Why is the movement of an action potential unidirectional?
Unlike graded potentials, the propogation of an action potential is unidirectional, because the absolute refractory period prevents the initiation of an AP in a region of membrane that has just produced an AP.
How do action potentials travel in the heart?
Action potentials from the SA node propagate to the atria. From there, the impulse travels to the atrioventricular (AV) node and through specialized conduction fibers (Purkinje fibers) into the ventricular muscle.
What causes unidirectional block?
Unidirectional block can be induced by application of a test stimulus inside vulnerable window. Distance and time are mea- sured from site and time of initial stimulation.
What are the two types of action potential in the heart?
The first type is characterized by a prominent phase 1 repolarization and a relatively negative plateau level. The second type resembles the action potential elicited in ventricular cells isolated from the same hearts. It shows little phase 1 repolarization and a relatively positive plateau level.
What happens during repolarization?
Repolarization is a stage of an action potential in which the cell experiences a decrease of voltage due to the efflux of potassium (K+) ions along its electrochemical gradient. This phase occurs after the cell reaches its highest voltage from depolarization.
In which structure in the heart is there a plateau in the cardiac action potential?
The work cells have a large stable resting membrane potential and display a prolonged action potential with a plateau phase. The pacemaker cells have smaller unstable resting potentials and spontaneously depolarize, generating the intrinsic electrical activity of the heart.
How does depolarization occur in the heart?
Depolarization occurs in the four chambers of the heart: both atria first, and then both ventricles. The sinoatrial (SA) node on the wall of the right atrium initiates depolarization in the right and left atria, causing contraction, which corresponds to the P wave on an electrocardiogram.
What is meant by unidirectional block?
u·ni·di·rec·tion·al block block that prevents passage of an impulse when it approaches from one direction but not from the other, as when block in the AV node prevents anterograde conduction to the ventricles whereas retrograde conduction to the atria remains intact.
How does reentry occur?
Reentry, due to a circuit within the myocardium, occurs when a propagating impulse fails to die out after normal activation of the heart and persists as a result of continuous activity around the circuit to re-excite the heart after the refractory period has ended; it is the electrophysiologic mechanism responsible for …
Why is plateau important in cardiac action potential?
This plateau phase prolongs the action potential duration and distinguishes cardiac action potentials from the much shorter action potentials found in nerves and skeletal muscle. Repolarization (phase 3) occurs when gK+ (and therefore IKr) increases, along with the inactivation of Ca++ channels (decreased gCa++).
Why is hyperpolarization important in action potential?
Hyperpolarization is important because it prevents any stimulus that has already been delivered up an axon from generating another action potential in the reverse direction.
How do action potentials spread throughout the heart?
If it can re-excite the tissue, a circular (counterclockwise in this case) pathway of high frequency impulses (i.e., a tachyarrhythmia) will become the source of action potentials that spread throughout a region of the heart (e.g., ventricle) or the entire heart.
What are the phases of the ventricular action potential?
The ventricular action potential is generally split into 5 phases ( phases 0-4 ). Rather confusingly, phase 4 is the baseline that the membrane potential begins and ends at. Like any action potential, each phase is driven by the opening and closing of a variety of specific ion channels.
What is an action potential?
An action potential is a reversible change of this membrane potential resulting from a “ripple effect” – an activation of currents generated by the sudden diffusion of ions across the membrane lowers the electrochemical gradient.
How does an action potential travel through a block?
In such a block, impulses can travel retrograde (from branch 3 into branch 2) but not orthograde. When this condition exists, an action potential will travel down the branch 1, into the common distal path (branch 3), and then travel retrograde through the unidirectional block in branch 2 (blue line).