Autonomic nervous system Lec 17

The Autonomic Nervous System (ANS)

Overview

• The ANS controls involuntary bodily functions.

• It regulates organs not typically under voluntary control.

Goals of the Study

• Describe the basic anatomy of the ANS.

• Compare and contrast the parasympathetic and sympathetic systems.

• Discuss their roles in "fight or flight" vs. "rest and digest" responses.

• Provide physiological examples of antagonistic and complementary functions of the SNS (Sympathetic Nervous System) and PNS (Parasympathetic Nervous System).

Chapter Reference

• Chapter 11 from the 8th edition of Silverthorn: "Autonomic Nervous System."

Outline of Content

1. Introduction

2. Anatomy of the ANS

   • Sympathetic branch

   • Parasympathetic branch

   • Cranial nerves/vagus nerve

3. Cellular Physiology of the ANS

   • Receptors

   • Neurotransmitters

   • Convergence and divergence

   • Effects on target organs

4. Balancing SNS and PNS for homeostasis

   • Dual innervation

   • Control without dual innervation

   • CNS centers contributing to autonomic regulation

1.1 Introduction

• Central Nervous System (CNS): Composed of brain and spinal cord; processes information from the periphery.

• Peripheral Nervous System (PNS) is divided into:

  • Sensory (afferent) division

  • Motor (efferent) division:

    • Somatic nervous system

    • Autonomic nervous system (ANS)

1.2 Autonomic Nervous System (ANS)

• Involuntary Control: Regulates organs like smooth muscle, cardiac muscle, and glands.

• Reflex Mechanisms:

  • Reflexes involve sensory input to the CNS (afferent), processing (integration), and autonomic output (efferent).

  • They maintain homeostasis and respond to stimuli through negative feedback or feedforward mechanisms.

• Functions of ANS: Important for maintaining homeostasis in body systems.

Features of ANS

• Autonomic indicates self-governed activities that occur without conscious intent.

• Reflexes support involuntary reactions to internal stimuli.

2. Anatomy of the ANS

General Structure

• Nerve pathways consist of a two-neuron chain:

  1. Preganglionic fiber: Synapses with the second neuron.

  2. Postganglionic fiber: Innervates the target organ.

• Ganglion: A mass of neuronal cell bodies where the synapse occurs.

2.1 Sympathetic Nervous System (SNS)

• Originates from the thoracic and lumbar regions (T1-L2).

• Short preganglionic fibers and long postganglionic fibers.

• Preganglionic fibers release acetylcholine (ACh); most postganglionic fibers release norepinephrine (NE).

Pathways in SNS

• After exiting the spinal cord, SNS fibers can:

  1. Synapse in a sympathetic chain ganglion.

  2. Pass through to synapse in the adrenal medulla.

  3. Pass through and synapse in a collateral ganglion.

2.2 Parasympathetic Nervous System (PNS)

• Origination from cranial and sacral areas of the CNS.

• Long preganglionic fibers and very short postganglionic fibers.

• Both types of fibers release ACh.

2.3 Cranial Nerves with Parasympathetic Functions

• Four cranial nerves involved:

  • Oculomotor nerve (III): Controls lens and pupil.

  • Facial nerve (VII): Innervates tear glands, salivary glands, and nasal glands.

  • Glossopharyngeal nerve (IX): Innervates salivary glands.

  • Vagus nerve (X): Contains 70-90% of parasympathetic fibers, affecting many viscera.

The Vagus Nerve

• Tenth cranial nerve, known for its extensive branching and "wandering" nature, innervates almost every organ except for the adrenal medulla.

3. Cellular Physiology of the ANS

3.1 Autonomic Neurotransmitters

• SNS: Preganglionic neuron releases ACh, affecting nicotinic receptors, while postganglionic neurons release NE at adrenergic receptors.

• PNS: Both pre and postganglionic neurons release ACh, impacting muscarinic and nicotinic receptors.

3.2 Convergence and Divergence

• SNS exhibits more divergence than PNS; one preganglionic neuron may synapse with many postganglionic neurons, leading to amplification of responses.

3.3 Autonomic Effects

• SNS generally produces longer-lasting effects than PNS due to slower breakdown mechanisms for NE, which can remain in circulation.

3.4 Receptors on Target Organs

• SNS Receptors: Involves alpha and beta-adrenergic receptors affecting various cellular responses.

• PNS Receptors: Nicotinic and muscarinic receptors play significant roles in signaling.

4. Balancing SNS and PNS for Homeostasis

Dual Innervation

• Most organs receive input from both systems which may provide opposing or cooperative effects.

Effects of Autonomic Stimulation

• Summary of effects on different organs:

  • Heart: SNS increases rate and force; PNS decreases.

  • Blood Vessels: SNS causes constriction; PNS dilates a few specific vessels.

  • Lungs: SNS dilates airways; PNS constricts airways.

  • Digestive Tract: SNS decreases activity; PNS stimulates.

  • Urinary Bladder: SNS relaxes; PNS contracts.

Control without Dual Innervation

• Examples include adrenal medulla and sweat glands that are innervated only by the SNS.

CNS Centers Contributing to Autonomic Regulation

• Limbic System: Integrates sensory and emotional responses.

• Hypothalamus: Major control center for hunger, thirst, and emotional states affecting autonomic output.

• Brainstem and Spinal Cord: Mediate various autonomic responses and reflexes for micturition and defecation.