Question

During contraction of heart muscle cells ________. A) all of the calcium required for contraction comes from storage in the sarcoplasmic reticulum B) some calcium enters the cell from the extracellular space and triggers the release of larger amounts of calcium from intracellular stores C) the action potential is prevented from spreading from cell to cell by gap junctions D) calcium is prevented from entering cardiac fibers that have been stimulated

Answer 1

During the contraction of heart muscle cells, some extracellular calcium enters the cell and triggers the release of additional calcium from the sarcoplasmic reticulum, which is essential for cardiac muscle contraction.

Step-by-step explanation:

During contraction of heart muscle cells some calcium enters the cell from the extracellular space and triggers the release of larger amounts of calcium from intracellular stores. This process is known as the calcium-induced calcium release (CICR) mechanism, which is essential for the contraction of cardiac muscles. The initial influx of calcium through the slow calcium channels during the action potential depolarization phase, accounts for a small portion (approximately 20%) of the calcium required for contraction. The majority of the calcium used in contraction is then released from the sarcoplasmic reticulum (SR).

Unlike skeletal muscle, where almost all of the calcium comes from the SR, cardiac muscle fibers rely on both the influx of extracellular calcium and the release from the SR to initiate contraction. This mechanism allows for a longer contraction time in cardiac muscles, which is necessary for the proper functioning of the heart.

Answer 2

C) Some calcium enters the cell from extracellular space and triggers the release of larger amounts of calcium from intracellular stores

Step-by-step explanation:

The contraction of the heart shows that calcium ions are involved in the contraction of the heart.

When the action potential is generated in the Sinoatrial node, the potential travels down through the cells via the gap junctions. As the potential travels, the calcium stored in the sarcomere is released. These calcium ions along with action potential also move across the cells. So the calcium enters from the space and the some is released from the sarcomere.

Thus, the selected option is correct.