We know that muscle fibers are composed of myofibrils, and each one of those is a series of small contractile units called sarcomeres. Within each sarcomere, there are contractile proteins (actin and myosin) which interact to shorten the sarcomere and therefore contract the muscle. My questions are:
+ Is there a certain number of myofibrils in each muscle fiber ? Could their number increase or decrease in response to training or detraining?
+ Is there a certain number of contractile proteins in each sarcomere ? Could their number increase or decrease in response to training or detraining?
The general rule of thought is that muscles hypertrophy/atrophy by increasing/decreasing fiber diameter and not by changing fiber number. However, there is some evidence for fiber number changing, mainly under extreme loading/unloading conditions. Fiber diameter increases/decreases by adding/degrading myofibrils in parallel to the existing ones. Regarding your last question, I would think that the amount of contractile protein per sarcomere would stay constant and not change with training/detraining. I am somewhat guessing on this because I have never heard of sarcomeres or myofibrils changing in size.
In regards to your last question, as far as I know the number of myosin molecules in a thick filament of humans should remain constant unless there is a pathological condition, in which case I would expect to see a reduction of the number of myosin heads. There is a hypothesis that says that the number of myosin in a certain thick filament is defined by the number of super-repeats in titin´s A-Band. This area is highly conserved even among different species. Not sure if it has been proved, though.
Myofibrils are added in parallel during muscle fiber hypertrophy, such as during strength training, as Gordon said. I also agree with him that myofibril diameter doesn't change with training (at least not that I've ever heard); any apparent claims in the literature to the contrary seem to be typographical errors - people typing "myofibril" instead of "myofiber."
The number of proteins per sarcomere do not change as a function of training/fiber type.
To my knowledge and measurements, one myofibril has a diameter of about 1 micrometer. A hypertrophied muscle means more myofibrils within the striated muscle cell (i.e., single muscle fiber for skeletal muscle, and a cardiomyocyte for the cardiac muscle). In non-pathologic situation of the striated muscle, there should not be an overexpression of sarcomeric proteins.
The number of sarcolemmas is determined genetically. In Homo Sapiens, only hypertrophy is observed (an increase in the diameter of contractile proteins), an increase in the number of sarcolemmas (hyperplasia) is observed only in some small mammals.
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