MUSCLE CONTRACTION 2016 Paul Billiet ODWS Muscle tissue

Muscle tissue Muscle Tendon Fibres Myofibril Z line Striations Sarcolemma Nucleus Sarcomere Tendon ©

Striated muscle viewed at x 400

Striated muscle Relaxed muscle fibrils showing sarcomeres (Z to Z) and A, H and I bands

Muscle contraction observed Contraction. TEM view

The sliding filament theory Cross bridges Myosin Cross bridges Z line Actin RELAXED Actin H band I band A band CONTRACTED © 2016 Paul Billiet ODWS H band I band disappear when the muscle is fully contracted

The motor end plate is the terminal button of a motor neurone that makes contact with a muscle cell ¢ The motor end plate releases the neurotransmitter acetylcholine into the synapse ¢ Acetyl choline is broken down in the synaptic cleft by choline esterase enzyme ¢ The products of this breakdown are reabsorbed by the motor end plate. ¢ © 2016 Paul Billiet ODWS

The motor end plate (neuromuscular junction) biology. clc. uc. edu/fankhauser/Labs/Anatomy_. . .

7. Action potential generated which travels across the muscle cell. 8. Neurotransmitter destroyed by

The motor end plate (neuromuscular junction) ¢ ¢ ¢ An impulse arrives at the motor end plate from the axon of a motor neurone Acetyl choline is released Acetyl choline diffuses across the synapse and Acetyl choline binds with the receptor sites on the sarcolemma (the muscle cell membrane) The sarcolemma depolarises An action potential is created (from – 90 m. V to +40 m. V) once threshold of the muscle cell is reached (all-or-nothing response). © 2016 Paul Billiet ODWS

The muscle cell plasma membrane © 2016 Paul Billiet ODWS

The muscle cell sarcoplasmic reticulum covers the myofibrils and stores Ca 2+ ¢ Infoldings from the sarcolemma (T-tubules) connect to sarcoplasmic reticulum ¢ Depolarisation of the sarcolemma leads to depolarisation of the sarcoplasmic reticulum ¢ The depolarised sarcoplasmic reticulum becomes permeable to Ca 2+ ¢ © 2016 Paul Billiet ODWS

Muscle contraction Ca 2+ diffuses into the cytosol (cytoplasm of the muscle fibre) from the sarcoplasmic reticulum ¢ Troponin/tropomyosin protein complex blocks actin filament stops myosin head groups from binding to it ¢ Ca 2+ lifts the blockage. ¢ © 2016 Paul Billiet ODWS

ATP is hydrolysed myosin reaches out and attaches to actin ADP is released the myosin head moves When myosin picks up an ATP molecule myosin detaches itself from actin © 2016 Paul Billiet ODWS Scitable Nature Publishing

The power stroke ¢ ¢ ¢ ¢ Myosin is an actin-activated ATPase When it binds with actin it hydrolyses ATP to ADP and inorganic Phosphate (Pi) As ADP & Pi is released the conformation (shape) of the myosin head changes ATP hydrolysis is coupled to movement at a molecular level This change pushes the myosin along the actin filament (the “power stroke”) By 8 -10 nm The muscle contracts. © 2016 Paul Billiet ODWS

Muscle relaxation Contraction continues as long as Ca 2+ levels remain high in the cytosol ¢ Ca 2+ is rapidly pumped back across the sarcoplasmic reticulum out of the cytosol (this also requires ATP) … ¢ and the muscle relaxes. ¢ © 2016 Paul Billiet ODWS