These signals from your brain normally control how much testosterone your testicles make and release. Any issue with your testicles, hypothalamus or pituitary gland can cause low testosterone. There are several possible causes of low testosterone. Sexual symptoms of low testosterone are the most specific. Low testosterone symptoms (low T symptoms) can vary a lot. It helps male features like a deepening voice and body hair growth develop. You can have different symptoms depending on your age. Through a process known as Translation, those blueprints are sent to a Ribosome, which is like a muscle protein-building factory that manufactures a string of amino acids based on the blueprint from the mRNA. From the mechano-sensors, a signal gets sent to a beast mode molecule called mTOR, which is a major regulator of cellular growth in general. Then there are Filaments, which bind to the famous actin proteins that slide during contraction, making them a really good candidate for sensing tension. When it comes to muscle growth, there isn’t just one pathway with one outcome, but rather many different interconnected pathways with many different downstream effects. We know that mechanical tension is the main thing driving muscle growth, but what happens next? Also, paying attention to things like the mind-muscle connection, at least on certain exercises, and eccentric control should also help, as those aspects of lifting have been shown to increase intramuscular tension. This means we need to lift with good consistent technique while using the acute training variables and progressive overload to push the level of intramuscular tension up over time. Mitochondrial Ca2+ overload induces the collapse of mitochondrial membrane potential and the mitochondrial opening of permeability transition pore (mPTP), which triggers the release of pro-apoptotic factors and leads to cell death. Mitochondrial fission separates the damaged part from the healthy mitochondrial network or occurs during cell mitosis, producing two new mitochondria to meet cell division needs (33, 37). Mitochondrial proliferation is achieved by increasing nuclear and mitochondrial-encoded proteins, mtRNA, and other components (i.e., mitochondrial biogenesis), followed by mitochondrial fission. The TOM complex recognizes and binds to the mitochondrial targeting sequence of the incoming protein, while the TIM complex facilitates the translocation of the protein across the inner mitochondrial membrane (32). Along with mitochondrial alteration, age-related decreases in sex steroid hormones in both men and women have been implicated as critical factors in the development of sarcopenia (21, 22). Hence, the maintenance of functioning muscle mass is a complex process, and malfunction of any of the above elements can lead to muscle deterioration. Hypertrophy is different from hyperplasia—an increase in cell number—which rarely occurs in human skeletal muscle. The second contact site, MAM, between the SR and mitochondria, has the functional role of synchronizing the mitochondrial energy production rate with the metabolic demand of muscle fibers. The historical knowledge that the terminal cisternae of the SR engage with the T-tubules in forming the triad, which represents the first identified membrane contact site, is now expanded by evidence that in skeletal muscle, the SR participates in the assembly of at least two more types of membrane-contact sites. Emphasize progressive overload by consistently adding weight or reps to maintain training stimulus. This intensity range provides sufficient mechanical tension to trigger myofibrillar adaptations while still allowing enough volume for growth. Applying it effectively requires specific training strategies tailored to each type of adaptation. This is why bodybuilders often look their biggest right after a high-volume workout—their muscles are literally swollen with fluid. There’s also this other path that’s triggered by amino acids in the protein we eat. From there, mTOR goes to the nucleus and tells the DNA machinery to produce a messenger RNA (mRNA) strand, which you can think of as a set of blueprints for building new muscle. "Conclusively identifying major hypertrophy stimuli and their sensors is one of the big remaining questions in exercise physiology." Testosterone levels naturally drop in men, so this is a common condition. Special cells (called Leydig cells) in your testicles make this hormone. Additionally, in this case, the molecules that anchor mitochondria to these domains of muscle fibers remain to be identified. Mitochondria, like many other organelles, in skeletal muscle, are localized with a well-defined pattern with respect to triads and sarcolemma, a pattern that differs in slow- and fast-twitch fibers. This highly precise pattern, evident in EM micrographs for about a half-century, is waiting for the identification of the proteins that tether triads to the sarcomere with such a regularly repetitive precision. Altogether, these new findings are changing the traditional view of the SR as a static organelle by depicting an unexpected dynamic ability to extend its complex structure to support muscle function. MG53 expression increases during myogenesis, and, during membrane damage repair, it is recruited at damaged membrane sites by cavin-1 where, by binding to phosphatidylserine, it acts as a scaffold to recruit additional proteins, such as Cav-3, DYSF, and annexin V, to start vesicle trafficking 49,59,60. The goal of strength training is to induce muscle hypertrophy from straining the muscles to cause damage. We propose that age-related decline in both sex hormones may trigger sarcopenia by initially impairing mitochondrial function rather than being an independent factor. We contend that mitochondrial-regulated apoptosis is central to initiating the signal for age-related skeletal muscle deterioration. Accumulating evidence reveals that 17β-estradiol and testosterone play important roles in regulating mitochondrial homeostasis in skeletal muscle. Castration leads to a decrease in mtDNA copy number in the skeletal muscle of male pigs, suggesting that testosterone is required to maintain mitochondrial copy number (146). Alternatively, MFN2 deficiency in muscle during aging reduces autophagy, which contributes to the accumulation of damaged mitochondria and triggers age-related mitochondrial dysfunction (84). In line, overexpression of Parkin attenuates aging-related loss of muscle mass and strength, along with improved mitochondrial biogenesis and enzymatic activities (95). Loss of Parkin causes a decline in muscle force in mice, as well as impaired mitochondrial respiratory function and increased sensitivity to mPTP in skeletal muscle (94). Fragmented and atypically enlarged mitochondria were often observed in aged skeletal muscle (81, 82), indicating that mitochondrial dynamics are compromised in advanced age. Once PGC-1α is activated, it powerfully upregulates the expression of several proteins (e.g., NRF1, NRF2, ERRα, ERRβ, ERRγ, TFAM) encoded by both nuclear and mitochondrial genomes, leading to an increase in mitochondrial mass (75). However, additional studies are warranted to understand the role of UPS in degrading mitochondrial membrane proteins and the importance of UPS in sarcopenia. Protein quality control in the mitochondrial intermembrane space is also critical to safeguard a proper mitochondrial function and skeletal muscle funciton.
