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6.4Contraction of Muscle Cells

The contractile apparatus in skeletal and cardiac muscles is a specialized structure for contraction, and consists of basic unit called sarcomere (Fig. 6-10A). These are mainly composed of thick myosin filaments, consisting of polymerized myosins, and thin actin filaments. Six thin actin filaments surrounds one thick myosin filament (Fig. 6-10B), and strong contractility occurs through the interaction between two types of filaments.

Contractile apparatus in muscle cellsContractile apparatus in muscle cells

Fig. 6-10 Contractile apparatus in muscle cells

A) An electron micrograph showing the contractile apparatus in amphibian skeletal muscle, and its model. Muscle cells are filled with structures called myofibrils, each of which is composed of numerous sarcomeres connected side by side. Sarcoplasmic reticulum (endoplasmic reticulum that store Ca2+) surround the myofibrils. Actin and myosin filaments are anchored at the Z line and M line, respectively.
B) An electron micrograph showing the arrangement of actin and myosin filaments that constitute sarcomeres, and its model.

Muscle cell contraction occurs through the interaction between myosin heads and actin filaments (Fig. 6-11A). The process occurs as follows: (1) When a myosin head binds ATP, it detaches from the actin filament. (2) When ATP is hydrolyzed, a structural change (a rotating movement) occurs in the myosin head. (3) When a phosphate is released, the myosin head binds to the actin filament. (4) When the ADP is released, the myosin head structure changes (reverting to its original shape) while remaining attached to the actin filament, during which (5) the myosin and actin filaments exhibit a slight sliding movement against each other. Strong muscle contraction is achieved through high-speed repetition of these steps.
Muscle cells contract in response to neural stimuli. The protein complex that regulates this contraction exists in the contractile apparatus (Fig. 6-11B). Regulatory proteins bind to actin filaments to keep myosin heads from binding to them. In this condition, actin filaments and myosin heads cannot interact, thus disabling the contraction of muscle cells. However, if the intracellular Ca2+ concentration increases, Ca2+ binds to the regulatory protein and changes its structure to enable the actin-myosin interaction. Such an increase in Ca2+ levels is caused by the transmission of nerve excitation to muscle cells.
Excitation from nerves is transmitted to muscle cells via chemical synapses - a special cell adhesion mechanism found between neurons and muscle cells (see in 11.4). When excitation is transferred from nerves to synapses, chemical transmitters (neurotransmitters) are released from neurons. If transmitters bind to the receptors of muscle cells, the ion channels of muscle cells open, thereby changing muscle cell membrane potential (to the plus side). The changes in the membrane potential (excitation) are transmitted to the sarcoplasmic reticulum in the muscle cells via T-tubules (tubular invaginations of the cell membrane extending inward). As a result, Ca2+ stored in the sarcoplasmic reticulum is released to cytoplasm (Fig. 6-12). This increased Ca2+ level causes the binding of Ca2+ to the regulatory protein, thus triggering the contraction of muscle cells.

Model of the contractile apparatus in muscle cellsModel of the contractile apparatus in muscle cells

Fig. 6-11 Model of the contractile apparatus in muscle cells

A) Muscle cell contraction occurs through the interaction of myosin and actin filaments. The process consists of a rotating movement of the myosin head caused by ATP hydrolysis, the binding of the myosin head to an actin filament and a rotating movement of the myosin head bound to the actin filament. As a result, the actin and myosin filaments slide against each other.
B) Regulation of the binding of myosin heads to actin filaments by intracellular Ca2+. If the Ca2+ concentration increases in a cell, Ca2+ binds to the regulatory protein. As a result, a structural change of the regulatory protein (which blocked the binding site for the myosin head) occurs, thus enabling the binding of the myosin head to the actin filament.

Transmission of excitation and the contraction of muscle cells

Fig. 6-12 Transmission of excitation and the contraction of muscle cells

Excitation from nerve fibers is transmitted to muscle cells via synapses. This excitation is then passed on to the sarcoplasmic reticulum via the T-tubules connecting to the cell membrane. As a result, Ca2+ stored in the sarcoplasmic reticulum is released into the cytoplasm, triggering the contraction of sarcomeres.

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