Muscle Movement Flashcards
What is the A band
Made up of actin and myosin, in middle is H zone
What is H zone
Centre of A band made up of only myosin
What is I band
Only actin, at either end of A band
What is Z line
The end of each sarcomere
What happens to distance between Z lines during contraction
Distance gets shorter
What is actin
Globular protein coiled into 2 chains twisted around each other, appear lighter, surrounded by coil of tropomyosin
What is myosin
Heavier, long rods shaped fibres with bulbous heads projecting to the side, fibrous protein with globular heads
What is tropomyosin
Protein that forms fibrous strand around actin filament, covers myosin binding sites when muscle relaxed
How do actin and myosin appear under electron microscope
Actin lighter as thinner, myosin darker as thicker
What is bound to myosin heads
ADP
How do muscles contract
-action potential from motor neurone (at neuromuscular joint) stimulates muscle cell and depolarises sarcolemma, depolarisation spreads down T-tubules to sarcoplasmic reticulum
-causes sarcoplasmjc reticulum to release calcium ions and calcium channels open to allow Ca2+ across membrane and then bind to troponin binding sites stimulating tropomyosin to change shape and position on the actin
-myosin binding sites exposed on actin molecule and globular myosin heads bind to these to form cross-bridges between two filaments
-formation of crossbridges causes myosin heads to change angle and shape releasing ADP and Pi
-pulls actin filament towards centre of sarcomere and causes muscle to contract
How do myosin heads return to their original position
-ATP binds to myosin heads (once bound to binding sites) producing change in shape that causes myosin heads to release from actin filaments
-enzyme ATPase hydrolyses ATP into ADP and Pi, causes myosin heads to move back to their og position (recovery stroke) and ADP binds to myosin head
-myosin heads then able to bind to new binding sites on actin filaments (closer to Z disc) snd myosin heads move again pulling actin filaments closer to centre of sarcomere
What do calcium ions do
Activate ATPase enzyme and bind to tropomyosin protein which causes them to change position
What happens when excitation stops
-calcium ions leave their binding sites on the tropomyosin molecules and are moved by active transport back into the sarcoplasmic reticulum (needs ATP)
-troponin molecules return to their og shape pulling the attached tropomyosin molecules with them= blocks the myosin binding sites again
-muscles aren’t contracted as no myosin heads are attached to actin filaments
-the actin filaments slide back to their relaxed position which lengthens the sarcomere
What happens to calcium ions when sarcolemma no longer depolarised
Taken back up by sarcoplasmic reticulum by active transport
