Single Fibre Twitch

A muscle fibre responds to a single stimulus with a single brief contraction mediated by the release of Ca2+. This is called a twitch. Isometric and isotonic twitches are illustrated in Figure MP.10a and b. The following characteristics can be seen:

• There is a latent period between stimulus and onset of developed tension. This varies between fibre types

• Contraction time is that time between onset of tension development and maximum tension, which differs between fibre types

• The shortening profile of isotonic contractions depends on the applied load

Figure MP.10 Single fibre-twitch response

Relaxation follows with the immediate re-uptake of Ca2+ by the sarcoplasmic reticulum. This re-uptake starts before maximal contraction has taken place, so the muscle fibre does not develop the strongest force of which it is capable. Muscle tissue has the properties of absolute and relative refractoriness seen in most excitable tissue, but the time periods of de- and repolarizations are shorter than the time usually taken for contraction and relaxation.

Repeated action potentials produce different results according to the interval between stimuli. If a second stimulus may come along before complete re-uptake of the Ca2+, it elicits a contraction stronger than the first (Figure MP.11a).

A series of stimuli repeated rapidly produces a gradual augmentation of contraction until the tension developed reaches a maximum which is then sustained. This is called a tetanic contraction and it is significantly stronger than that produced by a single twitch (Figure MP.11b).

u0 200 45O 600

u0 200 45O 600

(o) Repeoted isometric twitches of single muscle fibre

(o) Repeoted isometric twitches of single muscle fibre

(b) High frequency Switches of single muscle fibre giving tetany

Figure MP.11 Single fibre-repeated twitch response

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