splanchnic Nerves: Bundles of preganglionic fibers innervating collateral ganglia.

Neurotransmitters and Receptors

Sympathetic pathways use both acetylcholine (ACh) and norepinephrine (NE). The specificity of the response is determined by the type of receptor present on the target cell.

• Preganglionic Neurons: Release ACh (cholinergic synapses).

• Postganglionic Neurons: Mostly release NE (adrenergic synapses); some release ACh or nitric oxide (NO).

• Adrenal Medullae: Secrete epinephrine (E) and NE into the bloodstream, producing widespread and prolonged effects.

Functional Effects

• Increased metabolic rate

• Increased heart rate and blood pressure

• Increased respiratory rate

• Inhibition of digestive and urinary functions

• Stimulation of sweat glands

Parasympathetic Division

Structural Organization

The parasympathetic division is referred to as the craniosacral system because its preganglionic neurons originate in the brainstem and sacral spinal cord segments (S2–S4).

• Preganglionic Neurons: Long axons, synapse in ganglia near or within target organs (intramural or terminal ganglia).

• Postganglionic Neurons: Short axons, innervate target organs.

• Vagus Nerve (Cranial Nerve X): Major parasympathetic nerve, innervates heart, lungs, digestive organs, and more.

Neurotransmitters and Receptors

• Both preganglionic and postganglionic neurons release acetylcholine (ACh).

• Receptors:

  • Nicotinic Receptors: Excitatory, found on ganglion cells.

  • Muscarinic Receptors: Excitatory or inhibitory, found on target cells.

Functional Effects

• Decreased metabolic rate

• Decreased heart rate and blood pressure

• Increased activity of digestive glands

• Stimulation of urination and defecation

• Constriction of pupils

Chemical Signaling in the ANS

Neurotransmitter Release and Varicosities

Autonomic neurons release neurotransmitters from varicosities, which are swellings along their axons. This allows neurotransmitters to be distributed over a wide area of the target cell surface.

• Sympathetic: NE and E effects last longer due to slower inactivation.

• Parasympathetic: ACh effects are short-lived due to rapid breakdown by acetylcholinesterase.

Autonomic Tone and Dual Innervation

Concept of Autonomic Tone

Autonomic tone refers to the baseline level of activity in autonomic motor neurons, allowing for fine adjustments in organ function. Dual innervation enables precise control, especially in organs like the heart, where both divisions influence heart rate.

• Example: Heart rate is regulated by both sympathetic (increases rate) and parasympathetic (decreases rate) input.

• Blood Vessel Diameter: Controlled mainly by sympathetic tone.

Visceral Reflexes and Enteric Nervous System

Somatic vs. Visceral Reflexes

Reflexes in the nervous system can be classified as somatic (involving skeletal muscles) or visceral (involving glands, smooth muscle, or cardiac muscle).

• Somatic Reflexes: Involve lower motor neurons and skeletal muscles.

• Visceral Reflexes: Involve preganglionic and postganglionic neurons, affecting visceral effectors.

• Long Reflexes: Involve CNS processing; coordinate activities of entire organs.

• Short Reflexes: Bypass CNS; localized effects, especially in the digestive tract.

Enteric Nervous System

The enteric nervous system is a network of neurons in the digestive tract capable of independent function, though it is influenced by both sympathetic and parasympathetic divisions.

• Controls digestive functions without direct CNS input.

• Contains sensory neurons, interneurons, and motor neurons.

Central Control of the ANS

Higher-Level Regulation

Central control centers such as the hypothalamus, pons, and medulla oblongata coordinate complex autonomic functions. These centers integrate input from the limbic system, thalamus, and cerebral cortex, allowing emotional states and conscious thought to influence autonomic responses.

• Hypothalamus: Main integrative center for autonomic activity.

• Pons and Medulla: Regulate cardiovascular, respiratory, digestive, and urinary functions.

Comparison of Sympathetic and Parasympathetic Divisions

Feature	Sympathetic	Parasympathetic
Origin	Lateral horns of thoracic/lumbar spinal cord	Brainstem and sacral spinal cord (S2–S4)
Ganglia Location	Near vertebral column	Near or within target organs
Preganglionic Fiber	Short, releases ACh	Long, releases ACh
Postganglionic Fiber	Long, releases NE, NO, or ACh	Short, releases ACh
Function	Fight or flight, increased alertness	Rest and digest, relaxation

Key Terms and Definitions

• Preganglionic Neuron: Neuron in CNS that sends axon to autonomic ganglion.

• Postganglionic Neuron: Neuron in ganglion that innervates target organ.

• Varicosity: Swelling along axon where neurotransmitter is released.

• Adrenergic: Refers to neurons releasing norepinephrine.

• Cholinergic: Refers to neurons releasing acetylcholine.

• Dual Innervation: Organ receives input from both sympathetic and parasympathetic divisions.

Relevant Equations

Neurotransmitter breakdown:

• Acetylcholine: $\text{ACh} \xrightarrow{\text{AChE}} \text{Acetate} + \text{Choline}$

• Norepinephrine: $\text{NE} \xrightarrow{\text{MAO or COMT}} \text{Inactive Metabolites}$

Summary

The autonomic nervous system is essential for maintaining homeostasis through involuntary control of organ systems. Understanding its structure, neurotransmitters, and central regulation is crucial for interpreting physiological responses and clinical scenarios.