I left-off in my first blogpost on the book Essentials of Strength Training and Conditioning talking about human musculature and the skeleton. Now I want to talk about what actually causes muscles to contract. Personally, I wouldn’t worry too much about the non-bolded names… understanding the PROCESS is what’s important (at least at the simplified level at which I’m presenting it).
The signals which make a muscle move originate in a specialized nerve cell called a Motor Neuron. Each muscle is composed of many, many muscle cells which, due to their appearance (and to tradition), we call Muscle Fibers (but please keep in mind that all muscle “fibers” are really just very, very elongated cells).
Motor neurons can be promiscuously connected to several hundred muscle fibers, but each muscle fiber is monogamously dedicated to only one motor neuron. Only in those muscles requiring incredible precision (such as the eye) will there be muscle neurons completely dedicated to a single muscle cell/fiber. It makes sense that a muscle fiber would only receive instructions from one motor neuron– multiple signals received from multiple neurons would interfere with, and probably contradict, each other.
The motor neuron and its harem of muscle fibers comprise what is called a Motor Unit. When a motor neuron sends out its signal, all the hundreds of muscle fibers connected to it contract together.
When muscle fibers receive the signalling electric current from their motor neuron, a neurotransmitter is released (Acetylcholine) and collected until there’s so much that it triggers the muscle fiber to contract. This build-up happens in what I call “microscopic time”– in which the pace of activity is pretty much beyond human imagining, and a thousand-linked chain-reaction can occur in the blink of an eye.
Each muscle fiber has hundreds of myofibrils running lengthwise. Myofibrils are where the myosin and the actin components (which I’m about to talk about) of the muscle fiber are located. These filaments (the myosins and actins) are what most directly cause muscle contraction.
Myosins and Actins are arranged in alternating columns inside the myofibrils. These columns interlace, so that each myosin overlaps on either end between two actins (so if you’re picturing it, the rows of the overlapping columns would go: actin-mysosin-actin, actin-mysosin-actin… as you can see, there are twice as many actins). In a state of rest, the actins and mysosins are NOT bound to each other, and there is no tension in the muscle. It is relaxed… however…
Once a sufficient amount of Acetylcholine is built-up, calcium ions are released inside the muscle cell. This calcium ions bond to the troponin layer covering the actin, causing the actins to grab hold of the little bridges hanging from the myosin (aptly called the myosin bridges).
When the interlacing columns contract, they will now– due to formation of these cross bridges– pull each other closer, like if you interlaced the tips of your fingers and then moved your hands together so that the fingers go farther and farther between each other. The energy for the contraction is supplied by the hydrolysis (breakdown via water) of Adenosine Triphosphate (ATP) into Adenosine Diphosphate (ADP). The enzyme ATPase facilitates this process.
The more cross-bridges formed between actins and myosins, the greater the force of the contraction. Our authors inform us that “the amount of force produced by a muscle at any time is directly related to the number of myosin cross-bridge heads bound to actin filaments cross-sectionally at that instant.”
Once the muscle fiber receives no more stimulation by its motor nerve boss, the calcium ions are pumped out, and the actins and myosins disassociate (“recock” to put it in action terms). To be ready to flex again however, the actin and myosin filaments must not only de-couple, but the ADP must be replaced by another ATP molecule for fuel. Also, calcium ions must continue to be available to bind to the troponin of the actin so that the actin-myosin cross-bridges can be formed again.
As with so much in Nature, when it comes to human movement, much of the action, fundamentally speaking, comes down to a binary choice– yes or no, on or off… or in the case of muscles: to contract or not to contract.
The muscle fiber contraction is all or none– less nerve-signal stimulation does NOT produce less contraction. Until the electrical impulses incoming from the motor neuron produce enough of the neurotransmitter Acetylcholine to cross the activation threshold, there is simply ZERO instruction given to the muscle… in other words, the muscle remains relaxed. Also important to keep in mind… a motor neuron either stimulates ALL of its harem of muscle fibers or none– there’s never a situation in which a motor neuron would only stimulate SOME of the muscle fibers under its direction.
That said… It is entirely possible to COMPOUND signals from the motor neuron. This occurs when the signals which the neuron are sending come so fast and furiously that the muscle fiber contractions (or “twitches“) “merge and eventually completely fuse,” creating a maximum force event which is known as “tetanus.” Obviously, this is not something you want to go on interminably, so make sure to get your tetanus shot and try not step on any rusty nails.
As I previously stated, muscle fibers are actually just really, really long cells. Each stretched-out muscle cell is so long that it contains more than one nucleus– many nuclei, in fact. The nuclei divvy-up the labor of running the lengthy cell, with each nucleus possessing a domain inside of which it’s the boss. Personally, I can’t help but feel that muscle fibers were once upon a time single cells that eventually got re-coded in evolutionary history to merge together.
Like all animal cells, each muscle cell/fiber is filled with cytoplasm. For whatever reason, the cytoplasm of muscle cells is called sarcoplasm.
Muscle fibers can be very different from each other, and there are several equally logical ways we could categorize them. One of the most intriguing ways is the division of muscle fibers into Slow Twitch and Fast Twitch.
Slow Twitch muscle cells are fatigue resistant and have a high capacity for aerobic activity.
Fast Twitch muscle fibers have low aerobic power and fatigue easily. On the other hand, Fast twitchers possess a lot of anaerobic power and can produce rapid force. Also, fast twitch fibers appear to increase in size more readily than slow twitchers. As our authors point out, “athletes who genetically possess a relatively large proportion of fast-twitch fibers may have a greater potential for increasing muscle mass.”
A muscle fiber (which is single multi-nucleated cell, remember), is about the diameter of a human hair, and it can sometimes run the entire length of a muscle. These fibers are grouped in bundles called fasciculi.
Lastly –and this may prove especially interesting to weightlifters who are aware of the importance of the “recruitment” of motor units (muscle neuron-and-fibers groups, you’ll recall) during exercise (I’ll talk about this in a future post)– there are sensory receptors in muscles, tendons, and joints called proprioceptors which are sensitive to pressure and tension and proved the central nervous system with information concerning the position of the parts of the body in gravitized space.
How this relates to weightlifting is that specialized proprioceptors called muscle spindles run parallel to the other muscle fibers. Since they lengthen as the muscle lengthens, they can send information to the spinal cord about the rate of change of muscle length (rate of contraction). If the muscle fibers are contracting a lot, that’s a good sign the muscle is having trouble moving an object and might need some help from additional motor units. Thus, our authors tell us, spindles “indicate the degree to which the muscle must be activated in order to overcome a given resistance.”
And this last bit I just found interesting… Muscle spindles are the source of the “knee-jerk” reaction. Tapping the knee tendon causes the muscle spindles to stretch, which sends the signal to the nervous system to contract the muscle back into place… the contracting muscle pulls on the tendon which pulls on the bone and– there you have it– movement. I’m always blown away by just how FAST all this stuff is happening inside our bodies.