Vagus Nerves

Vagus Nerves and Cardiovascular Center Overview

  • Function of Vagus Nerves

    • Supply various organs in the body.

    • Focus on branches of vagus nerves related to cardiovascular center in the medulla oblongata.

Cardiovascular Center

  • Location

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

    • The medulla oblongata is the inferior part of the brain, fused to the spinal cord.

    • Vagus nerves exit from this area as they are classified as cranial nerves.

Parasympathetic Axons in Vagus Nerves

  • Role of Parasympathetic Nervous System

    • Vagus nerves contain parasympathetic axons.

    • They aid in maintaining or returning the body to a resting and digesting state.

Impact of Vagus Nerves on Heart Rate

  • Branches of Vagus Nerves

    • Primarily supply the sinoatrial (SA) node and atrioventricular (AV) node.

    • This implies that vagus nerves predominantly influence heart rate.

  • Mechanism of Action

    • When vagus nerve signals are transmitted to the cells in the SA and AV nodes, the result is a decrease in heart rate.

    • Acetylcholine is released from vagus nerves near these nodes, which inhibits the action of these cells.

  • Effects on Cardiac Output

    • As heart rate decreases during resting and digesting, cardiac output also reduces, trending towards resting cardiac output levels.

Stroke Volume Changes

  • Stroke Volume During Resting and Digesting

    • Stroke volume decreases in this state, similarly returning toward resting levels.

  • Reasons for Decreased Stroke Volume

    • The sympathetic nerves (cardiac accelerator nerves) do not stimulate the myocardium (heart muscle) during this period.

    • Lack of stimulation leads to decreased forcefulness of contraction in the ventricles.

    • Resulting reduction in myocardial contractility contributes to decreased stroke volume.

Summary of Vagus Nerves Effects

  • Main Effects

    • Vagus nerves significantly decrease heart rate but have little to no impact on myocardial contractility and stroke volume.

Neurotransmitter Release

  • Acetylcholine

    • Vagus nerves release acetylcholine as a neurotransmitter.

    • Acetylcholine acts as an inhibitory neurotransmitter for cardiac muscle cells.

  • Mechanism of Acetylcholine Action

    • Acetylcholine binds to muscarinic receptors on the sarcolemma (cell membrane of cardiac muscle).

    • The binding results in various mechanisms inhibiting the cardiac cells.

  • Inhibition via Potassium Ion Channels

    • One explanation for inhibition includes the opening of potassium ion channels.

    • The outflow of potassium ions causes the cell to become inhibited, thus affecting cardiac muscle function.