Physiology of Muscle Contraction: Electromechanical Coupling

Single muscle twitch

The curve of a single muscle twitch includes three consecutive phases reflecting the transition from electrical excitation to the mechanical response of the muscle fiber.

• Latent period (LP) — the time from the application of the stimulus to the onset of shortening, during which the action potential develops. Duration is about 10 ms.
• Contraction period — the phase of active muscle contraction in response to excitation. Duration is about 50 ms.
• Relaxation period — the return of the muscle to its original state. Duration is about 50 ms.

Excitation-contraction coupling

Electromechanical coupling — the concept explaining the conversion of electrical excitation of the muscle fiber membrane into a mechanical contraction. The process begins at the neuromuscular junction and ends with the sliding of myofilaments.

1. Acetylcholine is released in the presynaptic terminal and enters the synaptic cleft.
2. Acetylcholine binds to muscle-type nicotinic acetylcholine receptors (N_M) on the postsynaptic membrane.
3. Local depolarization occurs, which is sufficient to activate voltage-gated sodium channels.
4. An action potential develops, propagating along the sarcolemma and further into the T-tubules — membrane invaginations deep into the fiber.
5. Dihydropyridine receptors are activated in the T-tubules, mechanically linked to type 1 ryanodine receptors of the terminal cisternae of the sarcoplasmic reticulum (L-tubules).
6. Through ryanodine receptors, calcium massively exits from the cisternae into the sarcoplasm, sharply increasing the intracellular concentration of Ca²⁺.

Molecular mechanism of contraction

Contraction is realized through the interaction of two key contractile proteins — actin and myosin. At rest, the active sites of actin are covered by the tropomyosin–troponin regulatory complex, which prevents spontaneous contraction.

• Calcium binds to troponin C, causing conformational changes in the complex.
• Tropomyosin shifts, exposing the binding sites on actin for myosin heads.
• ATP hydrolysis occurs on the myosin head to form ADP and inorganic phosphate with the release of energy.
• The energy of hydrolysis shifts the myosin head from a 45° to a 90° position, providing the "power stroke" movement.
• The myosin head attaches to the actin filament and pulls it — the filaments slide relative to each other, and the muscle shortens.
• Detachment of the head from actin requires the binding of a new ATP molecule; without it, dissociation is impossible.

Relaxation and rigor mortis

Muscle relaxation requires energy and the active transport of calcium back into the sarcoplasmic reticulum. A new ATP molecule binds to the myosin head and ensures its detachment from actin, returning it to its original position.

In the absence of ATP (e.g., after death of the organism), myosin heads remain firmly bound to actin — rigor mortis develops, in which muscles become rigid and stiff.

Tetanus and its types

Tetanus — a prolonged muscle contraction occurring with frequent rhythmic stimulation, when new stimuli arrive before the fiber has fully relaxed. Several variants are distinguished depending on the frequency of the stimulus.

• Incomplete tetanus (serrated tetanus) — the stimulus hits during the relaxation phase; partial relaxations between contractions are visible on the curve.
• Complete tetanus (smooth tetanus) — the stimulus hits during the contraction phase; the muscle does not have time to relax, and contractions merge into one continuous one.
• Optimum point — the maximum contraction amplitude when the stimulus hits the exaltation phase (period of increased excitability).
• Pessimum point — the stimulus hits the absolute refractory phase when sodium channels are inactivated; the muscle does not respond to the stimulus.

Key points

• A single twitch consists of a latent period, a contraction period, and a relaxation period.
• Electromechanical coupling links the action potential to mechanical contraction via the T-tubule system and sarcoplasmic reticulum calcium channels.
• T-tubule dihydropyridine receptors activate type 1 ryanodine receptors, releasing Ca²⁺.
• Calcium binds to troponin, shifting tropomyosin and exposing actin binding sites.
• ATP hydrolysis provides the energy for the myosin head power stroke.
• A new ATP molecule is necessary for myosin detachment from actin and muscle relaxation.
• The absence of ATP leads to the development of rigor mortis.
• Types of tetanus (incomplete, complete) and the phenomena of optimum and pessimum depend on the stimulation frequency and excitability phase.