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Skeletal muscles are stimulated to contract by nerves and act as effectors
· Three types of muscle
o Smooth muscle
§ Is generally found in tubular organs such as the intestines blood vessels and reproductive system where its function is peristalsis.
o Cardiac muscle
§ Is only found in the heart
o Skeletal muscle
§ Is attached to bones where is function is to produce movement and maintain posture.
· Skeletal muscle
o Key facts
§ An individual muscle is made up of hundreds of cylindrical muscle fibres about 50um in diameter and ranging in length from a few millimetres to several centimetres.
§ Muscle fibres are not composed of individual cells- the cell membranes have broken down and so each fibre has many nuclei.
§ Each fibre is surrounded by a modified cell membrane called the sarcolemma.
§ Each muscle fibre is composed of many long, cylindrical myofibrils consisting of a repeating arrangement of proteins, which causes the banding pattern.
§ Each repeated pattern of proteins is called a sarcomere
§ The two main proteins involved in muscular contraction are actin ( thin) and myosin (thick) filaments.
§ Muscles contract when the actin fibres are pulled over the myosin fibres
§ Two other proteins involved are tropmyosin and troponin.
o Tropomyosin winds round preventing it binding to myosin
o Troponin moves tropomyosin out of the way
o Set up of muscular contraction:
1. An impulse arrives down a motor nerve and terminates at the neuromuscular junction ( a modified synapse)
2. The synapse secretes acetylcholine
3. Acetylcholine fits into receptor sites on the motor end plate.
4. The binding causes a change in the permeability of the sarcoplasmic reticulum, resulting in an influx of calcium ions into the myofilament.
5. The calcium ions bind to the troponin, changing its shape.
6. Troponin displaces the tropomysoin, so that the myosin heads can bind to the actin.
7. The myosin head pulls backwards so the actin is pulled over the myosin. This is the power stroke.
8. An atp molecule becomes fixed to the myosin head, causing it to detach from the actin.