HOW SKELETAL MUSCLE WORKS

Each skeletal muscles is made up of hundreds of long cells called fibres. Each fibre is filled with tiny protein threads. There are two types – thick filaments called myosin and thin filaments called actin. They are both contractible proteins, capable of shortening.

These actin & myosin filaments are arranged lengthwise in units called sarcomeres. The overlapping of the filaments gives muscle a striped/striation pattern. The sarcomere is the contractible unit of the muscle – interaction between these two filaments in the sarcomere produces muscle contraction.

During contraction, heads on the myosin proteins briefly form cross-bridges with the actin and then pull the filaments sliding over each other (like the power-stroke of a rowing boat). This shortens the total length of the sarcomere and happens simultaneously in neighbouring sarcomeres all along the length of the muscle, which will shorten & become thicker.

(Like an extensible ladder that is closed up – all the parts remain the same length but the total length of the ladder is shorter due to more overlapping.)

Once the cross-bridges are released, the sarcomeres lengthen again passively. For this lengthening, some force is required, usually from an antagonistic muscle or gravity.

The sequence of physiological events which brings about this process of contraction is as follows:

1. The nerve impulse is conveyed to the muscle cell

2. The nerve impulse stimulates the release of calcium within the cell

3. The calcium stimulates the Actin & Myosin to slide over each other

ENERGY FOR MUSCLE CONTRACTION

The process of muscle contraction requires energy. There are various sources.

1. Immediate energy is provided by a molecule in the cell called ATP (adenosine triphosphate) which is stored in very small amounts & runs out after a few seconds of strenuous exercise. This chemical reaction requires oxygen and is therefore referred to as aerobic

2. Once ATP is depleted, a back-up energy compound called creatine phosphate is broken down but this will run out after a short time (it is suitable for a 100 metres race). This does not require oxygen and is therefore anaerobic

3. Next, glycogen is broken down into glucose, which is oxidised to make ATP. This is aerobic (Glycogen is a long-term energy storage found in the liver & muscles)

4. If there is not enough oxygen to oxidise the glucose, it can still be broken down aerobically but less effectively. Less ATP is produced, leading to weaker muscle contraction and the build-up of waste products (lactic acid, which causes pain, discomfort, & further fatigue)