Properties of cardiac Muscle and Electrical activity of the Heart

OJO AO

DEPT. OF PHYSIOLOGY

COLLEGE OF HEALTH SCIENCES

BOWEN UNIVERSITY

Introduction

Cardiovscular physiology involves the study of the funtioning of heart and blood vessels, the function is basical how blood is supplied the body. The heart is composed of contractile muscle, similar to our skeletal muscle.The heart acts as a pump for our bodies’ blood supply.Blood is pumped to the lungs via the right ventricle for oxygenation.Blood is pumped to the tissue via the left ventricle to distribute oxygen throughout the body.Heart consists of 4 chambers (2 atria & 2 ventricles). Atria are smaller than ventricles, left ventricle bigger than right ventricle.

Blood flows in the following order: . Right atrium - Right ventricle - Lungs - Left atrium - Left ventricle- Rest of Body- Right atrium

The Heart

The heart is a hollow, muscular organ about the size of a clenched fist.

It lies in the thoracic (chest) cavity about midline between the sternum (breastbone) anteriorly and the vertebrae (backbone) posteriorly.

Its mass is between 250 and 350 grams

• The heart is enclosed in a double-walled sac called the PERICARDIUM.

The loosely fitting superficial part of this sac is the fibrous pericardium- it is tough, dense connective tissue layer (1) protects the heart, (2) anchors it to surrounding structures, and (3) prevents overfilling of the heart with blood.

• Deep to the fibrous pericardium is the serous pericardium, a thin, slippery, double-layer serous membrane. Its parietal layer lines the internal surface of the fibrous pericardium

• On the surface of the heart muscle is the visceral pericardium, often called the epicardium (“upon the heart”), which is an integral part of the heart wall

• Between the parietal and visceral layers is the slit-like pericardial cavity, which contains a film of serous fluid.

• The serous membranes, lubricated by the fluid, glide smoothly past one another during heart activity, allowing the mobile heart to work in a relatively friction-free environment.

The heart wall has three distinct layers:

• A thin inner layer, the endothelium, a unique type of epithelial tissue that lines the entire circulatory system

• A middle layer, the myocardium, which is composed of cardiac muscle and constitutes the bulk of the heart wall

• A thin external layer, the epicardium, that covers the heart

lumenlearning.com/wm-biology2/chapter/structure-of-the-heart/

The Cardiac MuscleBranched

• Centrally located nucleus

• Muscle cells of the heart-more commonly called myocytes or myofibrils.• Outside membrane is called sarcolemma.

• Cardiac muscles have the same arrangement of actin and myosin, and the same bands, zones and Z discs as skeletal muscles forming sarcomeres.

• They do have less sarcoplasmic reticulum than skeletal muscles and require Calcium from extra cellular fluid for contraction as T-tubules are not well organized.

Although the cardiac muscles interdigitate & branch, there is no anatomical continuity b/w the individual muscle fibers. • Cardiac muscle are branched, have a single

nucleus and are interconnected to each other, end to end by specialized structures called as INTERCALATED DISCS. The intercalated discs are further composed of:

1. Gap Junctions2. Desmosomes

• Heart consists of 4 chambers • (2 atria & 2 ventricles). • Atria are smaller than ventricles, left ventricle bigger than right ventricle.• Blood flows in the following order: 1.Right atria2. Right ventricle3. Lungs4. Left atria5. Left ventricl6. Rest of Body

lumenlearning.com/wm-biology2/chapter/structure-of-the-heart/

• Even thought the heart is a single organ, the left and the right side of the heart is anatomically and functionally separate. • This is done with the help of the

interventricular spetum. • It ensures that the blood from

the left and right side of the heart does not mix. • Although the left and the right

sides are separated, the heart contracts in a co-ordinated fashion: the atria contract together and the ventricles contract together….

O2 poor blood returns from the body thru the Superior & Inferior Vena Cava

↓Enters the Right atrium

↓Right ventricle

↓Pulmonary artery

↓Lungs

↓Blood is Oxygenated

↓Pulmonary Veins

↓Left Atrium

↓Left Ventricle

↓Aorta

↓Circulated to the body

Properties of cardiac Muscle

1. Automaticity/Rhythmicity• Automaticity means the ability of the cell to undergo depolarization

spontaneously causing the production of electrical impulses• Ability to generate self propagted impulse• Rhythmicity means that spontaneous depolarization occurs at

regular intervals

2. Conductivity

• Conductivity is the ability to propagate an impulse.• Normally impulses are conducted in one direction• Conductivity may be increased or decreased under various

circumstances

3. Contractility

• Cradiac muscle contracts in response to the electrical impulse generated by the SA node

4. Refractory Period

• The refractory period of the myocardial fibers is of much longer duration than that of skeletal muscle fibers and lasts approximately as long as the cardiac contraction

5. Excitability

• The heart muscle responds to stimuli which may be mechanical, electrical or chemical

6. All or None Law

• Heart is a functional syncytium therefore all its fibers act as a single fiber• Heart either does not contract at all or it contracts with full force

7. Distensibility

Conducting System of the heart• The Sinoatrial node (SA node), is a group of autorhythmic cells (main

pacemaker of the heart) in the right atrium near the entry of the superior vena cava.

• An internodal pathway connects the SA node to the atrioventricular node (AV node), a group of autorhythmic cells found near the floor of the right atrium.

• From the AV node action potentials move into fiber known as the bundles of his or atrioventricular bundle. The bundle passes from the AV node into the wall of the septum between the ventricles.

• A short way down the septum the bundle divides into left and right bundle branches.

• These fibers continue downward to the apex where they divide into many small purkinje fibers that spread outward among the contractile cells.

http://www.heart.org/HEARTORG/Conditions/Arrhythmia

• Sinoatrial Node: The sinoatrial (SA) node is located in the right atrial wall, just inferior to the entrance of the superior vena cava.

• The SA node generates impulses about 75 times every minute. (However, its inherent rate in the absence of extrinsic neural and hormonal factors is closer to 100 times per minute)

• Atrioventricular Node: From the SA node, the depolarization wave spreads via gap junctions throughout the atria and via the internodal pathway to the atrioventricular (AV) node.

• At the AV node, the impulse is delayed for about 0.1 s, allowing the atria to respond and complete their contraction before the ventricles contract.

• Atrioventricular Bundle: From the AV node, the impulse sweeps to the atrioventricular (AV) bundle (also called the bundle of His) in the superior part of the interventricular septum.

• Right and left bundle branches. The AV bundle persists only briefly before splitting into two pathways—the right and left bundle branches, which course along the interventricular septum toward the heart apex

• Purkinje fibers: The Purkinje fibers complete the pathway through the interventricular septum, penetrate into the heart apex, and then turn superiorly into the ventricular walls.

• Because the left ventricle is much larger than the right, the Purkinje

network is more elaborate in that side of the heart

Myocyte Action Potentials

• Fast and Slow• Fast = non-pacemaker

cells• Slow = pacemaker cells

(SA and AV node)

• Phase 0• Stimulation of the

myocardial cell• Influx of sodium• The cell becomes

depolarize• Inside the cell = +20 mV

Ventricular Myocyte Action Potential

• Phase 1

• Ions

• Efflux of potassium

• Partial repolarization

• Phase 2

• Ions

• Sodium

• Efflux of potassium

• Influx of calcium

• Plateau

• Phase 3

• Ions

• Efflux of potassium*

• Influx of calcium

• Repolarization (slower process than depolarization)

• Phase 4

• Interval between repolarization to the next action potential

• Pumps restore ionic concentrations

Ion 0 1 2 3 4

Nainflux

influx

pump

Kefflux

efflux

efflux*

pump

Cainflux

influx pump

• Absolute refractory period - phase 1 - midway through phase 3

• Relative refractory period - midway through phase 3 - end of phase 3

SA NODE ACTION POTENTIAL• Funny” currents (phase 4);

slow Na channels that initiate spontaneous depolarization

• No fast sodium channels

• Calcium channels (slow)• Long-lasting, L-type• Transient, T-type

• Potassium channels

Phase 4: Pacemaker Potential: • Opening of voltage-gated Sodium channels

called Funny channels (If or f channels ).

• Closure of voltage-gated Potassium channels.• Opening of Voltage-gated Transient-type

Calcium (T-type Ca2+ channels) channels .

Phase 0: The Rising Phase or Depolarization:• Opening of Long-lasting voltage-gated

Calcium channels (L-type Ca2+ channels).• Large influx of Calcium.

Phase 3: The Falling Phase or Repolarization:• Opening of voltage-gated Potassium channels• Closing of L-type Ca channels.• Potassium Efflux.

• non-pacemaker action potentials can change into pacemaker cells under certain conditions. E.G hypoxic condition,

• the membrane depolarizes,

• closes fast Na+ channels.

• At a membrane potential of about –50 mV, fast Na+ channels are inactivated.

• This may elicit actionpotentials, inward current are carried by Ca++ (slow inward channels) exclusively.

• These action potentials resemble those found pace maker cells located in the SA node,and can sometimes display spontaneous depolarization and automaticity.

• This mechanism may serve as the electrophysiological mechanism behind certain types of ectopic beats and arrhthymias particularly in ischemic heart disease and following myocardial infarction