Calcium is needed for contraction in both skeletal and cardiac muscle cells, however, there are some differences in where it comes from and what it does. Which of these choices correctly describes how calcium works in cardiac muscle cells?
a) Calcium binds to troponin, causing tropomyosin to expose active sites on actin filaments.
b) Calcium is released from the sarcoplasmic reticulum and binds to troponin, causing tropomyosin to expose active sites on actin filaments.
c) Calcium is transported into the cell through voltage-gated channels and directly binds to myosin, initiating muscle contraction.
d) Calcium is released from the sarcoplasmic reticulum and directly binds to myosin, initiating muscle contraction.
