Now factors that promote protein synthesis such as IGF-1, growth hormone (GH), testosterone and some prostaglandins can commence the growth process. Protein synthesis occurs because a genetically-coded subtsance called 'messenger RNA' (mRNA) is sent out from the nucleus to the ribosomes. The nucleus is believed to release increased mRNA in response to tension and/or myofibrillar damage done as a result of insufficient cycling of actin-myosin cross-bridges during intense muscular contractions, though this mechanism is not fully understood. The mRNA contains the 'instructions' for the ribosomes to synthesize proteins, and so the process of constructing contractile (actin and myosin) and structural proteins (for the other components of the cell) from the amino acids taken into the cell from the bloodstream is set off. Several substances can influence this process. A short overview of the major ones are found below:
In addition, there are other intracellular bodies whose growth and/or proliferation would fall under the category of sarcoplasmic hypertrophy. These would be organelles such as the ribosomes, which are involved in protein synthesis. As in the case of neural drive machinery, in most cases they would increase in size or number only to support sarcomere hypertrophy. They would have little impact on overall muscle size.
Looking to gain muscle and recover faster after working out
It was mentioned in the The Neuromuscular System Part I: What A Weight Trainer Needs To Know About Muscle article that muscle cells have many nuclei and other intracellular organelles. This is because nuclei are intimately involved in the protein synthesis process (don't forget, actin and myosin are proteins), and a single nuclei can only support the manufacturing of a limited amount of protein. If muscle cells didn't have multiple nuclei they would be very small muscle cells indeed. So if a muscle is to grow beyond its current size (i.e. synthesize contractile proteins - actin and myosin) it has to increase the number of nuclei that it contains (called the 'myonuclei number'). How does it do this?