Steps in excitationcontraction coupling in smooth muscle

- The mechanism of excitation-contraction coupling is different from that in skeletal muscle.

- There is no troponin; instead, Ca2+ regulates myosin on the thick filaments.

1. Depolarization of the cell membrane opens voltage-gated Ca2t channels and Ca2+ flows into the cell down its electrochemical gradient, increasing the intracellular [Ca2*].

2. The Ca2+ that enters the cell may cause the release of additional Ca2+ from the SR through Ca2+-gated Ca2+ channels. Hormones and neurotransmitters also directly release Ca2+ from the SR through inositol 1,4,5-triphosphate (IP3)-gated Ca2+ channels.

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