Electrical Activity In The Heart And The Electrocardiogram Notes

Electrical activity in the heart and the electrocardiogram

Excitation contraction coupling

The excitation of a myocyte leads to the opening of L type voltage gated calcium ion channels- this leads to an influx of calcium ions

-extracellular calcium concentration is essential in triggering contraction- binds to ryanodine receptors- opening of channels which leads to the efflux of calcium out of the Sarcoplasmic reticulum

-the calcium binds to troponin C and relieves the steric inhibition and allows the actin and myosin filaments to undergo cross bridge cycling.

Requirements in heart contraction

-contraction must be coordinated

-atria must contract before the ventricles which allows the ventricles to be filled first before they can eject blood.

Gap junctions

-in cardiac muscle the myocytes are electrically and mechanically coupled as a synctium by gap junctions.

-Gap junctions connect the cytoplasm of two cells and consist of 2 connexons (hemi channels). Each connexon is made up of 6 membrane spanning proteins- connexins

-the number of Gap junctions present determines the conduction velocity

Spread of excitation: conduction pathways

-the heart is myogenic. The SAN, specialised knot of myocytes in the posterior wall of the right atrium, it is one of three pacemakers of the heart but as it has the fastest spontaneous rhythm is determines the heart rate (chonotropy) and overrides the other potential pacemaker regions (ectopic pacemakers)

-the exciation generated in the SAN spreads rapidly through the atria (1 m/s )- this is because the SAN and the atria are well coupled as there are large number of gap junctions which means there is a fast conduction velocity.

-the excitation is prevented from spreading immediately to the ventricles because the annulus fibrosus isolates the atria from the ventricles. The insulating region has no gap junctions so there is no spread of excitation.

-The electrical impulses rapidly reaches the AV node- this is a small mass of cells and connective tissue in the lower posterior region of the atrial septum- the AV node is the only electrical connection across the annulus fibrosus- there is a delay of 0.1 seconds as there is a low conduction velocity as there are a few number of gap junctions. Current flow is also slow because the myocytes are very small, narrow diameter and are arranged perpendicular to current flow. (0.05m/s) This delay allows the atria to contract first which allows the ventricles to be filled to be greater volume before they contract.

-the excitation then spreads rapidly through the bundle of his which transmits depolarisation across the annulus fibrosus and along the interventricular septum.

-The bundle of His divides into anterior and posterior, left and right bundle branches that passes down the left or right side of the interventricular septum. From here impulses are transmitted to the endocardial regions

-Fibres from the left and right bundles branches transmit impulses to the Purkinje fibres comprising large diameter cells that conduct electrical impulses very rapidly (3 - 5m/s). The myocytes in the purkinje fibres have large diameters- allows for a rapid current flow, also the myocytes in this region are very electronegative which allows for fast influx of ions into the cell- all parts of ventricles contract simultaneously

-Purkinje fibres transmit electrical impulses rapidly to the subendocardial myocytes

Electrocardiogram- measure electrical activity in the heart

-ECG is used clinically to see abnormalities in electrical activity of the myocardium

-recording of potential changes at the skin surface that is due to depolarisation and repolarisation of the myocardium ,heart muscle

-the spread of cardiac excitation results in extracellular currents which causes extracellular potential changes and can be treated as two moving dipoles in opposite direction. Each dipole is an aggregation of depolarised (negative) and hyperpolarised (positive) regions of the heart. Charge flows between these two dipoles and it is the potential due to these minute currents that can be picked up as potential differences at the skin.

-The spread of action potentials can be represented by a negative pole to a positive pole (electrical dipole)- this causes a deflection in an ECG reading. When there is a uniform excitation there is no potential difference and reading is 0 due to the atria being isoeletric.

- The direction of the negative charge in the extracellular space can be measured with electrodes- often measured from the atria and ventricles as they produce large enough extracellular currents in order to be detected.

Measuring ECG

-ECG is measured using 3 limb electrodes- one on each arm and one on the left leg- three points make the Einthoven’s triangle. ECG trace is generated by resolving the electrical vector arising from the electrical dipole onto one of the three leads connecting the three electrodes

-Lead 1: right arm (-) to left arm (+)- horizontal

Lead 2: Right arm (-) to left leg (+)- 60 degrees below horizontal

Lead 3: left arm (-) to left leg (+) 120 degrees below horizontal

-during cardiac cycle the electrical dipole changes in magnitude and direction so the resolution onto each of the three leads is changing

-standard leads are designed so that positive deflection results in a positive dipole which points towards the left arm (lead 1) or left leg (lead 2 or 3)

-electrical activity is measured due to the cardiac dipole- in a resting myocyte extracellular charge is positive whereas in a depolarised myocyte the extracellular charge is negative.

-the extracellular electrodes measures the difference in negative charge. The flow of negative charge towards the positive electrode represents an upward slope, the flow of negative charge from right...

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