Muscle contraction

This section concerns the contraction of striated muscle

• this type of muscle tissue makes up many muscles in the body that are attached to the skeleton
• striated muscle only contracts when stimulated by impulses from motor neurones
• muscle tissue like this is described to be neurogenic
• we already know about cardiac muscle which is myogenic - contracts and relaxes automatically with no need for impulses from neurones
• the third type of muscle tissue is smooth muscle which is found throughout the body in organs such as walls of veins, arteries and alimentary canal
• however smooth muscles in arteries also contracrs when it it stretched by pressure of blood surgung through them - without input from nervous system
• this type of muscle is called smooth because, unlike the other two it has no striations
• smooth muscle does not form smooth lining of tubular structures like trachea and arteries; lining of these structures is always formed by an epithelium

It is very important that the activities of the different muscles in our bodies are coordinated

• when a muscle contracts, it exerts a force on a particular part of the body like a bone resulting in a particular response
• the nervous system ensures that behaviour of each muscle is coordinated with all the other muscles so they bring about desired movement without causing damage to any parts of the skeletal or muscular system

The structure of striated muscle

• a muscle e.g. bicep, is made up of thousands of muscle fibres
• each muscle fibre is a very specialised 'cell' with a highly organised arrangement of contractile proteins in the cytoplasm, surrounded by a cell surface membrane
• because each unit conains many nuclei, rather than a cell it is called syncytium to describe the mutinucleate muscle fibre
• parts of the fibre are known by different terms;
  • cell surface membrane - sarcolemma
  • cytoplasm - sarcoplasm
  • endoplasmic reticulum - sarcoplasmic reticulum (SR)
• the cell surface membrane has many deep infoldings into the interior of the muscle fibres, called transverse system tubules (T-tubules)
• these run close to the sarcoplasmic reticulum
• membrane of the SR have huge numbers of protein pumps that transport calcium ions into the cisternae of the SR
• the sarcoplasm contains a large number of mitochondria, tightly packed between the microfibrils
• these carry out aerobic respiration, generating ATP required for muscle contraction

Muscle fibre stripes or striations are produced by a very regular arrangement of many myofibrils in the sarcoplasm

With an electron microscope microfibrils can be seen to have smaller components called filaments

• parallel groups of thick filaments lie between groups of thin ones
• both thick and thin filaments are made up of protein
• thick filaments are made mostly of myosin, whilst the thin ones are made mostly of actin
• this helps to explain what causes the stripes
• the darker parts of the stripes, the A bands, correspond to thick (myosin) filaments
• the ligheter parts…