Answered: Explain the structure and function of…

Human Anatomy & Physiology (11th Edition)

11th Edition

ISBN:9780134580999

Author:Elaine N. Marieb, Katja N. Hoehn

Publisher:Elaine N. Marieb, Katja N. Hoehn

Chapter1: The Human Body: An Orientation

Section: Chapter Questions

Problem 1RQ: The correct sequence of levels forming the structural hierarchy is A. (a) organ, organ system,...

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Question

Explain the structure and function of the conducting
system of the heart.

Expert Solution

Step 1

Introduction:

The cardiovascular or the circulatory system is the body's transport system via which the nutrients are conveyed to places where these are used, and the waste products (metabolites) are conveyed to appropriate places from where these are expelled. The conveying medium is the blood, a liquid connective tissue that flows in the tubular channels referred to as the blood vessels. The central pumping organ maintains the circulation, referred to as the heart.

Step 2

Answer:

CARDIAC CONDUCTION SYSTEM

Some cardiac tissues are concerned with the initiation (impulse generation), and propagation (conduction) of the heartbeat is referred to as the pacemaker tissues. These are the conducting system of the heart. They include the following:

SINOATRIAL NODE (SAN):

Location: It is present on the heart's posterior aspect at the superior vena cava junction with the right atrium.
Dimensions: 15 mm in length, 2 mm in width, and 1 mm in thickness.
Structure: The cells' outline is ill-defined, highly vascular, comprises thin, elongated muscle fibers, rich in glycogen and mitochondria, fusiform shaped with longitudinal striations. These are referred to as the P cells or pacemaker cells. These fibers can normally discharge and generate impulses more rapidly than any other pacemaker tissue, and their discharge rate determines the rate at which the heart beats. This is why the SA node is referred to as the cardiac pacemaker.
Innervation: It develops from the structures present on the embryo's right side. Because of this, in adults, SAN is innervated by the right vagus nerve. It predominantly receives the sympathetic nerve fibers of the right side from the cervical sympathetic ganglia through the cardiac nerves.

ATRIOVENTRICULAR NODE (AVN):

Location: It is present on the posterior aspect of the interatrial septum's right side near the coronary sinus opening.
Structure: It is similar to that of the sinoatrial node.
Innervation: It is the embryo's left-sided structure. Thus, in adults, it has the innervation of the left vagus nerve. It also primarily receives the sympathetic nerve supply from the left side.
Transmission of the impulses generated at SAN towards the AVN: The modified muscle fibers from the coronary sinus region collects fanwise and combine with the atrioventricular node, referred to as the internodal atrial pathway. They are responsible for the conduction of impulses from the sinoatrial node to the atrioventricular node by arranging themselves into three categories of bindles:

Bachmann's anterior internodal tract: It can conduct impulses directly from the sinoatrial node to the left atrium.
Middle inbternodal tract of Wenckebach.
A posterior internodal tract of Thorel.

THE BUNDLE OF HIS OR THE ATRIOVENTRICULAR BUNDLE:

It is originated from the atrioventricular node and then travels upwards to the membranous intraventricular septum's posterior margins and then forward inferior to it, isolated and unsheathed in a canal. At the membranous septum's anterior region, the bundles bifurcate into the left and the right branch.
The left branch causes the membrane's piercing and is placed on the muscular septum's upper border and then divides into a posterior and anterior fascicle. The right branch passes down the septum's right side.
Both the branches divide continuously to form a fiber network lying subendocardial in the ventricles.

PURKINJE FIBERS:

They originate from the terminal divisions of the left and right branches of the atrioventricular bundle to penetrate the ventricular wall.
These fibers are thicker and larger as compared to sinoatrial node fibers. Their length is 10 to 46 μm and is 70 to 80 μm in diameter.
Due to the larger diameter and extremely high level of permeability of the gap junction at the intercalated disc, they transmit the impulse at s faster velocity of 4 meters per second compared to other conducting tissues. This permits the most rapid transmission of the heart impulses throughout the whole ventricular system.