Muscle contraction

Structure of skeletal muscle

• Individual muscles are made up of millons of tiny muscle fibres called myofibrils- individually they produce nearly no force force but collectivily they can be extremely powerful
• If muscles were made up of individual cells joined end to end they couldn't be very effective- this was because the junction between adjectet cells would be a point of weakness
• The seperate muscle cells fuse together into muscle fibres- these muscle fibres share nuclei and also a cytoplasm- called a sarcoplasm
• Within the sarcoplasm theres a large concentration of mitrochondria and endoplasmic reticulum 

Microscopic structure of skeletal muscle

Myofibrils are made up of two types of protein filament:

• actin- thinner and consists of two strands twisted around one another
• myosin- thicker and consists of long rod-shaped fibres with bulbous heads that project to the side

Myofibrils appear striped due to the alternating light-coloured and dark-coloured bands. The light bands are called isotropic bands (I-bands)- they appear lighter because the actin and myosin filaments do not over lap. The dark bands are called anisotropic (A-bands)- they appear darker becuase the actin and myosin filaments overlap.

At the centre of each anisotropic band is a lighter-coloured region called the H-zone, and at the centre of each isotropic is a line called tge Z-line- the distance between each adjecent Z-line is called a sarcomere.

Two other important proteins are found with muscles:

• tropmyosin- forms a fibrous strand around the actin filament
• a globular protein (troponin) involved in muscle contraction

Types of muscle fibre

• Slow-twitch fibres- contract more slowly and provide less powerful contractions over a long period of time. They are adapted for aerobic respiration in order to avoid lactic acid build up. These adaptations include having:
• a large store of myoglobin (molecule that stores oxygen)
• a supply of glycogen to provide a source of metabolic energy
• a rich supply of blood vessels to deliver oxygen and glucose
• numerous mitochondria to produce ATP
• Fast-twitch fibres- contract more quickly and produce more powerful contractions but only for a short period. Adapted for anaerobic respiration in the following ways:
• thinker and more numerous myosin filaments
• a high concentration of enzymes involved in anareobic respiration
• a store of phosphocreatine, a molecule that can rapidly generate ATP from ADP in anareobic conditions and so provide energy for muscle contraction

Neuromuscular junctions

When a nerve impluse is recieved at a neuromuscular junction the synaptic vesicles fuse with the presynaptic membrane and release their acetylcholine. The acetylcholine diffuses to the post synampic membrane altering its permeability…