9.3 Muscle Fibre Contraction and Relaxation
Learning Objectives
By the end of this section, you will be able to:
- Describe the components involved in a muscle contraction
- Explain how muscles contract and relax
- Describe the sliding filament model of muscle contraction
The sequence of events that result in the contraction of an individual muscle fibre begins with a signal—the neurotransmitter, ACh—from the motor neuron innervating that fibre. The local membrane of the fibre will depolarise as positively charged sodium ions (Na+) enter, triggering an action potential that spreads to the rest of the membrane will depolarise, including the T-tubules. This triggers the release of calcium ions (Ca2+) from storage in the sarcoplasmic reticulum (SR). The Ca2+ then initiates contraction, which is sustained by ATP (Figure 9.3.1). As long as Ca2+ ions remain in the sarcoplasm to bind to troponin, which keeps the actin-binding sites “unshielded,” and as long as ATP is available to drive the cross-bridge cycling and the pulling of actin strands by myosin, the muscle fibre will continue to shorten to an anatomical limit. Muscle contraction usually stops when signalling from the motor neuron ends, which repolarises the sarcolemma and T-tubules, and closes the voltage-gated calcium channels in the SR. Ca2+ ions are then pumped back into the SR, which causes the tropomyosin to re-shield (or re-cover) the binding sites on the actin strands. A muscle also can stop contracting when it runs out of ATP and becomes fatigued (Figure 9.3.6 ).
The molecular events of muscle fibre shortening occur within the fibre’s sarcomeres (see Figure 9.3.2). The contraction of a striated muscle fibre occurs as the sarcomeres, linearly arranged within myofibrils, shorten as myosin heads pull on the actin filaments.
The region where thick and thin filaments overlap has a dense appearance, as there is little space between the filaments. This zone where thin and thick filaments overlap is very important to muscle contraction, as it is the site where filament movement starts. Thin filaments, anchored at their ends by the Z-discs, do not extend completely into the central region that only contains thick filaments, anchored at their bases at a spot called the M-line. A myofibril is composed of many sarcomeres running along its length; thus, myofibrils and muscle cells contract as the sarcomeres contract.
The Sliding Filament Model of Contraction
When signalled by a motor neuron, a skeletal muscle fibre contracts as the thin filaments are pulled and then slide past the thick filaments within the fibre’s sarcomeres. This process is known as the sliding filament model of muscle contraction (Figure 9.3.2). The sliding can only occur when myosin-binding sites on the actin filaments are exposed by a series of steps that begins with Ca2+ entry into the sarcoplasm.
Tropomyosin is a protein that winds around the chains of the actin filament and covers the myosin-binding sites to prevent actin fro