The Autonomic Nervous System (ANS)

Overview and Importance

The autonomic nervous system (ANS) is a critical component of the nervous system responsible for regulating involuntary functions such as heart rate, blood pressure, digestion, and glandular activity. Understanding the ANS is essential for anticipating the effects and side effects of drugs on patients.

Structural Organization of the Nervous System

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The PNS is further subdivided into sensory (afferent) and motor (efferent) divisions. The motor division includes the somatic nervous system (controls skeletal muscles) and the autonomic nervous system (controls smooth muscle, cardiac muscle, and glands).

Comparison: Autonomic vs. Somatic Nervous System

Both systems have motor fibers but differ in effectors, number of neurons between CNS and effectors, and neurotransmitter actions.

• Somatic Nervous System: Innervates skeletal muscles; uses a single, thick myelinated axon; neurotransmitter is acetylcholine (ACh), always excitatory.

• Autonomic Nervous System: Innervates cardiac muscle, smooth muscle, and glands; uses a two-neuron chain (preganglionic and postganglionic); neurotransmitters include ACh and norepinephrine (NE), effects can be excitatory or inhibitory.

Divisions of the Autonomic Nervous System

Parasympathetic and Sympathetic Divisions

The ANS consists of two main divisions:

• Parasympathetic Division: Promotes maintenance functions and conserves energy; known as the "rest-and-digest" system.

• Sympathetic Division: Mobilizes the body during activity; known as the "fight-or-flight" system.

Most visceral organs are served by both divisions, which have opposing effects to maintain homeostasis.

Functional Roles

• Parasympathetic: Directs digestion, diuresis, and defecation. During relaxation, heart rate and blood pressure are low, gastrointestinal activity is high, and pupils are constricted.

• Sympathetic: Activated during exercise, excitement, or emergency. Increases heart rate, dilates pupils and bronchioles, shunts blood to muscles, and causes liver to release glucose.

Anatomical Differences

• Sites of Origin: Parasympathetic fibers are craniosacral; sympathetic fibers are thoracolumbar.

• Fiber Lengths: Parasympathetic has long preganglionic and short postganglionic fibers; sympathetic has short preganglionic and long postganglionic fibers.

• Ganglia Location: Parasympathetic ganglia are near or within effectors; sympathetic ganglia are close to the spinal cord.

Parasympathetic Division

Cranial and Sacral Components

The parasympathetic division is also called the craniosacral division. Preganglionic fibers originate from the brainstem and sacral spinal cord, synapsing in terminal ganglia near or within target organs.

Cranial Nerves and Functions

• Oculomotor (III): Controls eye muscles, pupil constriction, and lens accommodation.

• Facial (VII): Stimulates glands in the head.

• Glossopharyngeal (IX): Stimulates parotid salivary glands.

• Vagus (X): Accounts for ~90% of parasympathetic fibers; serves thoracic and abdominal viscera.

Sympathetic Division

Thoracolumbar Origin and Complexity

The sympathetic division originates from spinal cord segments T1–L2 and innervates more organs than the parasympathetic division. Some structures, such as sweat glands and blood vessels, are innervated only by the sympathetic division.

Sympathetic Trunks and Ganglia

Preganglionic fibers pass through white rami communicantes to enter sympathetic trunk ganglia, which flank each side of the vertebral column. There are typically 23 ganglia per trunk.

Pathways of Sympathetic Innervation

• Synapse at the same level: Preganglionic and postganglionic neurons synapse in the same trunk ganglion.

• Synapse at a higher or lower level: Preganglionic axon ascends or descends to another trunk ganglion.

• Synapse in collateral ganglion: Preganglionic axon passes through trunk ganglion and synapses in collateral ganglia anterior to the vertebral column.

Summary Table: Autonomic Ganglia

The table below summarizes the main types of autonomic ganglia, their division, and location.

Sympathetic Pathways to Organs

• Head: Fibers emerge from T1–T4, synapse in superior cervical ganglion, innervate skin, blood vessels, iris, glands, and heart.

• Thorax: Fibers emerge from T1–T6, synapse in cervical trunk ganglia, innervate heart, thyroid, skin, lungs, and esophagus.

• Abdomen: Fibers from T5–L2 travel through splanchnic nerves, synapse in celiac and mesenteric ganglia, serve digestive organs.

• Pelvis: Fibers from T10–L2 serve pelvic organs, inhibit activity of muscles and glands.

• Adrenal Medulla: Preganglionic fibers stimulate chromaffin cells to release epinephrine and norepinephrine.

Visceral Reflex Arcs

Components and Differences

Visceral reflex arcs regulate involuntary functions and differ from somatic reflex arcs in having two consecutive neurons in the motor pathway, visceral sensory afferents, and effectors being smooth muscle, cardiac muscle, or glands.

Neurotransmitters and Receptors

Major Neurotransmitters

• Acetylcholine (ACh): Released by all ANS preganglionic axons and all parasympathetic postganglionic axons.

• Norepinephrine (NE): Released by most sympathetic postganglionic axons.

The effect depends on the type of receptor (cholinergic or adrenergic) present on the target organ.

Cholinergic Receptors

• Nicotinic receptors: Found on all postganglionic neurons, adrenal medulla, and skeletal muscle cells; always excitatory.

• Muscarinic receptors: Found on all effector cells stimulated by postganglionic cholinergic fibers; can be excitatory or inhibitory depending on the organ.

Adrenergic Receptors

• Alpha (α) receptors: Subclasses α1 and α2.

• Beta (β) receptors: Subclasses β1, β2, and β3.

Effects depend on which subclass predominates on the target organ.

Parasympathetic and Sympathetic Interactions

Dual Innervation and Antagonism

Most organs receive input from both divisions, which interact antagonistically to maintain precise control of visceral activity. Sympathetic increases heart rate and inhibits digestion; parasympathetic decreases heart rate and promotes digestion.

Sympathetic and Parasympathetic Tone

• Sympathetic tone: Maintains partial constriction of blood vessels; can increase or decrease blood pressure as needed.

• Parasympathetic tone: Dominates heart and digestive/urinary organs; slows heart rate and promotes normal activity.

Cooperative Effects

Some functions, such as control of external genitalia, require cooperation between both divisions: parasympathetic causes erection, sympathetic causes ejaculation.

Unique Roles and Effects of the Sympathetic Division

• Thermoregulation: Controls sweat glands and blood vessel dilation/constriction.

• Renin Release: Stimulates kidneys to release renin, affecting blood pressure.

• Metabolic Effects: Increases metabolic rate, blood glucose, and mobilizes fats.

Localized vs. Diffuse Effects

• Parasympathetic: Short-lived, localized effects due to rapid breakdown of ACh.

• Sympathetic: Longer-lasting, bodywide effects due to slower inactivation of NE and hormone release.

Control of ANS Activity

CNS Centers

The ANS is regulated by the brainstem, spinal cord, hypothalamus, and cerebral cortex. The hypothalamus is the main integrative center, with input from the limbic system and cerebral cortex.

Disorders of the Autonomic Nervous System

Common Disorders

• Hypertension: Overactive sympathetic response; treated with adrenergic blockers.

• Autonomic dysreflexia: Uncontrolled activation in spinal cord injury patients; risk of stroke.

• Raynaud’s disease: Exaggerated vasoconstriction in fingers and toes; treated with vasodilators.

Postural Orthostatic Tachycardia Syndrome (POTS)

POTS is characterized by an abnormal increase in heart rate upon standing, accompanied by vasodilation and low blood pressure. Symptoms include dizziness, fatigue, and cold extremities. Treatment involves dietary changes, exercise, and medications to block sympathetic receptors.

Developmental Aspects of the ANS

Origin and Aging

• Development: ANS preganglionic neurons derive from the neural tube; postganglionic neurons and ganglia from the neural crest.

• Aging: ANS efficiency declines with age, leading to constipation, dry eyes, orthostatic hypotension, and less responsive pressure receptors.