Cardiac Excitation Flashcards
What two properties allow normal, ordered excitation of the heart?
Ability of the heart to initiate its own beat - automaticity
Regularity of pacemaking activity - Rhythmicity
What is the sinoatrial node?
Initiator of electrical activation of the heart, pacemaker
APs originating here spread to the surrounding atrial myocytes and reach the AV node
What are the atrial internodal tracts?
Connect SA to the AV node: anterior, middle (Wenchkebach), and posterior (Thorel)
Connect right and left atria: Bachmann bundle - branch of the anterior internodal tract
What occurs at the AV node?
Delay in conduction
Important to allow complete atrial depolarization, contraction, and emptying of atrial blood
What is the Bundle of His?
Continuous with the AV node, gives off left bundle branch and continues as the right bundle branch
What are the purkinje fibers?
Carry signal from the right and left bundle branches throughout the ventricular myocardium
Once the impulse reaches the ends of the purkinje system it is transmitted through the muscle fibers themselves
How does sympathetic nervous activity raise heart rate?
NE activates B1 receptors
Produces an increase in funny current, therefore increasing the rate of phase 4 depolarization
SA node is depolarized more frequently and fires more APs per time
How does parasympathetic nervous activity decrease heart rate?
ACh activates M2 receptors in the SA node
Decreases funny current, decreasing the rate of phase 4 depolarization
Alsow enhances K efflux, allowing for hyperpolarization of the max diastolic potential
How does temp affect the rate of discharge by the SA node?
Increases discharge frequency
How does digitalis influence the heart?
Strenghten contractility
Can also decrease AV nodal conduction velocity
What are the latent pacemakers present in the conduction system?
AV node and bundle of HIS have intrinsic firing rates of 40-60 bpm
Pukinje system has rates of 15-40 bpm
What is the significance of the latent pacemaker sites?
These sites may initiate impulses and take over for the SA node when it slows or fails
Or if conduction between the SA node and AV node is prevented
What is an escape beat?
An impulse initiated by a latent pacemaker
What is an escape rhythm?
A continued series of escape beats
What is junctional escape?
Beats arise from the AV node or proximal bundle of His
E.g. moderate parasymp. stimulation slows the SA rate and allows pacemaker shift
What is ventricular escape?
Beats arise from a more distal point in the conduction system
E.g. Very strong parasymp. stimulation suppresses excitability at both the SA and AV nodes
What is an ectopic beat?
A latent pacemaker develops an intrinsic rate of depolarization faster than that of the SA node
A sequence is called an ectopic rhythm
What are some causes of ectopic beats?
High catecholamine concentration
Hypoxemia, ischemia, electrolyte disturbances and certain drug toxicities
What is abnormal automaticity?
Cardiac tissue injury can lead to myocardial cells outside the conduction system acquiring automaticity
What are early afterdepolarizations?
Changes of the membrane potential in the positive direction that interrupt normal repolarization
more likely to develop in conditions that prolong the action potential duration
Can lead to tachyarrhythmia
What are delayed afterdepolarizations?
Appear shortly after repolalrization is complete
Commonly develop in states of high intracellular calcium
What is conduction block?
When an impulse can’t propagate because it encounters a region of the hear that is electrically unexcitable
Can be transient or permanent
What results from AV block?
Prevents impusle propagation from the SA node to distal sites
Removes overdrive suppression, resulting in the emergence of escape beats from the distal sites
What is Reentry?
Reentry occurs when a conduction pathway is stimulated prematurely be a previously conducted action potential leading to a rapid, cyclical reactivation
Caused by a unidirectional block
