Overview of the Autonomic Nervous System

NURS 1231: Autonomic Nervous System Study Notes

Textbook and Resources

• Primary Text:

  • Human Anatomy & Physiology, 11th edition by Marieb and Hoehn.

• Additional Resources:

  • Information available on Blackboard.

  • Course Outline.

  • Figures sourced from the 11th edition of Marieb and Hoehn unless stated otherwise.

Chapter 14 Learning Objectives

1. General Innervation by the ANS

   • Understand what the Autonomic Nervous System (ANS) innervates (p. 532).

2. Comparison with Somatic Nervous System

   • Compare the neurotransmitters and synapses between the ANS and the somatic nervous system (p. 532-533).

3. Divisions of ANS

   • Outline the two divisions of the ANS, their roles, and major anatomical differences (p. 534-535).

4. Basic Anatomy of the Parasympathetic Nervous System

   • Describe the key features of the Parasympathetic Nervous System (p. 536-537).

5. Basic Anatomy of the Sympathetic Nervous System

   • Describe the key features of the Sympathetic Nervous System (p. 538-542).

6. Neurotransmitters and Receptors in ANS

   • Identify the neurotransmitters and receptors present in the ANS (p. 543-545).

7. Effects of Drugs on Receptors

   • Understand the effects of specific drugs on these receptors (p. 544).

8. Effects of ANS Divisions on Organs

   • Examine how the two divisions of the ANS affect various organs and their interaction (p. 545-546).

Autonomic Nervous System Overview

• The Autonomic Nervous System (ANS) consists of motor neurons that:

  • Innervate smooth muscle, cardiac muscle, and glands.

  • Make physiological adjustments to ensure optimal support for body activities (e.g., regulate blood flow, heart rate, blood pressure, digestive processes).

  • Operate under subconscious control, also known as the involuntary nervous system or general visceral motor system.

Comparison Between Autonomic and Somatic Nervous Systems

Key Differences

1. Effectors

   • Somatic Nervous System (SNS):

     • Innervates skeletal muscles.

   • Autonomic Nervous System (ANS):

     • Innervates cardiac muscle, smooth muscle, and glands.

2. Efferent Pathways and Ganglia

   • Somatic NS:

     • Cell body located in the Central Nervous System (CNS), with a thick myelinated axon extending directly to the skeletal muscle.

   • Autonomic NS:

     • Utilizes a two-neuron chain:

     • Preganglionic neuron: Cell body in CNS; lightly myelinated axon.

     • Postganglionic neuron: Cell body synapses with the preganglionic axon in an autonomic ganglion; non-myelinated axon.

3. Target Organ Responses to Neurotransmitters

   • Somatic Nervous System:

     • All motor neurons release Acetylcholine (ACh); the response is always stimulatory.

   • Autonomic Nervous System:

     • Preganglionic fibers release ACh; postganglionic fibers can release either Norepinephrine (NE) or ACh, resulting in either stimulatory or inhibitory effects depending on the receptor type.

Two Major Divisions of the Autonomic Nervous System

• Parasympathetic Division:

  • Functions to promote maintenance activities and conserve energy.

  • Known as the "Rest and Digest" system.

• Sympathetic Division:

  • Activates processes to mobilize body responses during stress ("Fight or Flight").

General Functions of Divisions

• Dual Innervation: Most visceral organs receive input from both divisions, producing opposite effects for the maintenance of homeostasis.

Parasympathetic Division Functions

• Directs activities associated with rest and energy conservation:

  • Digestion, diuresis, and defecation are prioritized.

  • Example: After a meal, a person reading will experience lower blood pressure, heart rate, and respiratory rates, while gastrointestinal activity is heightened.

  • Characteristics:

  • Pupils constricted.

  • Lenses accommodated for close vision.

Sympathetic Division Functions

• Activated during emergency situations, excitement, or physical activity:

  • Increases heart rate, dilates pupils, and diverts blood flow to essential organs (e.g., skeletal muscles).

  • Inhibits non-essential functions (e.g., digestion).

  • Other physiological changes include:

  • Shunting blood to skeletal muscles and heart.

  • Dilation of bronchioles for improved oxygen intake.

  • Liver releases glucose to provide energy.

Anatomical Differences Between Divisions

1) Sites of Origin

• Parasympathetic:

  • Fibers originate in the brain stem (cranial) and sacral spinal cord.

• Sympathetic:

  • Fibers originate in the thoracic and lumbar spinal cord (T1-L2).

2) Relative Lengths of Fibers

• Parasympathetic:

  • Long preganglionic fibers; short postganglionic fibers.

• Sympathetic:

  • Short preganglionic fibers; long postganglionic fibers.

3) Location of Ganglia

• Parasympathetic: Ganglia located within or near the visceral effector organs.

• Sympathetic: Ganglia located close to the spinal cord.

Summary of Autonomic Ganglia

Neurotransmitters in the ANS

• Major Neurotransmitters:

  • Acetylcholine (ACh) and Norepinephrine (NE).

  • ACh characteristics:

  • Released by all ANS preganglionic axons and all parasympathetic postganglionic axons.

  • NE characteristics:

  • Released by almost all sympathetic postganglionic axons, except those at sweat glands which release ACh.

Cholinergic Receptors

1) Nicotinic Receptors

• Found on:

  • All postganglionic neurons (both SNS and PNS).

  • Hormone-producing cells of the adrenal medulla.

• Effect of ACh at these receptors is always stimulatory, as ion channels open and depolarize the postsynaptic cell.

2) Muscarinic Receptors

• Found on effector cells stimulated by postganglionic cholinergic fibers in parasympathetic target organs and some sympathetic sweat glands.

• Can be either inhibitory or excitatory, depending on the receptor type and target organ (e.g., in cardiac muscle cells, ACh slows heart rate, while in intestinal smooth muscle, it increases motility).

Adrenergic Receptors

• Two major classes that respond to NE or epinephrine:

  • Alpha (α) Receptors: α1, α2.

  • Beta (β) Receptors: β1, β2, β3.

Effects of Drugs on Neurotransmitter Receptors

• Muscarinic Agents (e.g., Bethanechol): Enhance parasympathetic activity.

• Adrenergic Agents (e.g., Albuterol): Activate β2 adrenergic receptors, dilating bronchioles.

• Adrenergic Blockers (e.g., Propranolol): Block β adrenergic receptors, reducing blood pressure.

Summary of Drug Effects on ANS

Summary of ANS

autonomic nervous system provides dual innervation to visceral organs involving antagonistic or cooperative interactions which maintain homeostasis during various bodily states.[END OF NOTES]