The Autonomic Nervous System: Structure, Function, and Regulation

Chapter 14: The Autonomic Nervous System (ANS)

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, respiration, digestion, and glandular activity. The ANS operates without conscious control and is essential for maintaining homeostasis.

• Key Functions: Regulation of heart rate, blood pressure, respiratory rate, and digestive processes.

• Visceral Reflex Arcs: Involve sensory input from visceral organs, integration in the CNS, and motor output via autonomic ganglia to target organs.

Functions and Pathways of the ANS

Visceral Reflex Arcs

Visceral reflex arcs are neural pathways that control involuntary functions of internal organs.

• Sensory Input: Sensory information from visceral organs is transmitted by afferent sensory neurons to the spinal cord.

• Integration: The CNS integrates sensory input and sends motor output via efferent autonomic neurons.

• Motor Output: Efferent signals travel through autonomic ganglia to target organs, regulating their activity.

Comparison of Somatic and Autonomic Nervous Systems

The somatic and autonomic nervous systems differ in structure, function, and target tissues.

• Somatic Nervous System:

  • Controls voluntary movements of skeletal muscles.

  • Single motor neuron extends from CNS to muscle.

  • Neurotransmitter: Acetylcholine (ACh).

  • Motor neurons directly innervate target cells.

• Autonomic Nervous System:

  • Controls involuntary activity of smooth muscle, cardiac muscle, and glands.

  • Two-neuron chain: preganglionic and postganglionic neurons.

  • Neurotransmitters: Acetylcholine (ACh), Norepinephrine (NE), Epinephrine (E).

  • Motor neurons do not directly innervate target cells.

Divisions of the ANS

Sympathetic Nervous System

The Sympathetic Nervous System prepares the body for 'fight or flight' responses during stressful situations.

• Anatomy:

  • Preganglionic axons are usually short; postganglionic axons are usually long.

  • Cell bodies located in thoracic and upper lumbar spinal cord (thoracolumbar division).

  • Sympathetic chain ganglia run parallel to the spinal cord.

• Functions:

  • Increases heart rate and blood pressure.

  • Dilates pupils and bronchioles.

  • Inhibits digestive and urinary functions.

  • Stimulates sweat glands and adrenal medulla.

Parasympathetic Nervous System

The Parasympathetic Nervous System supports 'rest and digest' activities, conserving energy and maintaining homeostasis during restful states.

• Anatomy:

  • Preganglionic axons are usually long; postganglionic axons are usually short.

  • Cell bodies located in brainstem and sacral spinal cord (craniosacral division).

  • Major cranial nerves involved: Oculomotor (III), Facial (VII), Glossopharyngeal (IX), Vagus (X).

• Functions:

  • Decreases heart rate and blood pressure.

  • Constricts pupils and bronchioles.

  • Stimulates digestive and urinary functions.

  • Promotes secretion from glands.

Gross and Microscopic Anatomy of the Sympathetic Nervous System

Types of Sympathetic Ganglia

Type	Location	Function
Sympathetic Chain Ganglia	Along both sides of vertebral column	Innervate organs in thoracic cavity and body wall
Collateral (Prevertebral) Ganglia	Anterior to vertebral column	Innervate organs in abdominopelvic cavity

Neurotransmitters of the Sympathetic Nervous System

• Preganglionic neurons: Release Acetylcholine (ACh).

• Postganglionic neurons: Release Norepinephrine (NE), Epinephrine (E), or sometimes ACh.

Receptors of the Sympathetic Nervous System

• Cholinergic Receptors: Bind ACh.

  • Muscarinic receptors: Found on sweat glands; effect varies by tissue.

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

• Adrenergic Receptors: Bind NE and E; divided into alpha and beta types.

Receptor Type	Location	Effect
Alpha-1	Blood vessels, many organs	Vasoconstriction, increased BP
Alpha-2	Presynaptic terminals	Inhibits NE release
Beta-1	Heart	Increases heart rate and force
Beta-2	Bronchioles, blood vessels	Bronchodilation, vasodilation
Beta-3	Adipose tissue	Lipolysis

Pharmacology of Sympathetic Receptors

• Agonists: Stimulate receptor activity (e.g., alpha-2 agonists for hypertension).

• Antagonists (Blockers): Inhibit receptor activity (e.g., beta-blockers for hypertension).

Gross and Microscopic Anatomy of the Parasympathetic Nervous System

Parasympathetic Pathways

• Preganglionic neurons: Originate in brainstem or sacral spinal cord; synapse in terminal ganglia near or within target organs.

• Major cranial nerves: Oculomotor (III), Facial (VII), Glossopharyngeal (IX), Vagus (X).

• Vagus nerve: Provides ~75% of all parasympathetic outflow.

Neurotransmitters and Receptors of Parasympathetic Nervous System

• Cholinergic Receptors: Bind ACh.

  • Muscarinic receptors: Found on all parasympathetic target cells.

  • Nicotinic receptors: Found on all postganglionic neurons.

Effects of Parasympathetic Nervous System on Target Cells

• Decreases heart rate and blood pressure

• Pupil constriction

• Stimulates digestion and urinary function

• Increases glandular secretion

Homeostasis and Autonomic Tone

Innervation of Autonomic Divisions

• Most organs receive dual innervation from both sympathetic and parasympathetic divisions.

• Sympathetic division dominates during emergencies ('fight or flight').

• Parasympathetic division maintains homeostasis at rest ('rest and digest').

Autonomic Tone

• Both divisions maintain a baseline level of activity, allowing for rapid adjustments in organ function.

Additional info: The balance between sympathetic and parasympathetic activity is crucial for maintaining physiological stability and responding appropriately to internal and external stimuli.