I’m not sure but I’m guessing the answer would be the first one !
The link where the action
potential of the nerve that meets muscle and causes it to contraction is where excitation-contraction coupling occurs. The T_tubules the
invaginate into the sarcolemma of the muscle cells are the ones that carry the excitation into the muscles. Its depolarization
causes the lateral sacs of the sarcoplasmic reticulum to release ca2+
ions. The ca2+ then bind into their site on troponin and causes the tropomyosin to shift and allow actin to expose
the binding site in which myosin head
will bind and form a cross bridge that is
important in the contraction of the muscle.
I would say nucleus.
Hope this helps !
Photon
