The Autonomic Nervous System: Structure, Function, and Regulation

Chapter 15: The Autonomic Nervous System (ANS)

Introduction to the Autonomic Nervous System

The autonomic nervous system (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and glandular activity. This chapter explores the structure and function of the ANS, its neurotransmitters, and the differences between its sympathetic and parasympathetic divisions.

• Key Point: The ANS operates largely below the level of consciousness to maintain homeostasis.

• Key Point: The ANS is distinct from the somatic nervous system, which controls voluntary movements.

Comparison of Somatic and Autonomic Nervous Systems

Somatic Nervous System

• Sensory Neurons: Transmit information related to touch, pain, temperature, proprioception, and special senses (sight, hearing, taste, smell, equilibrium).

• Motor Neurons: Innervate skeletal muscles, allowing voluntary movement.

• Neural Pathway: A single, myelinated motor neuron extends from the CNS to the skeletal muscle fiber.

Autonomic Nervous System

• Sensory Input: Receives input from sensory receptors in organs, blood vessels, muscles, and the nervous system.

• Motor Pathways: Typically consist of two motor neurons in series:

  • Preganglionic Neuron: Cell body in the CNS; axon extends to an autonomic ganglion.

  • Postganglionic Neuron: Cell body in the autonomic ganglion; unmyelinated axon extends to the effector (smooth muscle, cardiac muscle, or gland).

• Divisions: The ANS is divided into the sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) divisions.

Anatomy of Autonomic Motor Pathways

General Organization

• Each division (sympathetic and parasympathetic) uses a two-neuron chain: preganglionic and postganglionic neurons.

• Preganglionic Neuron: Cell body in the brain or spinal cord.

• Postganglionic Neuron: Cell body and dendrites in an autonomic ganglion; synapses with preganglionic axon.

Sympathetic Division

• Origin: Preganglionic neuron cell bodies are located in the lateral horns of the gray matter in the 12 thoracic and first 2 or 3 lumbar segments of the spinal cord (thoracolumbar outflow).

• Sympathetic Ganglia:

  • Sympathetic Trunk Ganglia: Lie in a vertical row on either side of the vertebral column.

  • Prevertebral Ganglia: Lie anterior to the vertebral column, near large abdominal arteries.

• Pathways: After entering the sympathetic trunk ganglia, preganglionic axons may:

  • Synapse with postganglionic neurons in the first ganglion reached.

  • Ascend or descend to a different ganglion before synapsing.

  • Pass through the trunk ganglion to a prevertebral ganglion before synapsing.

  • Pass through both ganglia to the adrenal medullae (stimulating hormone release).

• Sympathetic Trunk Ganglia Numbers: Usually 2 cervical, 11 or 12 thoracic, 4 or 5 lumbar, 4 or 5 sacral, and 1 coccygeal ganglion.

Parasympathetic Division

• Origin: Preganglionic neuron cell bodies are in the nuclei of cranial nerves III, VII, IX, and X (brainstem) and in the lateral gray matter of sacral segments 2–4 (craniosacral outflow).

• Outflow:

  • Cranial Parasympathetic Outflow: Via cranial nerves III, VII, IX, X.

  • Sacral Parasympathetic Outflow: Via sacral spinal nerves S2–S4.

Autonomic Plexuses

• Major autonomic plexuses in the abdomen and pelvis are named after nearby arteries:

  • Celiac (solar) plexus

  • Superior mesenteric plexus

  • Inferior mesenteric plexus

  • Renal plexus

  • Hypogastric plexus

ANS Neurotransmitters and Receptors

Cholinergic and Adrenergic Neurons

• Cholinergic Neurons: Release acetylcholine (ACh).

• Cholinergic Receptors: Two main types:

  • Nicotinic Receptors

  • Muscarinic Receptors

• Adrenergic Neurons: Release norepinephrine (noradrenaline).

Receptors of the ANS

• Cholinergic receptors respond to acetylcholine; adrenergic receptors respond to norepinephrine and epinephrine.

• Adrenergic receptors are classified as alpha (α) and beta (β) types, with further subtypes (e.g., α1, α2, β1, β2, β3).

Physiology of the ANS

Sympathetic vs. Parasympathetic Divisions

• Sympathetic Division: Prepares the body for emergency situations (fight-or-flight response). Increases alertness, heart rate, blood pressure, and metabolic rate.

• Parasympathetic Division: Promotes rest, digestion, and energy storage (rest-and-digest response). Slows heart rate and increases digestive activities.

Autonomic Tone

• Autonomic tone is the balance between sympathetic and parasympathetic activity, regulated by the hypothalamus.

• Increased activity in one division is usually accompanied by decreased activity in the other.

Effects of the ANS on Target Organs

Glands

• Sympathetic stimulation generally inhibits secretion (except for sweat glands and adrenal medulla).

• Parasympathetic stimulation promotes secretion in most glands (e.g., salivary, digestive glands).

Cardiac Muscle

• Sympathetic stimulation increases heart rate and force of contraction.

• Parasympathetic stimulation decreases heart rate.

Smooth Muscle

• Sympathetic stimulation generally causes relaxation of smooth muscle in the walls of the digestive tract and constriction of sphincters.

• Parasympathetic stimulation increases motility and tone of the digestive tract.

Vascular Smooth Muscle

• Sympathetic stimulation causes vasoconstriction in most blood vessels, but vasodilation in skeletal muscle and heart (via β2 receptors).

• Parasympathetic effects on blood vessels are minimal.

Integration and Control of Autonomic Functions

Autonomic Reflex Arcs

Autonomic reflexes regulate functions such as heart rate, blood pressure, and digestion. These reflexes involve a receptor, sensory neuron, integrating center, motor neurons, and an effector.

The Nervous System and Homeostasis

• The ANS plays a critical role in maintaining homeostasis by adjusting the activity of various organ systems in response to internal and external stimuli.

Disorders of the Autonomic Nervous System

Autonomic Dysreflexia

• Exaggerated sympathetic response, often in individuals with spinal cord injury at or above T6.

• If untreated, may lead to seizures, stroke, or heart attack.

Raynaud Phenomenon

• Excessive sympathetic stimulation causes vasoconstriction in arterioles of the digits, leading to numbness and ischemia.

Summary Table: Comparison of Somatic and Autonomic Nervous Systems

Feature	Somatic Nervous System	Autonomic Nervous System
Effectors	Skeletal muscle	Smooth muscle, cardiac muscle, glands
Control	Voluntary (conscious)	Involuntary (unconscious)
Neural Pathway	One neuron from CNS to effector	Two neurons (preganglionic and postganglionic)
Neurotransmitters	Acetylcholine	Acetylcholine, norepinephrine
Effect of Nerve Impulse	Always excitatory	Excitatory or inhibitory

Summary Table: Sympathetic vs. Parasympathetic Divisions

Feature	Sympathetic Division	Parasympathetic Division
Origin	Thoracolumbar (T1–L2)	Craniosacral (Cranial nerves III, VII, IX, X; S2–S4)
Function	Fight-or-flight	Rest-and-digest
Neurotransmitters	Acetylcholine (preganglionic), norepinephrine (postganglionic)	Acetylcholine (both pre- and postganglionic)
General Effects	Increases heart rate, dilates pupils, inhibits digestion	Decreases heart rate, constricts pupils, stimulates digestion

Additional info: Some details, such as the classification of adrenergic receptors and the specific effects on target organs, were inferred and expanded for academic completeness.