28 - Physiological characteristics of the working myocardium. Excitation and contraction. Refractory periods. Extrasystoles, flutter and fibrillation. Myocardial metabolism.

sections

• physiological characteristics of heart (working myocardium)

• structure of myocardium

• excitation and contraction

• main electro physiological properties of contractive myocardium are

• extrasystoles

• atrial flutter

• atrial fibrillation

• physiological characteristics of heart (working myocardium)

• The heart contains 4 chambers

• the upper atria receives the venous blood to the two lower ventricles that eject blood into the arteries.

• The right ventricle pumps deoxygenated blood to the lungs, to become oxygenated and the left oxygenated blood goes rest of the body.

• The right atria and ventricle are separated from the left V and A by a septum- prevents mixture of the oxygenated and deoxygenated blood.

• The right atrium receives blood from the SVC and IVC

• left receives from the left the pulmonary veins

• structure of myocardium

Made of three layers of cells

1. purkinje cells

2. cardiomyocytes

3. myocardial endocrine cells

-cells of myocardium have actin and myosin filaments (involuntary skeletal muscle)

-They receive contact by sliding filament mechanism. Cells electrically linked by gap junction which allows electrical impulses to spread across the whole mass

• excitation and contraction

• Cardiac muscle fibers contract via excitation-contraction coupling,

• using a mechanism called calcium-induced calcium release.

The stages of contraction are:

1. Electrical signal starts in the SA node (the heart’s pacemaker) and travels to the AV node, then spreads to heart muscle cells through gap junctions.

2. Action potential causes Na⁺ and Ca²⁺ to enter the heart muscle cells via T-tubules.

3. Calcium binds to troponin-C, which moves the troponin complex and exposes actin binding sites.

4. Myosin binds to actin and pulls it inward using energy from ATP, causing muscle contraction.

5. Calcium is removed by the sarcoplasmic reticulum, troponin blocks actin again, and the muscle relaxes.

   -…………………………………………………………………………………

   Metabolism- ATP needed for contraction and Ca2+

• refractory periods

Refractory periods:

• absolute refractory period

• relative refractory period

-……………………………………………………………………………………………

• Absolute refractory period:I t begins at the start of depolarization and continues through most of repolarization. The cardiac muscle can not become excited during the whole period of systole and early diastole.

  • voltage-gated sodium (Na⁺) channels are either open or inactivated—and in their inactivated state, they cannot reopen until membrane returns to resting potential

  • This ensures each AP is one-way event preventing overlap or backward conduction

• Relative refractory period, is the interval after absolute refractory period during late repolarisation and hyperpolarisation in which a second action potential can be generated but only at a higher stimulus intensity.

  • Sodium channels able to open

  • But potassium channels still open, makes inside more negative than usual

  • MP further from threshold so larger than normal stimulus needed to trigger another AP

• extrasystoles

premature contractions of the heart which temporarily disturb cardiac rhythm – has lower amp of regular one and pause for next regular systole so next systole has higher amp

• atrial flutter

very rapid uniform contraction of the atria 250 to 350 bpm

• atrial fibrillation

irregular very rapid ineffective contractions of section of atrial muscles caused by impulses with a rate of 300 to 600 per min