20.9b Impulse Generation and Conduction

electrical conduction system

The key to the precise cardiac cycle is the (?)(Figure 20-26).

specialized cardiac tissue

This system is composed of (?) that generates electrical impulses and conducts them rapidly throughout the heart to ensure that the chambers contract in proper sequence.

SA node

The (?) is the heart’s dominant pacemaker.

action potentials

It spontaneously generates electrical impulses (?) that are propagated through intra-atrial pathways to the AV node where conduction is delayed momentarily.

delay

This (?) gives the ventricles time to fill completely.

bundle of His

The impulse then travels from the AV node throughout the ventricles via the (?) and the Purkinje network.

automaticity

All myocardial tissue, both contractile and conductive, has the ability to self-generate electrical impulses (?) and to propagate those impulses to surrounding tissue.

ions

It does this through the movement of (?) across the cell membrane.

polarized

At rest (when not stimulated), the cell membrane is (?) with a slight electrical charge.

positive ions

This charge is present because there are more (?) outside the cell than inside, resulting in a slight negative charge on the inside.

sodium (Na+)

The primary ions involved are (?) on the outside of the cell and potassium (K+) on the inside.

Calcium (Ca++)

(?), which is responsible for muscle contraction, is present in storage vesicles surrounding the cell.

sarcoplasmic reticulum

These vesicles are called the (?).

depolarize

The cell membrane is said to (?) when this charge is eliminated or reversed.

depolarization

When an impulse is generated and conducted to the muscle cells, the process of (?) and repolarization begins.

ion movements

Figure 20-27 depicts the sequence of (?) in the depolarization and repolarization of both slow and fast potentials.

Fast potentials

(?) occur in cardiac muscle tissue and in the ventricular conduction system; slow potentials occur in the pacemaker cells of the SA and AV nodes.

five phases

Cyclic activity in the fast potentials has (?).

Phase 0

(?), which represents depolarization, results from a rapid influx of Na+ ions into the cell.

more positive

This makes the inside of the cell (?) than the outside and is normally caused by the arrival of an impulse generated elsewhere in the heart, such as the SA node.

Sodium

Sodium stops entering the cell once the inside has become positive.

Phases 1 through 3

(?) represent repolarization.

K+

In phase 1, (?) begins to leave the cell, slowly returning the cell to its normal negative charge.

Ca++

Phase 2 interrupts with an influx of (?) into the cell.

calcium

Remember that the muscles are using (?) inside the cell for contraction.

plateau phase

This (?) delays repolarization and is important for medications that affect the strength of contraction.

Phase 3

(?) is marked by a cessation of calcium influx and the rapid efflux of potassium.

Phase 4

(?) is normally a flat stage representing the resting membrane potential.

pathologic states

However, in (?), phase 4 can include a slow influx of sodium that gradually makes the inside of the cell more positive.

threshold potential

When the interior of the cell reaches a point called its (?), the cell depolarizes without waiting for an impulse.

phase 4 depolarization

Many antiarrhythmics have their mechanism of action during this (?).

slow potentials

The (?), although similar to the fast ones, have several important distinctions.

dominant pacemakers

First, they are located in the (? ) of the heart.

depolarize

Second, they (?) differently.

phase 4

Notice in Figure 20-27 how (?) normally exhibits a gradually increasing slope toward the threshold potential.

gradual influx of calcium

Whereas sodium causes depolarization (phase 0) of the fast potentials, a (?) causes it in the slow potentials.

gradual, phase 4 depolarization

The slow potentials normally undergo a (?).

spontaneous generation of impulses

Although we do not know the exact mechanism, this gradual depolarization clearly is responsible for the (?) in the SA and AV nodes.

SA node

Although the AV node also has these slow potentials, the (?)’s rate of depolarization is faster, making it the heart’s dominant pacemaker.