Cardiac Conducting System

Introduction

The cardiac conducting system is a specialised network of cells within the heart responsible for controlling and coordinating the heart's rhythm and contractions. This system ensures that the heart beats in a regular and coordinated manner, allowing for efficient blood circulation throughout the body.

Structure of the Cardiac Conducting System

The cardiac conducting system consists of several key components:

1. Sino-Atrial Node (SAN)

• Location: The SAN, also known as the pacemaker, is located in the right atrium's wall.
• Function: The SAN generates electrical impulses that initiate each heartbeat. These impulses serve as the heart's natural pacemaker, setting the rhythm for the entire heart.

2. Atrio-Ventricular Node (AVN)

• Location: The AVN is situated in the centre of the heart.
• Function: The AVN acts as a relay station, receiving electrical signals from the SAN and transmitting them to the ventricles. It introduces a slight delay in the signal to allow the atria to contract fully before the ventricles do.

3. Conducting Fibres

• Location: Specialised conducting fibres extend from the AVN through the central wall of the heart and then up through the ventricles' walls.
• Function: These fibres rapidly transmit electrical impulses to the ventricular muscle cells, causing coordinated ventricular contraction (ventricular systole).

Control of Contraction and Timing

The cardiac conducting system ensures that the heart contracts rhythmically and in a coordinated manner. The process is as follows:

1. Initiation

The heartbeat originates in the heart itself, specifically in the SAN. The auto-rhythmic cells of the SAN spontaneously depolarise, generating electrical impulses that initiate each heartbeat.

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Cardiac Conducting System - Structure & Function

2. Atrial Contraction

The impulses from the SAN spread through the atria, causing atrial systole (atrial contraction). This phase ensures that blood is effectively transferred from the atria to the ventricles.

3. Transmission to Ventricles

After the atria contract, the electrical signals travel to the AVN. The AVN briefly delays the impulses to allow the ventricles to fill completely with blood from the atria.

4. Ventricular Contraction

Subsequently, the impulses are rapidly transmitted along the conducting fibres, causing ventricular systole (ventricular contraction). This phase forces blood into the aorta and pulmonary artery for distribution to the body and lungs, respectively.

Electrocardiogram (ECG) Interpretation

1. ECG

Impulses generated in the heart generate electrical currents that can be detected by an electrocardiogram (ECG), a diagnostic tool used to monitor the heart's electrical activity.

2. Heart Rate Calculation

Interpretation of an ECG involves calculating the heart rate by measuring the time intervals between consecutive heartbeats.

3. Identification of Waves

ECG interpretation also involves identifying specific waves that correspond to different phases of the cardiac cycle. These waves include the P-wave (atrial depolarisation, atrial systole), QRS complex (ventricular depolarisation, ventricular systole), and T-wave (ventricular repolarisation, ventricular diastole).

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Cardiac Conducting System-Structure & Function

Control of Heart Rate

1. The Medulla

Heart rate is regulated by the autonomic nervous system (ANS), specifically the medulla oblongata in the brainstem.

2. Sympathetic Nervous System

When the body requires an increased heart rate, the sympathetic nervous system releases the neurotransmitter noradrenaline, which stimulates the SAN to generate impulses more rapidly, leading to an elevated heart rate.

3. Parasympathetic Nervous System

Conversely, the parasympathetic nervous system releases acetylcholine, which slows down the heart rate by inhibiting the SAN's impulse generation.

Summary

The cardiac conducting system is essential for the heart's rhythmic and coordinated contractions. It consists of the SAN, AVN, and conducting fibres, each with specific roles. The SAN initiates the heartbeat, while the AVN delays the impulses to allow atrial contraction before ventricular contraction occurs. The conducting fibres transmit impulses to the ventricles, ensuring synchronised ventricular contraction. ECGs are used to monitor the heart's electrical activity, calculate heart rate, and identify specific waves corresponding to different phases of the cardiac cycle. The autonomic nervous system, through the sympathetic and parasympathetic branches, regulates heart rate to meet the body's demands. Understanding the cardiac conducting system is crucial for comprehending heart function and its control mechanisms.