It is essential that the basics of skeletal muscle is understood. If you are unfamiliar with the field of muscle physiology or merely need to brush up on it, please visit striated muscle and muscle contraction. If you are familiar with the biology of skeletal muscle, below is a more in depth look at muscle fibres.
The mammalian engine
A muscle fibre is like a car engine. First off, a muscle (for example, the bicep brachii) is comprised of thousands of muscle cells (also called fibres) and lie parallel to each other. These fibres are connected to bone via tendons.
Each muscle fibres is connected to a nerve and will only contract or relax in response to a nerve impulse. A number of these nerves coming from the fibres will fuse into one neurone and, together with the muscle fibres, will be known as a motor unit. Stimulation of the neurone of the motor unit will cause only those fibres to contract and produce force, whereas the other fibres will remain relaxed (or not take part in the contraction).
Each muscle fibre is made up of thousands of thick and thin filaments, also known as myosin and actin, respectively. With other proteins, they form the smallest contractile unit known as a sarcomere within a muscle fibre. It is also the myosin molecule, particularly the myosin heavy chain (MHC) protein, that determines the fibre type of one muscle fibre. These myosin heads are like the pistons of the engine that produce force and power.
No engine can function without fuel. Adenosine triphosphate (ATP) is the main fuel source of many cells. It is this ATP that is needed for the myosin to work. ATP cannot be stored, but can be replenished from other more complex fuel sources. These complex fuel sources are phosphocreatine, glucose, glycogen and fat. The first three fuels can be converted to ATP without oxygen.
A more efficient way is when glucose, glycogen or fat is metabolised with oxygen in the mitochondria. This yields a lot more ATP and the muscle fibre can resist fatigue much better due to the continuous supply of ATP.
The breakdown of the four fuels yields byproducts that can lead to premature muscle fatigue. These include carbon dioxide, hydrogen ions (that lowers the pH making the environment more acidic) and lactate – all are readily transported away from the muscle by the blood.
The next few sections will delve a little deeper into all these aspects to better understand muscle function. These include:
★ the fibre type concept,
★ fibre performance,
★ fuelling performance,
★ muscle fibre size and
★ the makeup of a muscle.