IP2: electrical activity of the heart

Which of these structures conduct(s) action potentials the slowest? a. AV node b. AV bundle c. Purkinje fibers d. Bundle branches

a. AV node Action potentials slow down as they pass through the AV node. This gives the atria time to finish contracting before the ventricles are depolarized.

A person notices his or her heart beat because he or she senses blood being pumped by the heart. Excessive caffeine intake can lead to irregular heart rhythms (arrhythmias) that patients perceive as "skipped beats." Given that caffeine is a stimulant, which of the following mechanisms best explains the reason for the feeling that the heart skipped a beat? a. Purkinje fibers initiate spontaneous action potentials, which cause the ventricles to contract early. b. Spontaneous action potentials in the SA node overlap such that the repolarizing phase of one action potential cancels out the depolarizing phase of the next. c. Action potentials are not delayed sufficiently at the AV node. d. Action potentials propagate into the ventricles before the contractile cells have repolarized from the previous heartbeat.

a. Purkinje fibers initiate spontaneous action potentials, which cause the ventricles to contract early. Action potentials normally originate in the SA node because the pacemaker cells there depolarize faster than pacemaker cells located elsewhere in the heart. However, certain drugs, such as caffeine, nicotine, or cocaine, can stimulate other pacemaker cells to speed up and temporarily "escape" the SA node rhythm. If this ectopic focus consists of Purkinje fibers, then ventricular contraction will occur prematurely, prior to ventricular filling. Without proper filling, this abnormal contraction pumps little blood and is not sensed. However, the following normal beat generated from the SA node has augmented filling and is sensed, resulting in the missed beat sensation. Return to Assignment

Which part of the intrinsic conduction system normally initiates the depolarizing impulse that causes a heartbeat? a. AV node b. SA node c. Internodal pathway d. AV bundle

b. SA node Like the rest of the intrinsic conduction system, the SA node contains pacemaker cells that spontaneously depolarize. The cells within the SA node, however, depolarize faster than the other cells within the system. This causes action potentials to initiate in the SA node rather than in any of the other structures.

The anatomy of the intrinsic conduction system causes contraction of the ventricles to begin at the apex and move superiorly. Why is this important? a. because the Purkinje fibers conduct action potentials away from the heart apex b. so blood is forced upward, toward the semilunar valves c. because the AV bundle is the only electrical connection between the atria and the ventricles d. so the atria can finish contracting before the ventricles contract

b. so blood is forced upward, toward the semilunar valves Beginning contraction at the apex causes blood to be forced upward, through the semilunar valves, and into the aorta and pulmonary trunk. This makes ventricular ejection more efficient, much like squeezing a tube of toothpaste up from the bottom is more efficient at dispensing the toothpaste than is squeezing from the top.

bundle brances function

conveys the impulse down the interventricular septum

purkinje fibers function

conveys the impulse throughout the ventricular walls

AV node function

delays occurs here while atra contract

AV bundle function

electrical links between atria and ventricles

Arrange the elements of the intrinsic conduction system in the order that a depolarizing impulse travels during a normal heartbeat.

first element depolarized to last element depolarized SA node, internodal pathways, AV node, AV bundle, bundle brances, purkinje fibers

internodal pathways function

links between the SA node and AV node

The AV node does contains what that can initiate the depolarizing impulses?

pacemaker cells However, under normal circumstances the impulses are initiated elsewhere.

SA node function

set(s) the pace for the entire heart