Skeletal muscle helps move the bones in our body
It is sometimes known as striated, or voluntary muscle (as we have control over its movement)
Each muscle is made from thousands of muscle fibres, each fibre is a very specialised structure called a syncitium (As it has many nuclei, thus not a cell)
Each fibre is surrounded by a sarcolemma (a plasma membrane)
The sarcolemma has deep infoldings in it which are called T tubules (these are used in the depolarisation of the sarcoplasmic reticulum)
The cytoplasm (often called the sarcoplasm) contains many tightly packed mitochondria which are used to generate the ATP necessary for muscle contraction)
Myofibrils are responsible for the striated appearance of skeletal muscle
The myofibrils are made up of even smaller components called filaments.
There are 2 types of filament, ones made from actin and ones made from myosin.
Actin filaments are thinner than myosin filaments and this is what causes the stripy pattern.
As the Z discs move closer together, the sarcomeres in each myofibril become shorter (this is known as the sliding filament model)
The myosin heads act as ATPases, creating the energy for the filaments to move
During contraction the troponin and tropomyosin heads change shape and bond to a different position on the actin filaments. The myosin heads can now bond to the newly exposed sites on the actin filaments, forming cross-bridges.
The myosin heads then tilt, pulling the actin filaments towards the centre of the sarcomere. They return to their original position by hydrolysing ATP (providing energy). This process can then repeat.
Stimulation of a Muscle
An action potential travelling down the axon of a motor neurone causes relase of acetylcholine at a neuromuscular junction. The postsynaptic membrane in this case is the sarcolemma which is then depolarised and an action potential sweeps across it, travelling down the T tubules and reaching the sarcoplasmic reticulum.
Calcium ions have been collected, by use of active transport, in the cisternae of the reticulum. When the action potential arrives, active transport stops and calcium ion channels open, releasing them in vast quantities down the concentration gradient.
The calcium ions then bond with the troponin and tropomyosin, changing their shape and moving them along the actin filaments, exposing new myosin binding sites.