The following factors affect excitability and contractility and therefore alter in various ways, the nature of the simple muscle curve:
Factor # 1. Strength and Duration of Stimuli:
For stimulation to occur, two factors are necessary—a minimum strength and an adequate duration of the stimulus. Chronaxie, which includes both these factors is the measure of excitability of a tissue.
Factor # 2. Effects of Two Successive Stimuli:
If the second stimulus is applied after sufficient intervals both first and second stimuli will cause contraction and two simple muscle curves will be recorded (Fig. 6.4, stage a). The second curve will be slightly higher than the first one due to the beneficial effect of contraction. If the second stimulus is applied in the relaxation period of the first, more or less two separate curves will be produced and the second curve will be higher (Fig. 6.3, stage b).
If the second stimulus is applied within the contraction period of the first one the second curve will be superimposed on the first one with a higher height of contraction (Fig. 6.3, Stage c). These are known as summation of effects (contraction). If the second stimulus is applied within the latent period but after refractory period of the first, then their effects are added together giving a simple muscle curve of larger height than either of them produced separately (Fig. 6.3, stage d).
This effect is known as summation of stimuli. By using maximal or submaximal stimuli, these summated effects can be obtained. This is true for either single muscle fibre or a whole muscle bulk. In case of submaximal stimuli, the summated, effects are due to activation of more nerves and muscle fibres, but in case of maximal stimuli, the greater response cannot be due to an increase in numbers of responding nerves and muscle fibres, but due to a difference in contractile mechanisms.
It is claimed that these effects are due to beginning of second twitch before the active state for the first twitch is complete. That is the duration of the active state of the first response makes possible the augmentation of the second response. Thus this effect goes against the law of all-or-none.
Factor # 3. Effects of Repetition of Stimuli:
Following phenomena are observed:
i. Staircase Phenomena:
When a freshly excised muscle is stimulated with a single induction shock of sufficient strength then a contraction of certain amplitude is recorded. If second stimulus of same strength is applied at an interval of about 1 second to the muscle after completion of the effect of first stimulus, then increased amplitude of contraction is recorded.
With a series of such stimuli (5 to 6 stimuli) but under the condition that each contraction is allowed to be completed before the next stimulus is applied, then a gradual increase in amplitude of contraction is obtained. This stair-like rise is called the staircase (treppe) phenomenon. Increased H+ ion concentration and an increase of temperature within the muscle create a favourable condition for more work (beneficial effect). Hence, the contraction becomes stronger.
ii. Clonus:
When repeated stimuli are applied, the type of response will vary according to frequency. When the frequency is such that each successive stimulus falls within the period of relaxation of the previous curve the record will show a series of wavy oscillations. This is called clonus or incomplete tetanus (Fig. 6.4, B-E).
iii. Tetanus:
When the frequency is more, so that the stimuli fall within the latent period of the previous curve, the record traces a clear steady line, which rises at first abruptly and then gradually, till maximum contraction takes place. This is called tetanus (Fig. 6.4, F). Here the fusion is complete and the muscle, instead of vibrating, exerts a steady pull.
Due to summation, the height of tetanic contraction is usually higher than that of a single twitch. Frequency of stimulation, required for the induction of tetanus, varies with the nature of the muscle. In external eye muscle it is about 350/sec., in gastrocnemius muscle it is about 100/sec.
Clonus may be described as summation of successive contraction, whereas tetanus is summation of successive stimuli. The mechanical movement in response to voluntary stimuli is neither a twitch nor tetanus. During voluntary movement, the skeletal muscles are stimulated at low frequency, which is less than fusion frequency, and also asynchronously, so that the infrequent contraction of a large number of fibres give the appearance of a smooth response.
iv. Fatigue:
When repeatedly stimulated, the muscle has lost its irritability, becomes gradually less excitable and ultimately ceases to respond. This phenomenon is called fatigue (Fig. 6.5, 6.6, 6.8). Muscular fatigue can be defined as the inability of the muscle to do further work. In the fatigue curve, all the periods are lengthened. The relaxation period is so much prolonged that the curve fails to reach the base line before the next stimulus arrives, thus leaving a contraction remainder (Fig. 6.5).
The causes of fatigue might be due to:
(a) Exhaustion of sources of energy of the muscle,
(b) Accumulation of the end products of chemical reactions, such as lactic acid, carbon dioxide, ketone bodies, and
(c) Decrease of local synthesis of acetylcholine—like substances during prolonged exercise.
Oxygen is required for the removal of these substances and so for recovery. Fatigued muscle left in nitrogen does not recover. In studying fatigue in muscle with circulation and without circulation, the muscle gets fatigue more earlier in case of the latter and does not recover on rest (Fig. 6.6A). But in case of muscle with circulation, fatigue comes later and the muscle recovers on rest (Fig. 6.6B).
This shows that oxygen, which is supplied through blood circulation, is required for recovery of fatigue. The seat of fatigue lies in the muscle when it is directly stimulated. But when it is stimulated through the motor nerve, the seat of fatigue is in the neuromuscular junction. In physiological exercise, the seat of fatigue is neither in the muscle nor in the neuromuscular junction but at the synapses in the central nervous system (central fatigue).
On comparison, it is seen that fatigue after voluntary work first appears in the synapses, then in the neuromuscular junctions and lastly in the muscle itself. In human subjects fatigue can be studied with the help of an instrument called Ergograph (Fig. 6.7).
Fatigue can be experimented by noxious stimulation at the foot of a spinal frog. If a reflex withdrawal of foot is obtained and by continuous stimulation the reflex contraction of the muscle is lost, the foot does not withdraw. At this stage if the flexor nerve is stimulated, then contraction of the muscle again occurs.
This cessation of contraction is due to changes in the spinal cord. This can also be demonstrated in man by protective mechanism. When the arm or leg muscles of a subject are used to contract repeatedly with a weight attached to the part and he is unable to lift weight voluntarily, then electrical stimulation of the motor nerve through the skin produces a powerful contraction.
Factor # 4. All-or-None Law:
It means that if a single muscle fibre contracts at all, it will contract to its maximum, provided the conditions remain constant. If internal and external conditions are changed, the amount of contraction will vary. This law holds good for a single muscle fibre and does not apply for the whole muscle, which is composed of innumerable muscle fibres.
Because, in the latter case, as the strength of the stimulus is increased, more and more muscle fibres will be affected and the degree of contraction will be raised (staircase phenomena) and a stage will be achieved when there will be no further rise (all-or-none law for whole muscle). But modern theory claims that the all-or-none law is applicable in case of development of the action potential but not for the activation of the contractile materials.
Factor # 5. Effects of Temperature:
Moderate warmth (25°C.) increases and cold (5°C.) depresses both excitability and contractility. The former shortens and the latter lengthens all the periods of the muscle curve (Fig. 6.9). Temperature above 42°C produces heat rigor due to coagulation of proteins present in the muscle.
Factor # 6. Effects of Load:
Load lengthens the latent period but reduces the periods of contraction and relaxation. It also reduces the degree of contraction, i.e., the height of the curve (Fig. 6.10). The effect of load on the work done by the muscle depends on the way in which the load is applied.
If the weight is allowed to stretch the muscle prior to its contraction, the muscle is said to be free-loaded but if the lever is supported then the muscle is only stretched when the contraction begins, the muscle is said to be after-loaded. The mechanical efficiency in free-loaded muscle is higher than that in after-loaded muscle. This is mostly related with the increase in initial length of muscle fibres.
