Muscle contraction is controlled by changing the concentration of calcium ions (Ca2+) in the muscle cell. The normal intracellular concentration is below about 10-7 M and it is maintained at this level by an intracellular membrane system called the sarcoplasmic reticulum, which envelops each myofibril.
Periodically along the length of the muscle fibre, invaginations in the sarcolemma form a network of tubules which are called the transverse or T-tubules because they run transverse to the direction of the fibres. The sarcoplasmic reticulum also contains small longitudinal tubules which run parallel to the fibre axis (Figure 5.10). T-tubules represent a channel for carrying electrical impulses from the nervous system through the muscle cell to the sarcoplasmic reticulum. The sarcoplasmic reticulum cell membrane is depolarised causing the release of calcium through specific calcium channels. The essence of this structure is that the concentration of the Ca2+ ions can be changed simultaneously over the whole cell to ensure that the contraction occurs synchronously along its length.
Events in muscle contraction
Nerve supply to the muscle fibres is via the perimysium. Contraction is triggered by a nerve impulse, the action potential moves from the motor end plate and is carried to the interior of the muscle cell via the T-tubules. Nervous innervation of muscle cells is shown in Figure 5.11.
The depolarisation of the membrane by the nerve impulse triggers the release of Ca2+ from the SR, its concentration in the myofibre rising from 10-7 to 10-5 M. This initiates a contraction cycle.