The Autonomic Nervous System and Homeostasis: ANP Study Notes

Chapter 14: The Autonomic Nervous System and Homeostasis

Overview of the Autonomic Nervous System (ANS)

The Autonomic Nervous System (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes, including heart rate, blood pressure, digestion, and urinary functions. It operates largely without conscious control and is essential for maintaining homeostasis through visceral reflex arcs.

• Key Functions: Oversees vital functions such as cardiac activity, vascular tone, and glandular secretions.

• Visceral Reflex Arcs: Sensory signals from the viscera and skin are sent by afferent sensory neurons to the CNS, integrated, and then motor impulses are sent via efferent neurons to autonomic ganglia and target organs.

Comparison of Somatic and Autonomic Nervous Systems

The motor division of the peripheral nervous system is divided into the somatic and autonomic (visceral) divisions, each with distinct characteristics:

• Somatic Motor Division: Controls voluntary movements via somatic motor neurons innervating skeletal muscle; neurotransmitter is acetylcholine (ACh); always excitatory.

• Autonomic (Visceral) Motor Division: Controls involuntary actions via a two-neuron chain (preganglionic and postganglionic neurons); neurotransmitters include ACh and norepinephrine; can be excitatory or inhibitory.

Divisions of the Autonomic Nervous System

The ANS is divided into the Sympathetic and Parasympathetic nervous systems, which generally have antagonistic effects on target organs.

• Sympathetic Nervous System: "Fight or Flight"; preganglionic neurons originate in the thoracic and lumbar spinal cord; ganglia are near the spinal cord; preganglionic axons are short, postganglionic are long.

• Parasympathetic Nervous System: "Rest and Digest"; preganglionic neurons originate in the brainstem and sacral spinal cord; ganglia are near or within target organs; preganglionic axons are long, postganglionic are short.

Sympathetic Nervous System: Structure and Function

The sympathetic nervous system prepares the body for emergencies and maintains homeostasis during physical activity and emotional stress.

• Preganglionic Neurons: Originate in the lateral horns of the thoracic and lumbar spinal cord; axons exit via the anterior root and travel through white rami communicantes to the sympathetic chain ganglia.

• Sympathetic Chain Ganglia: Series of ganglia along the vertebral column; postganglionic neurons extend to target organs.

• Collateral (Preaortic) Ganglia: Located near the aorta; splanchnic nerves synapse here to innervate abdominopelvic organs.

Sympathetic Neurotransmitters and Receptors

Sympathetic neurons use several neurotransmitters and receptors to mediate their effects:

• Preganglionic Neurons: Release acetylcholine (ACh) at excitatory synapses with postganglionic neurons.

• Postganglionic Neurons: Release norepinephrine (noradrenaline), epinephrine (adrenaline), or ACh; most release norepinephrine.

• Adrenergic Receptors: Bind norepinephrine/epinephrine; include alpha (α1, α2) and beta (β1, β2, β3) subtypes, each with specific tissue distributions and effects.

• Cholinergic Receptors: Bind ACh; include muscarinic (on sweat glands) and nicotinic (on all postganglionic neurons and adrenal medulla cells).

Effects of the Sympathetic Nervous System on Target Cells

The sympathetic nervous system triggers a range of physiological changes to support "fight or flight" responses:

• Cardiac Muscle: Increases heart rate and contractility, raising blood pressure and blood flow.

• Smooth Muscle: Vasoconstriction in digestive, urinary, and integumentary systems; vasodilation in skeletal and cardiac muscle; bronchodilation; contraction of sphincters; pupil dilation.

• Metabolic Effects: Increases ATP production by stimulating lipolysis, glycogenolysis, and glucagon release.

• Adrenal Medulla: Releases epinephrine and norepinephrine into the bloodstream, prolonging sympathetic effects.

• Other Effects: Increases mental alertness, blood clotting, skeletal muscle tension, and triggers "goose bumps" and ejaculation in males.

Pharmacology of the Sympathetic Nervous System

Drugs can target specific sympathetic receptors as agonists (mimic neurotransmitter action) or antagonists (block neurotransmitter action):

• Antagonists (Blockers): Lower blood pressure, treat benign prostatic hyperplasia, decrease heart rate and contractility.

• Agonists: Treat hypertension, opiate withdrawal, and asthma (by causing bronchodilation).

Parasympathetic Nervous System: Structure and Function

The parasympathetic nervous system is responsible for "rest and digest" activities, promoting maintenance functions such as digestion and urination.

• Cranial Nerves: Oculomotor, facial, glossopharyngeal, and vagus nerves contain parasympathetic preganglionic neurons.

• Sacral Nerves: S2–S4 contribute to pelvic splanchnic nerves, innervating pelvic organs.

• Terminal Ganglia: Located near or within target organs; postganglionic axons are short.

Parasympathetic Neurotransmitters and Receptors

• Both pre- and postganglionic neurons release ACh.

• Nicotinic Receptors: On all postganglionic neurons.

• Muscarinic Receptors: On all parasympathetic target cells.

Effects of the Parasympathetic Nervous System on Target Cells

The parasympathetic system maintains homeostasis during rest:

• Cardiac Muscle: Decreases heart rate and blood pressure.

• Smooth Muscle: Constricts pupils, accommodates lens for near vision, bronchoconstriction, stimulates peristalsis, relaxes sphincters, and causes vasodilation in genitalia.

• Glandular Effects: Stimulates secretion of tears, saliva, and digestive enzymes; little effect on sweat glands.

• Other Effects: No direct effect on metabolic rate or mental alertness; promotes storage of energy reserves.

Pharmacology of the Parasympathetic Nervous System

• Muscarinic Agonists: Stimulate gastrointestinal activity and bladder emptying; used post-surgery.

• Muscarinic Antagonists: Block parasympathetic effects; used to treat bradycardia, motion sickness, and hypersalivation.

• Anticholinergic Side Effects: Many drugs can cause dry mouth, constipation, urinary retention, and blurred vision by blocking ACh or its receptors.

Interactions and Balance Between Autonomic Divisions

The sympathetic and parasympathetic systems generally work antagonistically to maintain homeostasis. Most organs receive dual innervation, allowing precise regulation depending on the body's needs.

• Sympathetic Dominance: During exercise or emergencies.

• Parasympathetic Dominance: During rest and recovery.

Autonomic Tone

Autonomic tone refers to the constant baseline activity of each division:

• Sympathetic Tone: Maintains partial constriction of blood vessels.

• Parasympathetic Tone: Maintains resting heart rate and dominates in digestive and urinary systems.

Clinical Application: Postural Orthostatic Tachycardia Syndrome (POTS)

POTS is characterized by an abnormal increase in heart rate upon standing, often accompanied by vasodilation and a drop in blood pressure. Symptoms include dizziness, fatigue, and palpitations. Treatment may involve lifestyle changes and medications targeting sympathetic receptors.

Nervous System Control of Homeostasis

Both ANS divisions are crucial for homeostasis. The hypothalamus and brainstem reticular formation centrally regulate physiological variables by sending signals to autonomic centers, which control preganglionic neurons. Emotional states influence visceral functions via higher brain centers.

Summary Table: Comparison of Somatic and Autonomic Motor Pathways

Feature	Somatic Motor	Sympathetic Motor	Parasympathetic Motor
Number of Neurons	One	Two (pre- and postganglionic)	Two (pre- and postganglionic)
Neurotransmitter	ACh	ACh (preganglionic), NE/ACh (postganglionic)	ACh (both)
Effect on Target	Excitatory	Excitatory or inhibitory	Excitatory or inhibitory
Target Tissue	Skeletal muscle	Smooth/cardiac muscle, glands	Smooth/cardiac muscle, glands