How Long Does it Take to Earn a PhD? A PhD usually requires at least five to six years and many people take seven to nine years. Masters vs PhD: Which One Costs More? The old adage time is money was never so true as with graduate school; as a PhD takes longer to complete, it also requires more money.
Once the active site is exposed, the myosin head (which has a strong affinity for the active site) binds and the cross bridge cycle begins. Sources of Calcium in smooth muscle cells Unlike skeletal muscle, smooth muscle is dependent on two sources of calcium in order to initiate contraction.
The role ATP plays in muscle contraction is that ATP binds to sites  on myosin heads, inducing a conformational change in the actin  binding site and reducing the affinity f â¦ or the actin substrate.es, calcium is necessary for the release of neurotransmitters that cause a muscle to initiate a contraction. Calcium is also needed to bind to the troponin-tropomyosin comple â¦ x causing it to change position so the myosin head can attach to the actin molecule which results in contraction. 2 people found this useful.