Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 5

Excitable Tissue: Muscle 97

does not normally occur without the other, their physiologic
bases and characteristics are different. Muscle fiber membrane
depolarization normally starts at the motor end plate, the spe-
cialized structure under the motor nerve ending. The action
potential is transmitted along the muscle fiber and initiates the
contractile response.

THE MUSCLE TWITCH

A single action potential causes a brief contraction followed by
relaxation. This response is called a
muscle twitch.
In Figure
5–5, the action potential and the twitch are plotted on the
same time scale. The twitch starts about 2 ms after the start of
depolarization of the membrane, before repolarization is com-
plete. The duration of the twitch varies with the type of muscle
being tested. “Fast” muscle fibers, primarily those concerned
with fine, rapid, precise movement, have twitch durations as
short as 7.5 ms. “Slow” muscle fibers, principally those in-
volved in strong, gross, sustained movements, have twitch du-
rations up to 100 ms.

MOLECULAR BASIS OF CONTRACTION

The process by which the contraction of muscle is brought

about is a sliding of the thin filaments over the thick filaments.

Note that this shortening is not due to changes in the actual

lengths of the thick and thin filaments, rather, by their increased

overlap within the muscle cell. The width of the A bands is con-

stant, whereas the Z lines move closer together when the muscle

contracts and farther apart when it relaxes (Figure 5–3).

The sliding during muscle contraction occurs when the myo-

sin heads bind firmly to actin, bend at the junction of the head

with the neck, and then detach. This “power stroke” depends

on the simultaneous hydrolysis of ATP. Myosin-II molecules

are dimers that have two heads, but only one attaches to actin at

any given time. The probable sequence of events of the power

stroke is outlined in Figure 5–6. In resting muscle, troponin I is

bound to actin and tropomyosin and covers the sites where

myosin heads interact with actin. Also at rest, the myosin head

contains tightly bound ADP. Following an action potential

cytosolic Ca

2+

is increased and free Ca

2+

binds to troponin C.

This binding results in a weakening of the troponin I interac-

tion with actin and exposes the actin binding site for myosin to

FIGURE 5–4
The dystrophin–glycoprotein complex.
Dystrophin connects F-actin to the two members of the dystroglycan (DG) complex,
α
and
β
-dystroglycan, and these in turn connect to the merosin subunit of laminin 211 in the extracellular matrix. The sarcoglycan complex of four
glycoproteins,
α
-,
β
-,
γ
-, and
δ
-sarcoglycan, sarcospan, and syntropins are all associated with the dystroglycan complex. There are muscle disorders
associated with loss, abnormalities, or both of the sarcoglycans and merosin.
(Reproduced with permission from Kandel ER, Scwartz JH, Jessell TM [editors]:
Principles of Neural Science
, 4th ed. McGraw-Hill, 2000.)

α

Dystroglycans

Sarcoglycan

complex

Sarcospan

Syntrophins

β

β

γ

δ

Dystrophin

F-Actin

Functionally important

carbohydrate side chains

Laminin 2

α

α 2

β 1 γ 1

text continues on p. 100