Muscle Activation and Muscle Fiber Types

Chapter 1, Part 2/4: The Neuromuscular System (Part 1/2)

Introduction/Overview

Now that we’ve covered the muscular system, we will carry on into the neuromuscular system. Simply put, the neuromuscular system is the relationship of nerve cells (or motor neurons) and the muscle(s) they are connected to, as well as how they work together. Motor neurons are responsible for transmitting the electric current to the muscle, and are connected to the muscles via the motor end plate (aka the “neuromuscular junction”).
If you look at the picture below (retrieved from exerbotics.com), the neuromuscular junction is simply the finger-looking things latched on to the muscle fibers.

Each muscle has only one neuromuscular junction, however single nerve cells may simultaneously touch multiple (or even several hundred!) muscle fibers at once. The motor neuron and all the muscle fibers it touches are collectively called a motor unit, as you can see in the above picture. When stimulated by the motor neuron, all the fibers in a motor unit contract as one.

Activation of Muscles

The amount of control one has over his or her muscles depends on the number of muscle fibers within each motor unit. If you look at the image above, you can see that the motor neuron is visibly touching 4 different muscle fibers, for example. For muscles needing more precise control, like those in the eye, each motor neuron may only be assigned one single muscle fiber. On the other hand, for larger muscles needing less precision, such as the quadriceps, one single motor neuron may be in contact with hundreds of muscle fibers at once.

The action potential (or electric current) transmitted from the motor neuron through the neuromuscular junction and into the nerve cell is not, by itself, capable of exciting muscle fibers; chemical transmission does that. When the action potential passes through the neuromuscular junction, the neurotransmitter acetylcholine is released, exciting the sarcolemma, the muscle fiber’s surrounding membrane. Once adequate acetylcholine has been released, an action potential takes place across the sarcolemma, and the fiber will contract.

Every muscle follows what is called the all-or-none principle, which states that once the action potential takes place, every fiber within that muscle unit contracts with everything it has. A proper analogy brought up in the text is that of a gun: once the pressure you exert on the trigger is great enough to the fire the gun, it fires full speed ahead. Squeezing the trigger harder won’t affect the speed of the bullet, nor is there any such thing as a partial firing of the gun—the same is true for the muscle fiber.

Every action potential transmitted generates a twitch within the muscle fiber. In order to produce more force, multiple successive twitches need to be applied rapidly enough that their individual forces will summate to a greater concurrent force. Seeing as a twitch is a muscle fiber generating an increasing amount of force then decreasing to a relaxed state, greater force will be applied when successive twitches are closer together. If twitches are so close together that they merge or even fuse, a condition called tetanus exists, which is the maximal amount of force a motor unit can produce.

Muscle Fiber Types

Skeletal muscles are made up of fibers of differing characteristics. The most fundamental way to sort through these fibers (and the motor unit they’re a part of) is by classifying them according to their twitch time with the terms slow-twitch and fast-twitch. As the names suggest, a slow-twitch muscle fiber is one that develops force and then relaxes slowly, whereas a fast-twitch fiber develops force and relaxes rapidly. Histochemical staining is another technique used to identify fiber types, however staining identifies not just 2, but 3 (and more) different types: Type I (slow-twitch), Type IIa (fast-twitch), and Type IIb (fast-twitch).

[[Because research using gel electrophoresis has indicated that Type IIb muscle fibers in humans actually contain an isoform more closely resembling the myosin heavy chain IIx (MHC IIx), Type IIb muscle fibers are more often (are forevermore in this text) called Type IIx muscle fibers.]]

The difference in contraction speed between Type I and II muscle fibers is primarily due to the differing forms of myosin ATPase present within the fiber. If you remember from yesterday’s post, ATPase provides the energy needed to drive a contraction within a muscle cell. Type I fibers have a slower form of this ATPase, meaning the myosin cross-bridges cycle more slowly, while Type II fibers have a faster form, allowing them to contract more quickly. Type II fibers also have a more developed sarcoplasmic reticulum, allowing for a quicker and more efficient calcium delivery system and thus a greater speed of contraction. On average, Type II muscle fibers contract five to six times faster than Type I with three to five times greater force production (from Physiology of Sport and Exercise, Fifth Edition). It’s because of this that, all else equal, someone with a greater percentage of Type II fibers can sprint faster than someone with a greater percentage of Type I fibers.

So what do these differences look like in the real world? Type I muscle fibers are more fatigue resistant, more efficient, and have a high capacity for aerobic work. In real words this means you can use these muscles for a long period of time, such as when you’re on an easy jog or a walk, and they won’t give out on you. On the flip side, Type II muscle fibers are inefficient, fatigable, and less aerobically powerful, but they can develop force rapidly and like no other. Type II fibers are most often used in movements requiring only seconds (or less) of power, such as a 100m sprint or a power clean. In comparing Type IIa fibers to Type IIx, the main notable difference is that Type IIx are comparatively more fatigable yet more powerful, while Type IIa are less fatigable and less powerful. Remember, though, that both Type II types are more fatigable and more powerful than all Type I fibers. 

To sum things up, motor neuron neuromuscular junction motor unit acetylcholine all-or-none fast-twitch slow-twitch twitch. Yep. 

But for real, super cool stuff regarding motor units and how and when different muscle types are used. I planned to add more today, but this is just a lot. A lot of excellent material. I'll finish the neuromuscular system up either tonight or tomorrow, so hang on tight!

Today, as always, I used my textbook "Essentials of Strength Training and Conditioning" (Third Edition) to aid me in my writing. Most all of the useful information came from this text, while the jargon-less chit-chat was done by me. I also used a text called "Physiology of Sport and Exercise" (Fifth Edition) by Kenney, Wilmore, and Costill to gain more information on muscle fiber types. Thanks and gig'em.