Chapter 14 Objectives

The Autonomic Nervous System (ANS): Chapter 14

Overview of the Autonomic Nervous System

The autonomic nervous system (ANS) is a division of the peripheral nervous system responsible for regulating involuntary physiological functions, including heart rate, digestion, respiratory rate, and glandular activity. It operates largely below the level of consciousness and maintains homeostasis by controlling smooth muscle, cardiac muscle, and glands.

• Effectors: Cardiac muscle, smooth muscle, and glands

• Pathways: Involves two-neuron chains (preganglionic and postganglionic fibers)

• Neurotransmitters: Acetylcholine (ACh) and norepinephrine (NE)

Somatic vs. Autonomic Nervous Systems

Comparison of Somatic and Autonomic Nervous Systems

The somatic and autonomic nervous systems differ in their effectors, pathways, and neurotransmitters.

• Somatic Nervous System:

  • Effectors: Skeletal muscle

  • Neurotransmitter: Acetylcholine (ACh), always stimulatory

  • Pathway: Single neuron from CNS to effector

• Autonomic Nervous System:

  • Effectors: Cardiac muscle, smooth muscle, glands

  • Neurotransmitters: ACh (preganglionic), ACh or NE (postganglionic)

  • Pathway: Two-neuron chain (preganglionic and postganglionic)

Divisions of the Autonomic Nervous System

Parasympathetic Division

The parasympathetic division is often called the "rest and digest" system. It conserves energy and promotes maintenance activities.

• Origin: Craniosacral (brainstem and sacral spinal cord)

• Fiber Length: Long preganglionic, short postganglionic fibers

• Ganglia Location: Near or within target organs

Sympathetic Division

The sympathetic division is known as the "fight or flight" system. It mobilizes the body during activity.

• Origin: Thoracolumbar (thoracic and lumbar spinal cord, T1-L2, lateral horn)

• Fiber Length: Short preganglionic, long postganglionic fibers

• Ganglia Location: Close to the spinal cord (paravertebral and collateral ganglia)

Autonomic Pathways and Neurotransmitters

Preganglionic and Postganglionic Fibers

• Preganglionic fibers: Release ACh (cholinergic)

• Postganglionic fibers: Release ACh (cholinergic) or NE (adrenergic)

Receptors

• Cholinergic Receptors:

  • Nicotinic: Always stimulatory

  • Muscarinic: Stimulatory or inhibitory (receptor-dependent)

• Adrenergic Receptors:

  • Alpha (α1, α2)

  • Beta (β1, β2, β3, β4)

Dual Innervation and Functional Effects

Dual Innervation

Most organs receive input from both sympathetic and parasympathetic divisions, often producing antagonistic effects but sometimes cooperative actions.

• Sympathetic tone: Maintains blood pressure

• Parasympathetic tone: Maintains normal digestive and urinary function

• Antagonistic effects: Opposing actions on the same organ

• Cooperative effects: Both divisions work together (e.g., sexual function)

Anatomical Structures of the Sympathetic Division

• White rami communicantes: Carry preganglionic fibers to the sympathetic trunk

• Sympathetic trunk (paravertebral ganglia): Chain of ganglia alongside the vertebral column

• Collateral ganglia: Located anterior to the vertebral column

• Gray rami communicantes: Carry postganglionic fibers to spinal nerves

Visceral Sensory Neurons and Reflexes

• Visceral sensory neurons: Monitor internal conditions

• Visceral reflex arcs: Control involuntary responses of organs

• Enteric nervous system: Intrinsic nervous system of the gastrointestinal tract

Control of the Autonomic Nervous System

• Brainstem and spinal cord: Basic autonomic functions

• Hypothalamus: Main integration center for ANS activity

• Cerebral cortex: Modulates ANS through limbic system

Clinical Disorders of the Autonomic Nervous System

• Hypertension: High blood pressure due to excessive sympathetic activity

• Raynaud's disease: Excessive vasoconstriction in response to cold or stress

• Autonomic dysreflexia: Uncontrolled sympathetic response, often in spinal cord injury patients

Effects of the ANS on Major Organs

Organ	Sympathetic Effect	Parasympathetic Effect
Heart	Increases rate and force	Decreases rate
Blood Vessels	Vasoconstriction (increases BP)	Little or no effect
Gastrointestinal Tract	Decreases activity	Increases activity
Lungs	Bronchodilation	Bronchoconstriction
Adrenal Medulla	Secretes epinephrine and norepinephrine	No effect
External Genitalia	Ejaculation, vaginal contraction	Erection, increased lubrication

Pharmacology: Cholinergic and Adrenergic Drugs

• Cholinergic drugs: Mimic or inhibit ACh effects (e.g., muscarinic agonists/antagonists)

• Adrenergic drugs: Mimic or inhibit NE effects (e.g., beta-blockers, alpha agonists)

• Clinical importance: Used to treat hypertension, asthma, arrhythmias, etc.

Key Definitions

• Cholinergic fibers: Neurons that release acetylcholine

• Adrenergic fibers: Neurons that release norepinephrine

• Nicotinic receptors: Found on all postganglionic neurons; always excitatory

• Muscarinic receptors: Found on effector cells; effect depends on receptor subtype

• Alpha and beta receptors: Subtypes of adrenergic receptors with varying effects

Visceral vs. Somatic Reflexes

• Somatic reflexes: Involve skeletal muscle, usually conscious control

• Visceral reflexes: Involve smooth/cardiac muscle or glands, unconscious control

Summary Table: Somatic vs. Autonomic Nervous System

Feature	Somatic	Autonomic
Effectors	Skeletal muscle	Cardiac/smooth muscle, glands
Pathways	Single neuron	Two-neuron chain
Neurotransmitter	ACh (always stimulatory)	ACh or NE (stimulatory or inhibitory)

Additional info:

• Beta-4 adrenergic receptors are less commonly referenced in standard texts; main subtypes are β1, β2, and β3.

• Drugs affecting the ANS are widely used in clinical medicine for cardiovascular, respiratory, and gastrointestinal disorders.