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The mammalian engine


Any muscle works like an engine. It has a size, structure, requires fuel to operate, produces force from the burning of that fuel, and needs to be controlled. If you are unfamiliar as to how muscles work, or quickly needs to brush up on the mechanics, click here.


Let’s take the Bicep brachii. It is made up of thousands of muscle cells or fibres, which lie parallel to one other. These fibres are connected to bone via tendons. Each muscle fibre is connected to a nerve (called innervation) and will only contract when this nerve fires. 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 this neurone will cause only those fibres to contract and produce force, whereas the other fibres will stay relaxed and not take part in 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 the fibre. These myosin heads are able to move and are like the pistons of an engine that can 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, blood glucose, glycogen and fat. The first three fuels can be converted to ATP without oxygen (O2). However, a more efficient way of generating ATP is when glucose, glycogen or fat are metabolised in the presence of with oxygen in the mitochondria. This yields a lot more ATP and the muscle fibre can contract for longer, and 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 (CO2), hydrogen ions (H+ – that lowers the pH making the environment more acidic) and lactate. All these byproducts 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.

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