Every muscle fiber consists of many
fibrils (myofibrils) that contain special
contractile proteins, actin and myosin,
in their cytoplasm. When Calcium ions
(Ca2+) are present in the cytoplasm, actin
and myosin cause a shortening of the
whole fibril. Calcium ions comes from a
special small organ in the cell called the
endoplasmatic reticulum. In muscle cells,
this organ is called the sarcoplasmatic
reticulum. The arcoplasmatic reticulum
releases Calcium ions when activated by
electrical impulse. After contraction, the
Ca2+-ions are pumped back into the
sarcoplasmatic reticulum which are waiting
for another electrical impulse. Then, the
nerves conduct electrical impulses to the
muscles.... or more precisely to the
myofibrils’ membrane (sarcolema). This
membrane is connected with the
sarcoplasmatic reticulum so when the nerve
carries an electrical impulse, the impulse
transfers to the sarcoplasmatic reticulum
and causes Ca2+-release and contraction.
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