Contraction in the simplest sense is shortening of a muscle fibre. When muscles receive stimulation from the nervous system, the required fibres simultaneously shorten to produce a contraction of the overall muscle. It seems simple enough, right? However, the mechanisms by which muscles are able to shorten is remarkably complicated.

How do muscle fibres shorten?

To understand how muscles are able to shorten, we must look at their ultrastructure, which is shown in figure 1 below.

figure 1. Ultrastructure of skeletal muscle

The contractile unit of muscle is called a sarcomere. It is the shortening of each individual sarcomere which results in the shortening, and thus contraction of muscle at a macroscopic level.

figure 2. Sarcomere ultrastructure

During a contraction the size of a sarcomere will decrease, bringing the Z lines (shown in figure 1) closer together. This is achieved through the interaction between actin and myosin.

Myosin is known as the thick filament within muscle and actin the thin filament respectively. During a muscle contraction, myosin heads bind to the thin actin filament. This causes a conformational change in the myosin heads, resulting in the myosin heads flicking inwards towards the M-line (figure 2). This causes the sarcomere to shorten. For clarity, this is shown in the animation below.

figure 3. Sarcomere shortening

ATP (which is used as an energy currency within cells) is then used to detach the myosin heads from their actin binding sites. Myosin can bind again to the binding sites on the thin filament for a continual contraction. It is interesting that energy is required to detach the myosin heads from actin after a contraction, rather than to initiate a contraction itself. This specific role of ATP is the reason muscles become rigid and tight after death in a process called rigor mortis. Following death, stores of ATP in the body are used up, resulting in myosin heads remaining bound to actin binding sites within muscles, giving a rigid state characteristic of rigor mortis.

Obviously, all the muscles in your body do not all contract simultaneously. This means there must be a mechanism that regulates when myosin heads are able to bind to actin filaments.

The solution to this problem is achieved by using calcium (Ca²⁺).

It makes sense that the rested state of muscles is to be relaxed. Therefore, under normal circumstances myosin heads are not able to bind to actin and no contraction will occur. This is accomplished because the binding sites are blocked by the protein tropomyosin. The tropomyosin itself is attached to another regulatory protein called troponin.

Upon the initiation of a muscle contraction Ca²⁺ is released and binds to troponin. This causes a conformational change which results in tropomyosin moving away from actin to reveal the myosin head binding sites, thus allowing a muscle contraction to occur. When a muscle is required to stop contracting the Ca²⁺ is simply removed from troponin to allow tropomyosin to block the binding sites once again.

How is Ca²⁺ released?

The Ca²⁺ released to bind to troponin to cause a muscle to contract is stored in a specialised organelle within skeletal muscle known as the sarcoplasmic reticulum. When a nervous impulse arrives at a muscle cell giving the information to contract, this electrical signal is sent to the sarcoplasmic reticulum down a process within muscle cells called the transverse (T) tubule. The arrival of this impulse causes Ca²⁺ to be released from the sarcoplasmic reticulum and to bind to troponin.

Stay strong, we have reached the end! Finally we know how muscles initiate, sustain and terminate a contraction. It seems such a complicated process, for simple actions that we are able to achieve even without thought.

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Here, we have only discussed skeletal muscles. The physiology that allows the contraction of cardiac muscle is similar and also requires actin and myosin. However, smooth muscle contraction is more complicated and many of its mechanisms are yet undiscovered.

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References:
Rigor mortis
Mechanism of muscle contraction
Troponin and tropomyosin