Vagus Nerves

Introduction to Vagus Nerves and Cardiovascular Function

  • The vagus nerves play a critical role in supplying many of our organs.

  • In particular, we focus on the branches of the vagus nerves that connect from the cardiovascular center in the medulla oblongata to the heart.

  • The cardiovascular center consists of groups of neurons located in the medulla oblongata.

    • Location: The medulla oblongata is the most inferior part of the brain and it is connected to the spinal cord.

  • The vagus nerves exit from the medulla oblongata, categorizing them as cranial nerves.

Role of Vagus Nerves in Parasympathetic Nervous System

  • The vagus nerves contain parasympathetic axons.

  • The parasympathetic system is responsible for signaling that helps maintain or return the body to a resting and digesting state.

Vagus Nerves and Heart Rate Regulation

Key Cardiac Structures

  • The vagus nerves have branches that primarily innervate:

    • Sinoatrial (SA) Node

    • Atrioventricular (AV) Node

  • Impact: The vagus nerves primarily influence heart rate by affecting the SA and AV nodes.

Mechanism of Action

  • When the vagus nerves send signals towards the cells in the SA and AV nodes:

    • Effect on Heart Rate: The firing of these signals results in a decrease in heart rate.

    • Neurotransmitter Involvement: The vagus nerves release a neurotransmitter called acetylcholine, which inhibits the cells of the SA and AV nodes.

Connection to Cardiac Output and Stroke Volume

Cardiac Output

  • A reduction in heart rate during resting and digesting states leads to a decrease in cardiac output, aligning it closer to resting values.

Stroke Volume Dynamics

  • During resting and digesting:

    • Stroke Volume Behavior: Stroke volume also decreases, trending back towards resting levels.

    • Mechanism Behind Decrease: This occurs because the sympathetic nerves (cardiac accelerator nerves) do not stimulate the myocardium of the heart chambers.

Myocardial Contractility

  • With the withdrawal of stimulation from cardiac accelerator nerves:

    • Decreased Contraction Force: The forcefulness of myocardial contraction decreases, leading to reduced myocardial contractility.

    • Impact on Stroke Volume: Consequently, a decrease in stroke volume is observed.

Summary of Vagus Nerve Functionality

  • In conclusion, the vagus nerves are primarily responsible for decreasing the heart rate due to their significant influence on the SA and AV nodes, while having a minimal impact on myocardial contractility and stroke volume.

Acetylcholine and Cardiac Muscle Inhibition

  • At the terminals of the vagus nerves, acetylcholine is released, which functions as an inhibitory neurotransmitter for cardiac muscle cells.

  • Mechanism of Action:

    • Acetylcholine binds to muscarinic receptors on the cardiac muscle cell's sarcolemma.

    • Effect of Binding:

    • It leads to the opening of potassium ion channels, resulting in an outflow of potassium ions.

    • This outflow causes the cardiac cell to become inhibited, thus contributing to the overall decrease in heart rate.

Conclusion

  • The complex interplay between the vagus nerve, acetylcholine release, and the neural regulation of cardiac functions illustrates the intricate balance maintained by the parasympathetic nervous system in controlling heart rate and overall cardiovascular dynamics.