HUMAN BIOLOGY

128 Chapter 7

right atrium

left atrium

left ventricle

Sinoatrial

(SA) node

Bundle of

conducting

muscle fibers

right ventricle

Purkinje

fibers

Atrioventricular

(AV) node

What is the cardiac conduction system?

• The cardiac conduction system consists of specialized cardiac
  muscle cells that stimulate heart contractions. The system’s
  signals stimulate a rhythmic cycle of contraction, first in the
  heart atria, then in the ventricles.

taKe-hoMe Message

atrioventricular (Av)
node Part of the cardiac
conduction system that
passes contraction signals
from the atria to the
ventricles.

cardiac conduction
system Self-exciting
heart muscle cells that
spontaneously generate and
conduct electrical signals.

sinoatrial (SA) node
Cluster of self-exciting cells
that establish a regular
heartbeat; also called the
cardiac pacemaker. the ventricles. At places along each bundle, cells called
Purkinje fibers pass the signal on to contractile muscle cells
in each ventricle. The slow conduction in the AV node is an
important part of this sequence. It gives the atria time to fin-
ish contracting before the wave of excitation spreads to the
ventricles.
Of all cells of the cardiac conduction system, the SA
node fires off impulses at the fastest rate and is the first
region to respond in each cardiac cycle. It is called the
“intrinsic cardiac pacemaker” because its self-generated
rhythmic firing is the basis for the normal rate of heartbeat.
People whose SA node chronically malfunctions may have
an artificial pacemaker implanted to provide a regular
stimulus for their heart contractions.

the nervous system adjusts heart activity

The nervous system can adjust the rate and strength of car-

diac muscle contraction. Stimulation by one set of nerves

can increase heart activity, while stimulation by another set

of nerves can slow it. The control centers for these adjust-

ments are in the spinal cord and parts of the brain. They are

discussed more fully in Chapter 13.

n Unlike skeletal muscle, cardiac muscle contracts—and the

heart beats—without nervous system orders.

n Links to Muscle tissue 4.3, Cell junctions 4.6

electrical signals from “pacemaker” cells

drive the heart’s contractions

Cardiac muscle cells branch, then link to one another at

their endings. Gap junctions called intercalated discs span

both plasma membranes of neigh-

boring cells (Figure 7.9). With each

heartbeat, signals for contraction

spread so fast across the junctions

that cardiac muscle cells contract

together, almost as if they were a

single unit.

Where do the signals for heart

contractions come from? About

1 percent of cardiac muscle cells

function as the cardiac conduction

system. These cells do not con-

tract. Instead, some of them are self-

exciting “pacemaker” cells—that is,

they spontaneously generate and

con duct electrical impulses. Those

impulses are the signals that stimulate contractions in the

heart’s contractile cells. Because the cardiac conduction

system is independent of the nervous system, the heart will

keep right on beating even if all nerves leading to it are cut!

Excitation begins with a

cluster of cells in the upper

wall of the right atrium (Fig-

ure 7.10). About seventy times

a minute, this sinoatrial (SA)

node generates signals that

stimulate waves of excitation.

Each wave spreads swiftly over

both atria and causes them to

contract. It then reaches the

atrioventricular (Av) node

in the septum dividing the

two atria.

When a stimulus reaches the

AV node, it slows but keeps

moving along bundles of con-

ducting fibers that extend to

Figure 7.9 intercalated discs form

communication junctions between

cardiac muscle cells. Signals travel

rapidly across the junctions and

cause cells to contract nearly in

unison. (© Cengage Learning)

intercalated

disc where

cardiac

muscle

cells meet

cardiac

muscle cell

cardiac

muscle cell

intercalated disc

R

P T

Q S

7. 4

Figure 7.10 Animated! in the cardiac conduction system,

pacemaker cells produce electrical signals. In the recording of a

heartbeat above, letters indicate three waves of electrical activity

that were caused by the spread of impulses across cardiac muscle.

how Cardiac Muscle Contracts

© Cengage Learning

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