USA Biology Olympiad (USABO) Practice Exam

In cardiac muscle physiology, through which structures can calcium ions transfer between adjacent cells?

1. Desmosomes

2. Tight junctions

3. Gap junctions

4. Hemidesmosomes

The correct answer is: Gap junctions

Calcium ions can transfer between adjacent cardiac muscle cells primarily through gap junctions. These specialized structures form intercellular channels that allow for the direct passage of ions and small molecules, facilitating communication between neighboring cells. This is critical in cardiac muscle physiology, as the coordinated contraction of cardiomyocytes relies on the efficient transfer of calcium ions, which play a key role in initiating muscle contraction. Gap junctions consist of connexin proteins that create channels, enabling the rapid spread of electrical impulses and calcium signals throughout the myocardial tissue. This ensures that the heart contracts effectively and synchronously, which is essential for its pumping action. The presence of gap junctions allows for the propagation of action potentials and is fundamental in maintaining rhythmic heartbeats. In contrast, desmosomes provide structural integrity and mechanical adhesion between cells but do not allow for the transfer of ions. Tight junctions are involved in maintaining the barrier function and regulating permeability between cells, while hemidesmosomes connect cells to the extracellular matrix and are not involved in intercellular communication. Thus, gap junctions are the correct answer regarding the transfer of calcium ions between adjacent cardiac muscle cells.