muscular cells. This property is the liberation of some of
the energy contained in the chemical compounds of the cells
in such a way as to give mechanical work. The
Muscle.
mechanical work is obtained by movement resulting
from a change, it is supposed, in the elastic tension of the
framework of the living cell. In the fibrils existing in the
cell a sudden alteration of elasticity occurs, resulting in an
increased tension on the points of attachment of the cell to the
neighbouring elements of the tissue in which the cell is placed.
These yield under the strain, and the cell shortens between
those points of its attachment. This shortening is called
contraction. But the volume of the cell is not
appreciably altered, despite the change of its shape,
Contracti-bility.
for its one diameter increases in proportion as its
other is diminished. The manifestations of contractility by
muscle are various in mode. By tonic contraction is meant
a prolonged and equable state of tension which yields under
analysis no element of intermittent character. This is manifested
by the muscular walls of the hollow viscera and of the
heart, where it is the expression of a continuous liberation of
energy in process in the muscular tissue, the outcome of the
latter’s own intrinsic life, and largely independent of any connexion
with the nervous system. The muscular wall of the
blood-vessels also exhibits tonic contraction, which, however,
seems to be mainly traceable to a continual excitation of the
muscle cells by nervous influence conveyed to them along their
nerves, and originating in the great vaso motor centre in the bulb.
In the ordinary striped muscles of the skeletal musculature, e.g.
gastrocnemius, tonic contraction obtains; but this, like the last
mentioned, is not autochthonous in the muscles themselves; it
is indirect and neural, and appears to be maintained reflexly.
The receptive organs of the muscular sense and of the semicircular
canals are to be regarded as the sites of origin of this
reflex tonus of the skeletal muscles. Striped muscles possessing
an autochthonous tonus appear to be the various sphincter
muscles.
Another mode of manifestation of contractility by muscles is the rhythmic. A tendency to rhythmic contraction seems discoverable in almost all muscles. In some it is very marked, for example in some viscera, the spleen, the bladder, the ureter, the uterus, the intestine, and especially in the heart. In several of these it appears not unlikely that the recurrent explosive liberations of energy in the muscle tissue are not secondary to recurrent explosions in nerve cells, but are attributable to decompositions arising sua sponte in the chemical substances of the muscle cells themselves in the course of their living. Even small strips of the muscle of the heart, if taken immediately after the death of the animal, continue, when kept moist and Warm and supplied with oxygen, to “beat” rhythmically for hours. Rhythmic contraction is also characteristic of certain groups of skeletal muscles, e.g. the respiratory. In these the rhythmic activity is, however, clearly secondary to rhythmic discharges of the nerve cells constituting the respiratory centre in the bulb. Such discharges descend the nerve fibres of the spinal cord, and through the intermediation of various spinal nerve cells excite the respiratory muscles through their motor nerves. A form of contraction intermediate in character between the tonic and the rhythmic is met in the auricle of the heart of the toad. There slowly successive phases of increased and of diminished tonus regularly alternate, and upon them are superposed the rhythmic “beats” of the pulsating heart.
“The beat,” i.e. the short-lasting explosive contraction of the heart muscle, can be elicited by a single, even momentary, application of a stimulus, e.g. by an induction shock. Similarly, such a single stimulus elicits from a skeletal muscle a single “beat,” or, as it is termed, a “twitch.” In the heart muscle during a brief period after each beat, that is, after each single contraction of the rhythmic series, the muscle becomes inexcitable. It cannot then be excited to contract by any agent, though the inexcitable period is more brief for strong than for weak stimuli. But in the skeletal, voluntary or striped muscles a second stimulus succeeding a previous so quickly as to fall even during the continuance of the contraction excited by a first, elicits a second contraction. This second contraction starts from whatever phase of previous contraction the muscle may have reached at the time. A third stimulus excites a third additional contraction, a fourth a fourth, and so on. The increments of contraction become, however, less and less, until the succeeding stimuli serve merely to maintain, not to augment, the existing degree of contraction. We arrive thus by synthesis at a summation of “beats” or of simple contractions in the compound, or “tetanic,” or summed contraction of the skeletal muscles. The tetanic or summed contractions are more extensive than the simple, both in space and time, and liberate more energy, both as mechanical work and heat. The tension developed by their means in the muscle is many times greater than that developed by a simple twitch.
Muscle cells respond by changes in their activity to changes in their environment, and thus are said to be “excitable.” They are, however, less excitable than are the nerve cells which innervate them. The change which excites them is termed a stimulus. The least stimulus which suffices to excite is known as the stimulus of Excitability. threshold value. In the case of the heart muscle this threshold stimulus evokes a beat as extensive as does the strongest stimulus; that is, the intensity of the stimulus, so long as it is above threshold value, is not a function of the amount of the muscular response. But in the ordinary skeletal muscles the amount of the muscular contraction is for a short range of quantities of stimulus (of above threshold value) proportioned to the intensity of the stimulus and increases with it. A value of stimulus, however, is soon reached which evokes a maximal contraction. Further increase of contraction does not follow further increase of the intensity of the stimulus above that point.
Just as in a nerve fibre, when excited by a localized stimulus, the excited state spreads from the excited point to the adjacent unexcited ones, so in muscle the “contraction,” when excited at a point, spreads to the adjacent uncontracted parts. Both in muscle and in nerve this spread is termed conduction. It is propagated along the muscle fibres of the skeletal muscles at a rate of about 3 metres per second. In the heart muscle it travels much more slowly. The disturbance travels as a wave of contraction, and the whole extent of the wave-like disturbance measures in ordinary muscles much more than the whole length of any single muscle fibre. That the excited state spreads only to previously unexcited portions of the muscle fibre shows that even in the skeletal variety of muscle there exists, though only for a very brief time, a period of inexcitability. The duration of this period is about 1600 of a second in skeletal muscle.
When muscle that has remained inactive for some time is excited by a series of single and equal stimuli succeeding at intervals too prolonged to cause summation the succeeding contractions exhibit progressive increase up to a certain degree. The tenth contraction usually exhibits the culmination of this so-called “staircase effect.” The explanation may lie in the production of CO2 in the muscle. That substance, in small doses, favours the contractile power of muscle. The muscle is a machine for utilizing the energy contained in its own chemical compounds. It is not surprising that the chemical substances produced in it by the decomposition of its living material should not be of a nature indifferent for muscular life. We find that if the series of excitations of the muscle be prolonged. beyond the short stage of initial improvement, the contractions, after being Well maintained for a time, later decline in force and speed, and ultimately dwindle even to vanishing point. This decline is said to be due to muscular fatigue. The muscle recovers on being allowed to rest unstimulated for a while, and more quickly on being washed with an innocuous but non-nutritious solution, such as ·6%, NaCl in water. The washing seems to remove excreta of the muscle’s own production, and the period of repose removes them perhaps by diffusion, perhaps by breaking them down into innocuous material. Since the
