Temporal relationship between excitation and contraction
Any single muscle contraction proceeds in several consecutive stages, each of which is closely related to changes in the excitability level of the cell membrane and the development of an action potential (AP).
- Latent period (LP) is the time elapsed from the moment of stimulus application to the onset of a visible mechanical response. At this stage, the action potential develops and the processes of electromechanical coupling take place. At this moment, the membrane is in a state of absolute refractoriness (complete unexcitability). Sodium channels are inactivated, so a repeated stimulus is unable to trigger a new AP or contraction.
- Shortening period (SP) is the phase of direct force buildup, or muscle systole. At this time, refractoriness is replaced by the exaltation phase—a period of increased (supernormal) excitability.
- Relaxation period (RP) is the phase of the muscle fiber returning to its initial length (muscle diastole).
Mechanisms of single contraction summation
Summation occurs only with repeated stimulation. If the second stimulus arrives after the cycle has completely finished (following the relaxation phase), the muscle has time to fully return to its initial state. In this case, two isolated single contractions of equal amplitude occur, and summation is absent.
The overlap effect directly depends on which specific phase of the first response the repeated impulse falls into:
- Falling into the relaxation period: the ongoing relaxation process is immediately interrupted and replaced by a new round of contraction. The final amplitude turns out to be greater than the initial one.
- Falling into the shortening period: the current shortening is prolonged, and the overall amplitude of the muscle response increases.
- Falling into the latent period: a new contraction does not occur, since the second stimulation coincides with the absolute refractory period of the first excitation.
Formation of tetanic contraction
If the frequency of applied stimuli increases, single responses begin to merge into continuous muscle tension. Such a fused contraction is called tetanus.
Depending on the stimulation frequency, two main types of tetanus are distinguished:
- Serrated (incomplete) tetanus. Formed when each subsequent impulse falls into the relaxation period of the previous single cycle. New tension is superimposed on the previous one at a time when the muscle has not yet fully relaxed. During graphical recording, individual peaks ("teeth") are visible, but the baseline tension level significantly exceeds that of a single response.
- Smooth (complete) tetanus. Develops with a further increase in frequency, when each subsequent stimulus arrives strictly in the shortening period of the previous response. Contractions completely merge without the slightest sign of relaxation. As a result, a single, powerful, and stable tension is formed, the amplitude of which is several times greater than a single one.
Optimum and pessimum of stimulation frequency
The amplitude of fused muscle tension cannot grow indefinitely. It strictly depends on the ratio of stimulation frequency and the measure of tissue lability.
Optimum is the stimulation frequency at which the maximum amplitude of tetanic contraction is recorded. The main condition for the optimum: each subsequent impulse must fall not just into the shortening period, but precisely into the exaltation phase (increased excitability) remaining from the previous cycle.
Pessimum is a sharp drop in amplitude (muscle relaxation) during rhythmic stimulation whose frequency exceeds the measure of tissue lability. This phenomenon is also known as Vvedensky inhibition. Condition for the pessimum: the frequency is so high that each new impulse falls into the latent period and coincides with the refractory period (unexcitability) from the previous action potential. The tissue is physically incapable of responding to the stimulus.
Briefly: answers to topical questions
What is the role of calcium ions in electromechanical coupling processes during muscle contraction?
Calcium ions are critically important for electromechanical coupling because their binding to troponin removes the blocking effect of tropomyosin and initiates the cross-bridge cycle.
The process includes the following stages:
- Release of calcium from the sarcoplasmic reticulum (in the myocardium, additionally from T-systems and the extracellular space).
- Ions bind to specific sites on troponin C.
- The conformation of troponin changes, and the tropomyosin bound to it shifts away from the myosin-binding sites of actin.
- The myosin head attaches to the actin filament, after which cross-bridge movement occurs using ATP energy.
The number of available interaction sites depends on the calcium concentration, and when calcium is removed, contractile activity is turned off.
What is the measure of lability in excitable tissue?
The measure of lability in excitable tissue is the maximum stimulation frequency that the tissue can reproduce without rhythm transformation.
This indicator characterizes the minimum duration of the excitation process and is quantitatively evaluated as the reciprocal of the absolute refractory period duration. Depending on the type of tissue, the values of the lability measure vary:
- Nervous tissue—up to 1000 imp/s.
- Skeletal muscles—100–200 imp/s.
- Neuromuscular synapses—50–100 imp/s.
If the stimulation frequency does not exceed the measure of lability, the stimulation optimum is observed, and when it is exceeded, a pessimum state develops, in which the response decreases sharply.
Due to what biochemical mechanisms is the relaxation period of the muscle fiber ensured?
The relaxation period of the muscle fiber is ensured by the active transport of calcium ions back into the sarcoplasmic reticulum and the detachment of cross-bridges from actin.
ATP plays a key role in these processes:
- Hydrolysis of ATP under the action of calcium ATPase of the sarcoplasmic reticulum membrane supplies energy for the active transport of calcium from the cytoplasm back into the cavity of the sarcoplasmic reticulum.
- Binding of ATP to myosin is accompanied by the detachment of cross-bridges from actin.
Upon a decrease in calcium concentration in the cytoplasm and its dissociation from troponin, the blocking effect of tropomyosin resumes, leading to muscle relaxation.
MNEUMONIC — Relaxation (Serrated tetanus occurs if the impulse falls into the Relaxation phase). EYE — Shortening (Smooth tetanus is formed when falling into the Shortening period).
Class and exam questions
How does fused tetanus differ from unfused tetanus?
Fused tetanus occurs when a stimulus falls during the shortening period (complete fusion without relaxation). Unfused tetanus occurs when it falls during the relaxation period, resulting in visible "notches" of partial tension drop on the graph.
Into which excitability phase must the stimulus fall for the optimum?
To achieve maximum amplitude (optimum), the subsequent stimulus must fall into the exaltation phase (supernormal excitability) of the previous cycle.
What else to study on this topic
- Processes of electromechanical coupling in the latent period
- Dynamics of sodium channel inactivation during refractoriness
- Mechanisms of Vvedensky inhibition (pessimum) formation
- The concept of excitable tissue lability measure
- Graphical analysis of excitability phases (exaltation and refractoriness)
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