Autonomic Nervous System: Structure, Function, and Clinical Relevance

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Autonomic Nervous System (ANS)

Definition and Components

The autonomic nervous system (ANS) is a division of the peripheral nervous system that regulates involuntary activities of smooth muscle, cardiac muscle, and certain glands. It consists of visceral efferent neurons and visceral afferent neurons with receptors. The ANS operates without conscious control and is regulated by centers in the brain, particularly the hypothalamus and medulla oblongata.

Components: Visceral efferent neurons, visceral afferent neurons, and receptors.
Function: Controls smooth muscle, cardiac muscle, and glandular activity.
Regulation: Centers in the brain (hypothalamus, medulla oblongata).

Comparison of Somatic and Autonomic Nervous Systems

Effectors, Efferent Pathways, and Neurotransmitters

The somatic and autonomic nervous systems differ in their effectors, efferent pathways, and neurotransmitter responses.

Somatic System	ANS
Stimulates skeletal muscles	Stimulates cardiac muscle, smooth muscle, and glands
Motor neuron cell body in CNS, one neuron extends to effector organ	Motor unit is 2 neuron chain: Preganglionic neuron cell body in CNS, synapses outside CNS, postganglionic axon extends to effector organ
Axons are myelinated. Conduction is fast.	Preganglionic axons are lightly myelinated. Postganglionic axons are unmyelinated. Conduction is slower.
All motor neurons secrete acetylcholine at synapse with effector.	Postganglionic neurons secrete acetylcholine or norepinephrine at synapse with effector.
Response is always excitation.	Response may be excitation or inhibition depending on the nature of receptors present on target organ.

Parasympathetic and Sympathetic Divisions

General Functions and Effects

The ANS is divided into two major arms: the parasympathetic and sympathetic divisions, which generally have opposite effects on target organs.

Parasympathetic Division: Conserves and restores body energy during rest ("resting and digesting"). Promotes digestive activity, lowers heart rate and blood pressure.
Sympathetic Division: Mobilizes the body during stress ("fight or flight"). Increases heart rate, dilates bronchioles, inhibits digestion, and redirects blood to skeletal muscles.

These divisions counterbalance each other to maintain homeostasis.

Anatomical Features

Parasympathetic: Originates from cranial and sacral regions of the spinal cord. Long preganglionic and short postganglionic fibers. Ganglia are near or within target organs.
Sympathetic: Originates from thoracolumbar region (T1-L2) of the spinal cord. Short preganglionic and long postganglionic fibers. Ganglia are close to the spinal cord.

Cranial and Sacral Outflow (Parasympathetic)

Cranial Outflow: Oculomotor (III), Facial (VII), Glossopharyngeal (IX), and Vagus (X) nerves provide parasympathetic innervation to the head and thoracic/abdominal organs.
Sacral Outflow: S2-S4 nerves innervate the lower large intestine and pelvic organs.

Sympathetic Division Pathways

Preganglionic fibers arise from T1-L2, synapse in sympathetic chain ganglia, and postganglionic fibers extend to target organs.
Sympathetic effects are more diffuse and longer-lasting than parasympathetic effects.

Visceral Sensory Neurons and Referred Pain

Function and Mechanism

Visceral sensory neurons transmit information about chemical changes, stretch, and irritation of the viscera. Referred pain occurs when visceral pain is perceived as originating from a somatic region due to shared neural pathways.

Example: Pain from a heart attack may be felt in the left arm or jaw.

Neurotransmitters of the ANS and Their Receptors

Major Neurotransmitters

Acetylcholine (ACh): Released by all preganglionic axons and most parasympathetic postganglionic axons.
Norepinephrine (NE): Released by most sympathetic postganglionic axons (adrenergic fibers).

Cholinergic Receptors

Nicotinic receptors: Found on skeletal muscle cells, all postganglionic neurons, and adrenal medulla. Always stimulatory.
Muscarinic receptors: Found on all effector cells stimulated by postganglionic cholinergic fibers. Can be stimulatory or inhibitory depending on the target organ.

Adrenergic Receptors

Two main classes: alpha (α) and beta (β) receptors.
Alpha receptor activation is usually stimulatory; beta receptor activation can be stimulatory or inhibitory depending on the organ.

Pharmacology: Drugs Affecting the ANS

Examples and Mechanisms

Anticholinesterase drugs (e.g., neostigmine): Inhibit breakdown of ACh, increasing its effect. Used in myasthenia gravis.
Tricyclic antidepressants: Prolong NE activity at synapses.
Adrenergic blockers (e.g., beta-blockers): Reduce heart rate and prevent arrhythmias by blocking beta receptors.

Autonomic Tone and Unique Features

Sympathetic and Parasympathetic Tone

Sympathetic tone: Maintains partial constriction of blood vessels; can be increased or decreased as needed.
Parasympathetic tone: Dominates the heart and smooth muscle of digestive and urinary tracts under resting conditions.

Localized vs. Diffuse Effects

Parasympathetic effects are short-lived and localized.
Sympathetic effects are longer-lasting and more diffuse due to NE and epinephrine release into the bloodstream.

Control Centers of Autonomic Functions

Central Regulation

ANS activity is regulated by the spinal cord, brain stem, hypothalamus, and cerebral cortex.
The hypothalamus is the main integration center for ANS activity, controlling heart rate, blood pressure, body temperature, water balance, and endocrine activity.

Homeostatic Imbalances: Hypertension and Raynaud's Disease

Definitions and Mechanisms

Hypertension: May result from overactive sympathetic vasoconstriction, increasing heart workload and arterial pressure.
Raynaud's disease: Characterized by intermittent attacks of skin blanching and pain in the fingers and toes due to exaggerated vasoconstriction, often triggered by cold or stress.