Chapter 14: The Autonomic Nervous System and Homeostasis - 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 responsible for regulating involuntary physiological functions, including heart rate, blood pressure, digestion, and urinary processes. The ANS operates largely autonomously and mediates homeostasis through visceral reflex arcs.

• Homeostasis: The maintenance of stable internal conditions.

• Visceral Reflex Arc: A series of events where a sensory stimulus leads to a predictable motor response.

• ANS Functions: Oversees heart rate, blood pressure, and urinary processes via reflex arcs.

• Reflex Arc Steps:

  1. Sensory signals from viscera and skin sent by afferent neurons to CNS.

  2. CNS integrates the stimuli.

  3. Motor impulses sent via efferent neurons to autonomic ganglia.

  4. Autonomic ganglia relay impulses to target organs, triggering motor responses.

Comparison of Somatic and Autonomic Nervous Systems

The somatic and autonomic nervous systems differ in their targets and mechanisms of action.

• Somatic Nervous System: Controls voluntary muscle contractions by directly innervating skeletal muscle fibers and releasing acetylcholine (ACh).

• Autonomic Nervous System: Controls involuntary actions, innervates smooth muscle, cardiac muscle, and glands. Uses a two-neuron chain: preganglionic (CNS) and postganglionic (PNS) neurons.

• Neurotransmitters: ANS uses ACh and norepinephrine, which may stimulate or inhibit target cells.

• Ganglion: Cluster of neuronal cell bodies in the PNS.

Divisions of the ANS

The ANS is divided into the sympathetic and parasympathetic nervous systems, which often have opposing effects.

• Sympathetic Nervous System: Thoracolumbar division; preganglionic axons are short, postganglionic axons are long. Maintains homeostasis during physical activity and mediates visceral responses to emotion.

• Parasympathetic Nervous System: Craniosacral division; preganglionic axons are long, postganglionic axons are short. Maintains homeostasis during rest, promotes digestion and urinary function.

• Balance: The two divisions antagonize each other to maintain homeostasis (e.g., heart rate regulation).

The Sympathetic Nervous System

Anatomy of the Sympathetic Nervous System

The sympathetic nervous system is essential for responding to stress and maintaining homeostasis during activity.

• Sympathetic Chain Ganglia: Series of ganglia along the vertebral column, from the superior cervical to the inferior sacral ganglion.

• Preganglionic Neurons: Originate in the lateral horns of the thoracic and lumbar spinal cord; exit via anterior root and travel through white rami communicantes (myelinated).

• Postganglionic Neurons: Cell bodies in sympathetic chain or collateral ganglia; axons travel via gray rami communicantes (unmyelinated) or directly to target organs.

• Collateral Ganglia: Located near the aorta (preaortic ganglia) or abdominopelvic organs; include celiac, superior mesenteric, and inferior mesenteric ganglia.

• Splanchnic Nerves: Preganglionic axons that synapse in collateral ganglia.

Sympathetic Neurotransmitters and Receptors

Neurons communicate via neurotransmitters, which bind to specific receptors on target cells.

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

• Postganglionic Neurons: Release norepinephrine (noradrenaline), epinephrine (adrenaline), or ACh at target cells.

• Adrenergic Receptors: Bind norepinephrine/epinephrine; two major types:

  • Alpha-1: Smooth muscle cells in blood vessels, skin, GI system, kidneys, arrector pili, uterus, pupil.

  • Alpha-2: Mostly on preganglionic neurons; hyperpolarization inhibits sympathetic response (negative feedback).

  • Beta-1: Cardiac muscle, kidney, adipose tissue.

  • Beta-2: Bronchioles, skeletal muscle, urinary bladder, blood vessels, liver, pancreas, salivary glands.

  • Beta-3: Adipose tissue, digestive tract.

• Cholinergic Receptors: Bind ACh; two types:

  • Muscarinic: Sweat glands.

  • Nicotinic: All postganglionic neurons, adrenal medulla.

Effects of the Sympathetic Nervous System on Target Cells

Activation of the sympathetic nervous system triggers widespread physiological changes.

• Cardiac Muscle: Norepinephrine binding to beta-1 receptors increases heart rate and force of contraction, raising blood pressure.

• Smooth Muscle:

  • Vasoconstriction: Alpha-1 receptors cause constriction of blood vessels in digestive, urinary, and integumentary systems, diverting blood to skeletal and cardiac muscle.

  • Bronchodilation: Beta-2 receptors relax bronchioles, increasing oxygen intake.

  • Vasodilation: Beta-2 receptors relax blood vessels serving skeletal and cardiac muscle.

  • Sphincter Contraction: Norepinephrine contracts urinary and digestive sphincters.

  • Digestive Tract Relaxation: Beta-3 receptors relax smooth muscle, slowing digestion.

  • Pupil Dilation: Alpha-1 receptors contract dilator pupillae muscles.

  • Exocrine Gland Constriction: Beta-2 receptors decrease secretion (except sweat glands).

• Metabolic Rate: Norepinephrine increases ATP consumption, lipid breakdown (beta-3), glucose release (beta-2), and glucagon secretion (beta-2).

• Sweat Glands: ACh increases secretion via muscarinic receptors, aiding temperature regulation.

• Adrenal Medulla: Preganglionic neurons stimulate adrenal medulla to release epinephrine (80%) and norepinephrine (20%) into the bloodstream, prolonging sympathetic effects and reaching non-innervated cells.

• Other Effects: Enhances mental alertness, increases blood clotting, elevates skeletal muscle tension, triggers goosebumps, and causes ejaculation.

Pharmacology of Sympathetic Nervous System Receptors

Drugs can modulate sympathetic activity by acting as agonists or antagonists at specific receptors.

Drug Type	Receptor Target	Effect	Clinical Use
Alpha-1 Blockers (Antagonists)	Alpha-1	Prevent vasoconstriction	Hypertension, benign prostatic hyperplasia
Alpha-2 Agonists	Alpha-2	Decrease sympathetic output	Hypertension, opiate withdrawal
Beta Blockers (Antagonists)	Beta-1	Decrease heart rate and contraction	Hypertension, cardiovascular disease
Beta-2 Agonists	Beta-2	Bronchodilation	Asthma

The Parasympathetic Nervous System

Anatomy of the Parasympathetic Nervous System

The parasympathetic nervous system is active during rest and promotes digestion, urinary function, and reproductive processes.

• Cranial Nerves: Oculomotor (III), Facial (VII), Glossopharyngeal (IX), Vagus (X) house preganglionic neurons.

• Sacral Nerves: S2-S4 supply pelvic organs via pelvic splanchnic nerves.

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

• Vagus Nerve: Provides majority of parasympathetic innervation to thoracic and abdominal viscera.

• Accommodation: Oculomotor nerve innervates ciliary muscle for near vision.

Parasympathetic Neurotransmitters and Receptors

Both preganglionic and postganglionic parasympathetic neurons release acetylcholine (ACh), which generally has excitatory effects.

• Nicotinic Receptors: Found on all postganglionic parasympathetic neurons.

• Muscarinic Receptors: Found on all parasympathetic target cells.

Effects of the Parasympathetic Nervous System on Target Cells

The parasympathetic nervous system maintains homeostasis during rest by counteracting sympathetic activity.

• Cardiac Muscle: Decreases heart rate and blood pressure via vagus nerve stimulation.

• Smooth Muscle:

  • Pupil Constriction: Oculomotor nerve innervates sphincter pupillae muscle.

  • Lens Accommodation: Oculomotor nerve contracts ciliary muscle for near vision.

  • Bronchoconstriction: Vagus nerve contracts bronchioles.

  • Peristalsis: Vagus nerve contracts digestive tract smooth muscle, promoting food movement.

  • Sphincter Relaxation: Vagus and pelvic splanchnic nerves relax urinary and digestive sphincters, promoting urination and defecation.

  • Genital Engorgement: Pelvic splanchnic nerves cause vasodilation in penis/clitoris.

• Glandular Epithelial Cells: Increases secretion from exocrine glands (except sweat glands).

• Other Effects: No direct effect on metabolic rate, mental alertness, skeletal muscle contraction, blood clotting, adipocytes, or most endocrine secretions.

Pharmacology of the Parasympathetic Nervous System

Drug Type	Receptor Target	Effect	Clinical Use
Muscarinic Agonists	Muscarinic	Mimic parasympathetic actions	Stimulate GI activity, bladder emptying
Muscarinic Antagonists	Muscarinic	Block parasympathetic actions	Treat slow heart rate, motion sickness, hypersalivation

Homeostasis: Interactions of Autonomic Divisions

Dual Innervation and Autonomic Tone

Most organs receive input from both sympathetic and parasympathetic neurons, allowing for precise regulation and maintenance of homeostasis.

• Dual Innervation: Both divisions innervate most organs, enabling antagonistic control.

• Autonomic Tone: Constant baseline activity from each division; includes sympathetic tone and parasympathetic tone.

• Parasympathetic Dominance: Normally dominant in the heart (average rate: 72 bpm), digestive, and urinary systems.

Summary Table: Sympathetic vs. Parasympathetic Effects

Target Organ	Sympathetic Effect	Parasympathetic Effect
Heart	Increases rate and force	Decreases rate
Bronchioles	Dilation	Constriction
Digestive Tract	Decreases motility	Increases motility
Pupil	Dilation	Constriction
Sweat Glands	Increases secretion	No effect
Exocrine Glands	Decreases secretion	Increases secretion
Urinary Sphincters	Contraction	Relaxation

Additional info: The ANS is crucial for maintaining homeostasis by balancing the activity of its two divisions. Pharmacological agents targeting ANS receptors are widely used in clinical medicine to treat cardiovascular, respiratory, and digestive disorders.