Skeletal muscles contract when their (A)__________ shorten, using the energy source ATP. A rise in the concentration of (B)__________ ions causes muscle contraction.
A fibers
B Calcium?
To answer this question, we need to understand the process of skeletal muscle contraction.
(A) In skeletal muscles, contraction occurs when the muscle fibers shorten. The basic unit of contraction in a skeletal muscle is called a sarcomere. Sarcomeres contain two types of protein filaments - thick filaments made of myosin and thin filaments made of actin. During muscle contraction, the thick and thin filaments slide past each other, causing the sarcomere to shorten, and ultimately resulting in muscle contraction.
(B) The rise in the concentration of calcium (Ca2+) ions causes muscle contraction. Normally, skeletal muscles are in a relaxed state, and the concentration of calcium ions within the muscle fibers is relatively low. However, when a nerve signal reaches the muscle fibers, it triggers the release of calcium ions from the sarcoplasmic reticulum (a specialized structure within muscle cells). The calcium ions then bind to specific sites on the thin filaments, which allows the thick and thin filaments to interact with each other. This interaction leads to the sliding of the filaments, sarcomere shortening, and muscle contraction.
In summary, skeletal muscles contract when their sarcomeres shorten, and this is facilitated by the rise in the concentration of calcium ions within the muscle fibers. The energy source ATP is required for the actual contraction process to occur, but it does not directly cause the contraction itself.