Cardiac Function Review: Action Potentials And Conduction Pathways

Front 1

Cardiac Muscle (Myocytes)

Back 1

Individual cells with a nucleus and plasma membrane (sarcolemma) joined end-to-end to form a long fiber. Each individual muscle fiber comprises a group of multiple parallel myofibrils. Myofibrils consist of two kinds of proteins, actin and myosin. During cardiac contraction myosic-actin interaction leads to the formation of crossbridges. As myofibrils slide past one another, force is generated. Intracellular Calcium is the trigger.

Front 2

Heart Action Potential:
Phase 0:

Back 2

Fast upstroke. Triggered ybr reaching the threshold of Na channels. Na channels open, causing rapid depolarization. Depolarizations stops when channels deactivate they close. Na current is slowed by class IA and IC antiarrythmic drugs. Ex: Quinidine

Front 3

Heart Action Potential:
Phase 1:

Back 3

Partial repolarization. Na channels close. K channels open and start repolarization

Front 4

Heart Action Potential:
Phase 2:

Back 4

Plateau Phase: Ca channels open, depolarization balances K channel repolarization. Class IV antiarrythmic drugs shorten this phase.

Front 5

Heart Action Potential:
Phase 3:

Back 5

Repolarization. Ca channels close, k channels are still open and re-polarize fiber. Class IB drugs shorten this phase, class III drugs lengthen it.

Front 6

Heart Action Potential:
Phase 4

Back 6

Forward current
In pacemaker cells (SA node), cells slowly depolarize source of automaticity. In pacemaker cells, an increase in threshold lengthens this phase. This phase corresponds to a quiet period (diastole). In non-pacemaker cells, phase 4 is flat (no depolarization). Ca that enters in Phase 2 is removed by mitochondria and by Na/Ca exchange. Na that enters in all other phases is removed by Na/K ATP-ase

Front 7

Systole

Back 7

A period of cardiac muscle contraction

Front 8

Diastoly

Back 8

A period of cardiac muscle relaxation.

Front 9

Frank-searling law

Back 9

States that within physiological range, the farther the muscle is stretched, the greater the force of contraction.

Front 10

Automaticity (Pacemaker cells)

Back 10

The property of some cells to spontaneously depolarize (phase 4) and to fire action potentials if threshold is reached

Front 11

Conductivity:

Back 11

The property of cells to conduct action potentials in either direction, also the rate at which action potentials propagate.

Front 12

Regractoriness

Back 12

A period after an action potential that a cell cannot fire another- a major reason why colliding action potentials cancel each other.

Front 13

Normal Conduction Path in the Heart

Back 13

SA node --> Atrial Wall --> AV node --> Bundle of HIS --> Purkinje Fibers -->Ventrical Myocardium

Front 14

P wave

Back 14

Depolarization of Atria

Front 15

QRS

Back 15

Depolarization of Ventricles and repolarization of atria, ventrical systole. Should be 0.3 seconds in length

Front 16

T wave

Back 16

Repolarization of ventricles

Front 17

Autonomic Nervous System Control:
Parasympathetic stimulation (vagal)

Back 17

Acetylcholine: muscarinic receptors
Decreases depolarization rates in SA node and AV node
Decreases heart rate

Front 18

Automonic Nervous Stystem
Sympathetic

Back 18

Norepinephrine: beta 1 receptors
Epinephrine from adrenal medulla: beta 1 receptors
Stimulate SA node, AV node, atrial and ventricular muscle, increased heart rate, increased force of myocardial contraction, increased conduction velocity.

Front 19

Heart Problems: Inadequate oxygen delivery

Back 19

Myocaridal ischemia, angina, myocardial infarction, cardiac failure, CHF, cardiac arrythmias