Muscle Contraction

Muscle Contraction

Structure of Skeletal Muscle

• Muscles are effector organs that respone to nervous stimulation by contacting that bring about movement.
• There are three types of muscle in the body:
• Cardiac muscle - found exclusively in the heart - not under concious control.
• Smooth muscle - found in the walls of blood vessels and the gut - not under concious control.
• Skeletal muscle - makes up the bulk of body muslce and is attached to bone - acts under voluntary and concious control.
• Individual muscles are made up of millions of tiny muscle fibres called myofibrils, which collectively are very powerful.
• Myofibrils are lined up paraellel to each other in order to maximise strength and force.
• Muscles are composed of smaller units bundled into progressively larger ones.
• Muscles are not made up of individual cells joined end to end, because they would not be able to perform the function of contraction verry efficiently, due to the junction betweem adjacent cells creating a point of weakness and so reduce the overall strength.
• Muscles have therefore developed a different structure, where separate cells have become fused toegether into muscle fibres, which share a nucleus and a cytoplasm, called a sarcoplasm. 
• Within each sarcoplasm is a large concentration of mitochondria and endoplasmic reticulum.

Microscopic Structure of Skeletal Muscle

Myofibrils are made up of two types of protein filament:

• Actin - thin and consists of two strands twised around one another.
• Myosin - thick and consists of long rod-shaped fibres with bulbous heads that project to the side.
• Myofibrils appear striped due to their alternating light and dark coloured bands.
• Dark coloured bands = anisotropic bands - they appear darker because the actin and muosin filaments overlap. Has a lighter colour region in the centre of each ban called the H-zone.
• Light coloured bands = isotropic bands - they appear lighter as the actin and myosin filaments do not overlap. Has a line in the centre called the Z-line. The distance between each adjacent Z-lines is called a sarcomere. 
• When a muscle contracts, theses sacromeres shorten and the pattern of light and dark bands changes.

Two other important proteins found in muscles:

• Tropomyosin - froms a fibrous strand around the actin filament.
• Troponin (a gobular protein) - involved in muscle contraction.

Types of Muscle Fibre

There are two types and the proportion of each vary from muscle to muscle and person to person. These two types are:

 Slow-twitch fibres:

• Contract slowly and provide less powerful contractions over a longer period.
• Adapted for endurance work, such as marathon running.
• More common in muscle like the calf muscle, which need to contract constantly to maintain the body in an upright position.
• Suited to their role by being adapted for aerobic respiration in order to avoid the build-up of lactic acid, which causes them to function less effectively.
• These adaptations include having:
• A large store of myoglobin, which is a bright red molecule that stores oxygen.
• A supply of glycogen to provide a source of metabolic…