If
you calculate the time it would take for extracellular Ca 2+ to
diffuse to the middle of a skeletal muscle fibre, the fastest predicted rate
for onset of contraction would be in the order of 100 msec after the arrival of
the motor neuron action potential. Why then is it possible for a muscle fibre
to start contracting only 10 msec after the arrival of the motor nerve action
potential? ( Lecture: skeletal muscle, chapter: excitation-contraction coupling)

Question

If
you calculate the time it would take for extracellular Ca 2+  to
diffuse to the middle of a skeletal muscle fibre, the fastest predicted rate
for onset of contraction would be in the order of 100 msec after the arrival of
the motor neuron action potential. Why then is it possible for a muscle fibre
to start contracting only 10 msec after the arrival of the motor nerve action
potential?  ( Lecture: skeletal muscle, chapter: excitation-contraction coupling)

Answer

The concentration difference between intracellular Ca 2+  stored in the sarcoplasmic reticulum  and sarcoplasmic [Ca 2+]   is so small (approx 1 to 1.5 times) that ATP powered Ca 2+  pumps are required to move Ca 2+  from the SR
into the cytoplasm.

Answer

This is possible because Ca 2+  is released from mitochondrial stores close to the contractile proteins.

Answer

The rate of onset of muscle contraction is dependent on the diffusion of intracellular ATP and not the diffusion of Ca 2+ .

Answer

The concentration difference between intracellular and extracellular Ca 2+  is so large (about 1 to 20,000) that the Ca 2+  diffuses much more rapidly than expected.

Answer

Membrane invaginations (t-tubular system) carry the action potential into the middle of the muscle fibre triggering an almost simultaneous release of Ca 2+  from the SR throughout the fibre.

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If you calculate the time it would take for extracellular Ca 2+ to diffuse to...

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