The most important property of skeletal muscles is its ability to contract. Muscle contraction occurs as a result of the interaction of myofibrils inside the muscle cells. This process either shortens the muscle or increases its tension, generating a force that either facilitates or slows down a movement.
There are two types of muscle contraction; isometric and isotonic. A muscle contraction is deemed as isometric if the length of the muscle does not change during the contraction, and isotonic if the tension remains unchanged while the length of the muscle changes. There are two types of isotonic contractions:
- Concentric contraction, in which the muscle shortens due to generating enough force to overcome the imposed resistance. This type of contraction serves to facilitate any noticeable movement (e.g. lifting a barbell or walking on an incline).
- Eccentric contraction, in which the muscle stretches due to the resistance being greater than the force the muscle generates. During an eccentric contraction, the muscle maintains high tension. This type of contraction usually serves to slow down a movement (e.g. lowering a barbell or walking downhill).
Motor neuron axonAxon motoneuronis1/2
The sequence of events that results in the contraction of a muscle cell begins as the nervous system generates a signal called the action potential. This signal travels through motor neurons to reach the neuromuscular junction, the site of contact between the motor nerve and the muscle. A group of muscle cells innervated by the branches of a single motor nerve is called the motor unit.
The incoming action potential from the motor nerve initiates the release of acetylcholine (ACh) from the nerve into the synaptic cleft, which is the space between the nerve ending and the sarcolemma. The ACh binds to the receptors on the sarcolemma and triggers a chemical reaction in the muscle cell. This involves the release of calcium ions from the sarcoplasmic reticulum, which in turn causes a rearrangement of contractile proteins within the muscle cell. The main proteins involved are actin and myosin, which in the presence of ATP, slide over each other and pull on the ends of each muscle cell together, causing a contraction. As the nerve signal diminishes, the chemical process reverses and the muscle relaxes.