Final answer:
The upstroke of the SA nodal action potential is due to the opening of voltage-gated calcium channels, making answer b (Primarily permeable to Ca²⁺) the correct option.
Step-by-step explanation:
The upstroke of the SA nodal action potential is characterized by a rapid influx of calcium ions (Ca²⁺). In the Sinoatrial (SA) node of the heart, this process is initiated by a slow pre-potential depolarization caused by a gradual influx of sodium ions (Na⁺). As the threshold is reached, voltage-gated calcium channels open and Ca²⁺ rushes into the cell, causing a further rapid depolarization of the membrane.
Afterward, these channels close and repolarization occurs when potassium channels open, allowing potassium (K⁺) ions to leave the cell, bringing the membrane potential back towards its initial state. The correct answer to the student's question is b. Primarily permeable to Ca²⁺, thus indicating that the upstroke of the SA nodal action potential is due to the opening of channels that are primarily permeable to calcium ions.
The upstroke of the SA nodal action potential is primarily produced by opening a channel that is permeable to Na⁺. When a stimulus reaches the SA node, voltage-gated sodium ion channels open, allowing an influx of sodium ions into the cell. This results in depolarization of the membrane and initiates the action potential. After the depolarization phase, potassium ion channels open, allowing potassium ions to flow out of the cell and repolarize the membrane.