What additional example of primary active transport is mentioned in relation to muscles?

The correct answer is the Ca++ pumps, which are an important example of primary active transport, especially in muscle tissue. This transport mechanism is crucial because it regulates the concentration of calcium ions (Ca++) within muscle cells. In the context of muscle contraction, an increase in intracellular calcium levels triggers contraction, while removal of calcium from the cytoplasm is necessary for muscle relaxation.

The Ca++ pumps, located in the sarcoplasmic reticulum of muscle cells, actively transport calcium ions against their concentration gradient, which requires energy in the form of ATP. This regulation is vital for muscle function, as precise control of calcium levels directly influences the ability of the muscle to contract and relax properly.

Other options, such as sodium channels or potassium channels, primarily facilitate facilitated diffusion rather than active transport, as they allow ions to move down their concentration gradients without the expenditure of energy. Chloride pumps, while also involved in transport mechanisms, do not play as direct a role in muscle contraction and relaxation compared to the critical function of Ca++ pumps in this context. Therefore, the Ca++ pumps are highlighted as a primary active transport example specific to the functioning of muscles.