Final answer:
Elevated levels of potassium ions in the extracellular fluid decrease the likelihood of skeletal muscle contraction. Calcium ion concentration, ATP availability, and the presence of acetylcholine at the neuromuscular junction are necessary for muscle contraction.
Step-by-step explanation:
Elevated levels of potassium ions in the extracellular fluid are likely to decrease the likelihood of skeletal muscle contraction. Hyperkalemia, an elevated potassium blood level, can impair the function of skeletal muscles, including their ability to repolarize. This can result in a partial depolarization of the plasma membrane of skeletal muscle fibers, inhibiting their ability to contract.
Increased calcium ion concentration in the muscle cell is necessary for skeletal muscle contraction. Calcium ions interact with shielding proteins, allowing actin-binding sites to be available for attachment by myosin heads, initiating muscle contraction.
Adequate ATP availability is also crucial for skeletal muscle contraction. ATP is required for myosin heads to form cross-bridges with actin filaments and produce muscle tension.
The presence of acetylcholine at the neuromuscular junction is essential for initiating muscle contraction. Acetylcholine binds to receptors on muscle cells, triggering a depolarization of the muscle fiber membrane and the release of calcium ions.