excitation-contraction coupling

Question 1

what is excitation-contraction coupling

Answer 1

the process by which excitation triggers the increase in intracellular calcium concentration
it is an increase in intracellular calcium concentration that triggers muscle contraction, the time during which it remains elevated determines the duration of muscle contraction

Question 2

how do action potentials affect excitation-contraction coupling

Answer 2

the skeletal muscle fibre is so large that action potentials spreading along its surface cause almost no current flow deep within the fibre
maximum muscle contraction requires the current to penetrate deeply into the fibre to the vicinity of separate myofibrils
this penetration is achieved by transmission of action potentials along transverse tubules (T tubules)

Question 3

what are T tubules

Answer 3

penetrate all the way through the muscle fibre, from one side to the other
exist in a triad along with the sarcoplasmic reticulum cisterna
formed from invaginations of sarcolemma

Question 4

what is the sarcoplasmic reticulum

Answer 4

the intracellular store of calcium

Question 5

what is the process of excitation-contraction coupling

Answer 5

1. membrane depolarisation opens the L-type calcium channel > DHP receptor
2. mechanical coupling between the L-type calcium channel and the calcium release channel (ryanodine receptor) causes a conformational change which then causes the calcium release channel to open
3. calcium exits the sarcoplasmic reticulum via the calcium release channel and activates troponin C, leading to muscle contraction
4. calcium entering the cell via L-type calcium channels can also activate the calcium release channel

Question 6

how is intracellular calcium levels lowered

Answer 6

1. sodium-calcium exchanger and calcium pump in the plasma membrane both extrude calcium from the cell
2. calcium pump sequesters calcium within the sarcoplasmic reticulum
3. calcium is bound in the sarcoplasmic reticulum by calreticulin and calsequestrin > reuptake of calcium ions

Question 7

what is the excitatory ‘pulse’ of calcium

Answer 7

resting intracellular calcium concentration is too little to elicit contraction
full excitation of the t-tubule/SR system causes release of calcium to increase intracellular calcium concentration
the SERCA calcium pump depletes intracellular calcium concentration
the pulse lasts 1/20 of a second
during this pulse contraction occurs

Question 8

what is relaxation of skeletal muscle contraction

Answer 8

the contractile process is turned off when calcium is returned to the sarcoplasmic reticulun when electrical activity stops
the SR expresses calcium-ATPase pumps, which actively transports calcium from the cytosol and concentrates it in the SR
the thin filaments then return passively to their resting position.
the muscle fibre has relaxed

Question 9

what is the complete overview of skeletal muscle contraction

Answer 9

1. an action potential arriving at a terminal button of the NMJ stimulates release of Ach, which diffuses across the cleft and triggers an action potential in the muscle fibre
2. the action potential moves across the surface membrane and into the muscle fibres interior through the T tubules. an action potential in the T tubules triggers release of calcium from the SR into the cytosol
3. calcium binds to troponin on thin filaments
4. calcium binding to troponin causes tropomyosin to change shape, physically moving it away from its blocking position; this uncovers the binding sites on actin for the myosin cross-bridges
5. myosin cross-bridges attach to actin at the exposed binding sites
6. the binding triggers the cross bridge to bend, pulling the thin filament over the thick filament toward the centre of the sarcomere. this power stroke is powered by energy provided by ATP
7. after the power stroke, the cross bridge detaches from actin. if calcium is still present, the cycle returns to step 5
8. when action potentials stop, calcium is taken up by the SR. with no calcium on troponin, tropomyosin moves back to its original position, blocking myosin cross-bridge binding sires on actin contraction stops and the thin filaments passively slide back to their original relaxed positions.