Between the finer fibrils of the muscle fibres are minute spaces filled with a semi-fluid substance called
sarcoplasm.
[1]The actual muscular movements are made by the fibres, the changes in the general condition or "tone" of the muscle, as it is called, are generally attributed to the sarcoplasm.
The following are generally considered examples of changes in muscular tone.
- Inability to write smoothly after a hard set of tennis or other muscular exercise,
- inability to prevent excessive movements after an hour of strenuous physical engagement, and
- the tenseness of the muscles as a result of emotional excitement
A distinction should be made here, however, in the last-named instance.
If the
tenseness is present during the emotional excitement it is probably a real muscular contraction,
a reflex resulting in a heightened efficiency to respond to any sudden stimulus.
It puts the person on the
qui vive.
[2]
It is only indirectly, therefore, a change in muscular tone. If the tenseness in the muscles persists, or if its reverse, a hyper-relaxation, sets in after the excitement has passed, the condition does show a change in muscular tone.
Since, however, the tenseness during excitement is uniformly a reflex, it has been customary to speak of this tenseness as heightened muscular tone.
On this basis muscular tone is greatest during any form of strenuous exercise or intense emotional strain, it is less during periods of relative inactivity, and it is least during sleep or the influence of anesthetics. The close relationship between what is called "tone" and the property of relaxation will become evident with the analysis of relaxation.
[1]Sarcoplasm: Sarcoplasm is the cytoplasm(the material or protoplasm within a living cell, excluding the nucleus) of a myocyte (muscle fiber). It is comparable to the cytoplasm of other cells, but it contains unusually large amounts of glycosomes (granules of stored glycogen) and significant amounts of myoglobin, an oxygen-binding protein. The calcium ion concentration in sarcoplasma is also a special element of the muscle fiber; it is the means by which the muscle contractions take place and are regulated. It contains mostly myofibrils (which are composed of sarcomeres), but its contents are otherwise comparable to those of the cytoplasm of other cells. It has a Golgi apparatus near the nucleus, mitochondria just inside the cell membrane (sarcolemma), and a smooth endoplasmic reticulum (specialized for muscle function and called the sarcoplasmic reticulum).