Muscle contraction

Students participating in the presentation: 1- naif aljabri yousif alessa faris abalkheel Ali Moshaba AL-Ahmary abdulelah bin numay

S KELETAL MUSCLE

S KELETAL MUSCLE & M USCLE FILAMENTS  Skeletal muscle  Contraction of skeletal muscle is under voluntary control.  each skeletal muscle cell is innervated by a branch of a motoneurons.  Muscle filaments  each muscle fiber behaves as a single unit  is multinucleate and contains myofibrils  the myofibrils are surround by sarcoplasmic reticulum are invaginated by transverse tubules ( t tubules)  each myofibril contains interdigitating thich and thin filaments.

T HICH FILAMENTS & T HIN FILAMENTS  Thich filaments  are comprised of a large molecular weight protein called myosin  Thin filaments  are composed of 3 proteins: 1- actin.2-tropomyosin.3-troponin.

Arrangement of thick and thin filaments in sarcomeres

CYTOSKELTAL PROTEINS

T RANSVERSE TUBULE AND THE SARCOPLASMIC RETICULUM  Transverse tubule and the sarcoplasmic reticulum  are continuous with the sarcolemmal membrane and invaginated deep into the muscle fiber, making contact with terminalcisternae of the sarcoplsmic reticulum.

E XCITATION - CONTRACTION COUPLING Excitation-contraction coupling  In skeletal muscle the method of excitation contraction coupling relies on the ryanodine receptor being activated by a domain spanning the space between the T tubules and the sarcoplasmic reticulum to produce the calcium transient responsible for allowing contraction.  The motor neuron produces an action potential that propagates down its axon to the neuromuscular junction. motor neuronaction potentialneuromuscular junction  The action potential is sensed by a voltage-dependent calcium channel which causes an influx of Ca 2+ ions which causes exocytosis of synaptic vesicles containing acetylcholine.voltage-dependent calcium channel exocytosissynaptic vesiclesacetylcholine  Acetylcholine diffuses across the synapse and binds to nicotinic acetylcholine receptors on the myocyte, which causes an influx of Na + and an efflux of K + and generation of an end-plate potential.synapsenicotinic acetylcholine receptorsend-plate potential

E XCITATION - CONTRACTION COUPLING  The end-plate potential propagates throughout the myocyte's sarcolemma and into the T-tubule system.T-tubule  The T-tubule contains dihydropyridine receptors which are voltage-dependent calcium channels and are activated by the action potential.dihydropyridine receptors voltage-dependent calcium channels  Opening of the Ryanodine receptors causes and flow of Ca 2+ from the sarcoplasmic reticulum into the cytoplasm.  Ca 2+ released from the sarcoplasmic reticulum binds to Troponin C on actin filaments, which subsequently leads to the troponin complex being physically moved aside to uncover cross-bridge binding sites on the actin filament.Troponin Cactin filamentstroponin complex

E XCITATION - CONTRACTION COUPLING  By hydrolyzing ATP, myosin forms a cross bridges with the actin filaments, and pulls the actin toward the center of the sarcomere resulting in contraction of the sarcomere.myosinsarcomere  Simultaneously, the sarco/endoplasmic reticulum Ca 2+ -ATPase actively pumps Ca 2+ back into the sarcoplasmic reticulum where Ca 2+ rebinds to calsequestrin.sarco/endoplasmic reticulum Ca 2+ -ATPase  With Ca 2+ no longer bound to troponin C, the troponin complex slips back to its blocking position over the binding sites on actin.  Since cross-bridge cycling is ceasing then the load on the muscle causes the inactive sarcomeres to lengthen.

M ECHANISM OF TETANUS Mechanism of tetanus  A single action potential result in the release of a fixed amount of ca+ from the sarcoplasmic reticulum which produce a single twist is terminated ( relaxation occurs ) when the sarcoplasmic  Step 1: At the end of the previous round of movement and the start of the next cycle, the myosin head lacks a bound ATP and it is attached to the actin filament in a very short-lived conformation known as the 'rigor conformation'.  Step 2: ATP-binding to the myosin head domain induces a small conformational shift in the actin-binding site that reduces its affinity for actin and causes the myosin head to release the actin filament.

M ECHANISM OF TETANUS  Step 3: ATP-binding also causes a large conformational shift in the 'lever arm' of myosin that 'cocks' the head into a position further along the filament. ATP is then hydrolysed, but the inorganic phosphate and ADP remain bound to myosin.  Step 4: The myosin head makes weak contact with the actin filament and a slight conformational change occurs on myosin that promotes the release of inorganic phosphate.  Step 5: The release of inorganic phosphate reinforces the binding interaction between myosin and actin and subsequently triggers the 'power stroke'. The power stroke is the key force-generating step used by myosin motor proteins; forces are generated on the actin filament as the myosin protein reverts back to its original conformation.  Step 6: As myosin regains its original conformation, the ADP is released, but the myosin head remains tightly bound to the filament at a new position from where it started, thereby bringing the cycle back to the beginning.

M ECHANISM OF TETANUS