Triggering and Reversing Contraction
The coupling process leading from electrical signal (excitation)
to contraction in skeletal muscle
The trigger for a muscle contraction is an electrical impulse. The electrical signal sets off a series of events that lead to crossbridge cycling between myosin and actin, which generates force. The series of events is slightly different between skeletal, smooth and cardiac muscle.
Let's take a look at what occurs within a skeletal muscle, from excitation to contraction to relaxation:
- An electrical signal (action potential) travels down a nerve cell, causing it to release a chemical message (neurotransmitter) into a small gap between the nerve cell and muscle cell. This gap is called the synapse.
- The neurotransmitter crosses the gap, binds to a protein (receptor) on the muscle-cell membrane and causes an action potential in the muscle cell.
- The action potential rapidly spreads along the muscle cell and enters the cell through the T-tubule.
- The action potential opens gates in the muscle's calcium store (sarcoplasmic reticulum).
- Calcium ions flow into the cytoplasm, which is where the actin and myosin filaments are.
- Calcium ions bind to troponin-tropomyosin molecules located in the grooves of the actin filaments. Normally, the rod-like tropomyosin molecule covers the sites on actin where myosin can form crossbridges.
- Upon binding calcium ions, troponin changes shape and slides tropomyosin out of the groove, exposing the actin-myosin binding sites.
- Myosin interacts with actin by cycling crossbridges, as described previously. The muscle thereby creates force, and shortens.
- After the action potential has passed, the calcium gates close, and calcium pumps located on the sarcoplasmic reticulum remove calcium from the cytoplasm.
- As the calcium gets pumped back into the sarcoplasmic reticulum, calcium ions come off the troponin.
- The troponin returns to its normal shape and allows tropomyosin to cover the actin-myosin binding sites on the actin filament.
- Because no binding sites are available now, no crossbridges can form, and the muscle relaxes.
As you can see, muscle contraction is regulated by the level of calcium ions in the cytoplasm. In skeletal muscle, calcium ions work at the level of actin (actin-regulated contraction). They move the troponin-tropomyosin complex off the binding sites, allowing actin and myosin to interact.
All of this activity requires energy. Muscles use energy in the form of ATP. The energy from ATP is used to reset the myosin crossbridge head and release the actin filament. To make ATP, the muscle does the following:
- Breaks down creatine phosphate, adding the phosphate to ADP to create ATP
- Carries out anaerobic respiration, by which glucose is broken down to lactic acid and ATP is formed
- Carries out aerobic respiration, by which glucose, glycogen, fats and amino acids are broken down in the presence of oxygen to produce ATP (see How Exercise Works for details).
Muscles have a mixture of two basic types of fibers: fast twitch and slow twitch. Fast-twitch fibers are capable of developing greater forces, contracting faster and have greater anaerobic capacity. In contrast, slow-twitch fibers develop force slowly, can maintain contractions longer and have higher aerobic capacity. Training can increase muscle mass, probably by changing the size and number of muscle fibers rather than the types of fibers. Some athletes also use performance-enhancing drugs, specifically anabolic steroids, to build muscle, although this practice is dangerous and is banned in most athletic competitions.