The Muscle Growth Process
Muscle fibers have lots of nuclei and other type of organelles and intracellular matter. The nuclei are the places where protein synthesis occurs. Just one nucleus has a limited potential for the production of new protein. If muscle fibers didn’t have multiple nuclei in them, they’d be very tiny cells. That’s why in order for them to grow beyond the current size, they need to increase the number of nuclei which are present in the area.
This is called a myonuclei number. There are myogenic stem cells or ‘satellite cells’ surrounding the muscle cells. These satellite cells are also known as myoblasts. Under the right circumstances, they transform into a certain type of muscle cell and give their nuclei to the newly formed muscle fibers, thus increasing the total number of myonuclei.
The following 3 things need to occur for the above process to take place:
1. The satellite cell number must increase; a process known as ‘proliferation’.
2. They need to transform into muscle cells; a process known as ‘differentiation’.
3. They need to fuse together with “needy” muscle cells.
When the wall of the muscle cell, known as ‘sarcolemma’ gets damaged from tension induced by training, several growth factors are created and released inside the muscle cells. There are several types of these, some of which include:
• Insulin-like Growth Factor 1 (IGF-1)
• Fibroblast Growth Factor (FGF)
• Transforming Growth Factor – Beta Superfamily (TGF-beta)
These there factors then exit the muscle cell and find their way out to the area nearby since the permeability of the sarcolemma has increased because of the damage incurred during the contractions. When they leave the muscle cells FGF and IFG-1 stimulate the satellite cells to start proliferating and differentiating, while the TGF-beta is in charge of mediating, in this case, inhibiting growth.
Then, the satellite cells fuse together with the muscle cells and give their nuclei to the muscle cells giving the latter a greater capacity to grow. Afterward, the factors in charge of promoting protein syntheses like the growth hormone (GH), IGF-1, the hormone testosterone and certain prostaglandins help start the muscle growth process. Protein synthesis occurs due to the existence of an essential compound which bears a certain code in the form of genes, also known as ‘messenger RNA’ which is sent from the nucleus to another cell organelle, the ribosome.
It is believed that the nucleus releases increased amounts of mRNA as a response to muscle tension and myofibrillar damage, which happens because of insufficient cycling of the actin/myosin cross-bridges which happen during very intense muscle contractions, even though the exact mechanism is not yet understood in detail. The mRNA carries the instructions to the ribosome as to how it needs to synthesize new proteins, by which the process of creating structural and contractile proteins from strands of amino acids taken inside the cells from the blood, is started.
In the 3rd part, we will discuss compounds that impact this process: GH, IGF-1, testosterone, and prostaglandins.