Contraction of Skeletal Muscle Sliding Filament Theory Copy Figure 1 from p187

Sliding Filament Theory 1. Myosin heads form a cross-bridge with the actin 2. This causes the head to be pulled back and to move along. ADP and Pi released. 3. As ATP attaches the head is released from the actin and the cross-bridge is broken 4. When the ATP is hydrolysed by ATPase this provides the energy necessary for the head to be moved back to its original position. ATPase works in the presence of Ca2+

Evidence for the Sliding Filament Theory When a muscle contracts the following changes happens in a sarcomere: The H-zone becomes narrower The I band becomes narrower The Z-lines move closer together (sarcomere shortens)

Relaxed and Contracted Muscle

Evidence for the Sliding Filament Theory The A-band remains the same width. This band is determined by the width of the myosin filaments. So the myosin filaments themselves do not become shorter. Compare sarcomeres before and after contraction.

Sliding Filament Mechanism of Muscle contraction Fill in the diagram using p190

Muscle Contraction Contraction occurs when an impulses from a motor neurone reaches the synapse at the junction with the muscle. If it is stronger than a threshold stimulus contraction will occur.

Overview of Muscle contraction 1. Acetylcholine, a neurotransmitter substance, is released into the synapse, diffuses across and attaches to specific receptors on the sarcolemma (the outer membrane of the muscle fibre). 2. The muscle sarcolemma is depolarised. 3. Depolarisation spreads along the fibre. 4. This causes calcium to be released from the sarcoplasmic reticulum into the sarcoplasm. 5. Calcium bind to troponin which displaces tropomyosin, thus uncovering the myosin binding sites on the actin filaments.

Muscle contraction – detailed account The Neuromuscular Junction:

Muscle Contraction – Detailed Account When the electrical impulse arrives at the end of the motor neurone (1) this causes Ca2+ ions to flow into the axon terminal (2) This causes synaptic vesicles to fuse with the presynaptic membrane and release their acteylcholine neurotransmitter into the synaptic cleft (3)

Muscle Contraction Acetylcholine diffuses across the synaptic cleft to bind with the receptors on the sarcolemma (4). This causes Na+ ion channels to open (5) and the nervous impulse travels down the sarcolemma (6). The acetylycholine is broken down by enzyme acetylcholinestearase. The resulting choline and ethanoic acid diffuse back into the neurone. (They are recombined back into acetylcholine using ATP from the mitochondria).

Muscle contraction: T-Tubules

Muscle Contraction – detailed account The action potential travels deep into the fibre through a system of tubules (t-tubules) that branch throughout the sarcoplasm of the muscle. The tubules are in contact with the SR of the muscle which has actively absorbed calcium ions from the sarcoplasm of the muscle.

Muscle Contraction – detailed account Calcium ions flow from the sarcoplasmic reticulum into the muscle cytoplasm down their concentration gradient. Ca2+ bind to troponin which causes the tropomyosin molecules that were blocking the binding sites on the actin filaments to move away so that the myosin heads can attach and form an actin-myosin crossbridge.

Muscle Contraction – detailed account ATP attaches to the myosin heads causes them to release from the actin. ATP is hydrolysed to ADP + Pi by ATPase. The energy released causes the heads to move back to their original position.

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