Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 5

Excitable Tissue: Muscle 101

for contraction to occur without an appreciable decrease in
the length of the whole muscle (Figure 5–9). Such a contrac-
tion is called
isometric
(“same measure” or length). Contrac-
tion against a constant load with a decrease in muscle length is
isotonic
(“same tension”). Note that because work is the prod-
uct of force times distance, isotonic contractions do work,
whereas isometric contractions do not. In other situations,
muscle can do negative work while lengthening against a con-
stant weight.

SUMMATION OF CONTRACTIONS

The electrical response of a muscle fiber to repeated stimula-
tion is like that of nerve. The fiber is electrically refractory only
during the rising phase and part of the falling phase of the
spike potential. At this time, the contraction initiated by the
first stimulus is just beginning. However, because the contrac-
tile mechanism does not have a refractory period, repeated
stimulation before relaxation has occurred produces addition-
al activation of the contractile elements and a response that is
added to the contraction already present. This phenomenon is

known as

summation of contractions.

The tension developed

during summation is considerably greater than that during the

single muscle twitch. With rapidly repeated stimulation, acti-

vation of the contractile mechanism occurs repeatedly before

any relaxation has occurred, and the individual responses fuse

into one continuous contraction. Such a response is called a

tetanus (tetanic contraction).

It is a

complete tetanus

when

no relaxation occurs between stimuli and an

incomplete teta-

nus

when periods of incomplete relaxation take place between

the summated stimuli. During a complete tetanus, the tension

developed is about four times that developed by the individual

twitch contractions. The development of an incomplete and a

complete tetanus in response to stimuli of increasing frequen-

cy is shown in Figure 5–10.

FIGURE 5–8 Relation of the T tubule (TT) to the sarcoplasmic
reticulum in Ca2+ transport. In skeletal muscle, the voltage-gated
dihydropyridine receptor in the T tubule triggers Ca2+ release from the
sarcoplasmic reticulum (SR) via the ryanodine receptor (RyR). Upon
sensing a voltage change, there is a physical interaction between the
sarcolemmal-bound DHPR and the SR-bound RyR. This interaction
gates the RyR and allows for Ca2+ release from the SR.

Dihydropyridine receptor

Ryanodine receptor

Extracellular

space

Cytoplasm

COOH

COOH

Lumen of SR

NH 2

+ + + +

TT

FIGURE 5–9 A) Muscle preparation arranged for recording iso-

tonic contractions. B) Preparation arranged for recording isometric

contractions. In A, the muscle is fastened to a writing lever that swings

on a pivot. In B, it is attached to an electronic transducer that measures

the force generated without permitting the muscle to shorten.

A

Pivot

Recorder

B

Force

transducer

Recorder

To stimulator

FIGURE 5–10 Tetanus. Isometric tension of a single muscle fiber during continuously increasing and decreasing stimulation frequency.
Dots at the top are at intervals of 0.2 s. Note the development of incomplete and then complete tetanus as stimulation is increased, and the return
of incomplete tetanus, then full response, as stimulation frequency is decreased.