Skeletal muscle fibers are divided into two main types: slow-twitch, which are also called slow oxidative fibers, and fast-twitch muscle fibers.
Fast-twitch muscle fibers are further subdivided into fast oxidative and fast glycolytic fibers. This classification is based on the speed of contraction and the metabolic pathway that’s used to make ATP, a molecule that stores energy for muscle contraction.
Most muscles possess a mix of slow-twitch and fast-twitch fibers, but the predominant one determines the primary function of the muscle.
Alright, now let’s take a look at a muscle cell, or myocyte - and specifically it’s sarcoplasm, which is the cytoplasm of a muscle cell.
The sarcoplasm is filled with stacks of long filaments called myofibrils.
Each myofibril has thick myosin and thin actin filaments that don’t extend through the entire length of the muscle fiber, instead they’re arranged into shorter segments called sarcomeres.
The myosin filaments have these small club-like extensions, which are called myosin heads.
The thin actin filament look like a pearl necklace that’s gently twisted. Each pearl represents one G-actin protein, which has an active site where the myosin head binds to during contraction.
Now, before myosin can bind actin, myosin needs to power up. Part of the myosin head is an ATPase, meaning that it can cleave an ATP molecule to ADP and phosphate ion and release some energy. The energy is used to cock the myosin head backwards, into its high energy position.
Next, the myosin head binds to the active site, and this triggers the release of the stored energy in the myosin head. When that happens, the myosin head launches pulling the thin filament along with it. This is called the power stroke.
The combined power strokes of all the myosin heads lead to sliding of the thin filament along the thick filament, and this results in the contraction of the skeletal muscle fiber.
Now, the speed of contraction depends on how quickly the ATPase enzyme cleaves a molecule of ATP. And there are actually two forms of this enzyme; slow twitch fibers have an ATPase that hydrolyzes ATP slowly, and fast twitch fibers have an ATPase that hydrolyzes ATP quickly.
Since myosin heads need ATP to reset, a lot of ATP is used by muscle fibers, and the main source for ATP is glucose.
Normally, some skeletal muscle cells take up glucose and store it as glycogen - using a process called glycogenesis. That way, when these muscles need energy, they can break down the glycogen to form glucose again - using a process called glycogenolysis.
To make ATP, glucose is put through glycolysis which is a set of biochemical reactions that take place in the sarcoplasm and results in 2 molecules of pyruvate and 2 ATP.
If oxygen and mitochondria are available, pyruvate is put through a process called aerobic respiration which is where pyruvate is converted into acetyl-COA which enters the mitochondria to produce NADH via the citric acid cycle.
NADH is then used to drive oxidative phosphorylation within the mitochondria, and the result is that each glucose molecule yields a total of 38 ATP.
In the absence of oxygen or mitochondria, there’s only anaerobic respiration, or anaerobic glycolysis, which is where glucose is broken down into pyruvate and that’s about it.
It’s quick, but we only get 2 ATP per glucose and the excess pyruvate gets converted into lactic acid, which goes into the blood, and then to the liver.
The liver can recycle lactic acid into pyruvate and then pyruvate into glucose by using 6 ATP.
The new glucose can then be sent to the muscles or other organs.
Now we have three different types of muscle fibers, slow oxidative, fast oxidative, and fast glycolytic twitch fibers. Let’s start with slow oxidative fibers, which are also called type I or slow-twitch muscle fibers.
In slow oxidative fibers, “slow” is for having the ATPase that hydrolyze ATP slowly, while “oxidative” stands for the aerobic respiration pathway for metabolizing glucose.
These slow twitch muscle fibers are relatively small, and produce the weakest contractions because they have fewer sarcomeres. But they’re well-supplied with blood vessels that bring them a plentiful supply of oxygen.
And within their sarcoplasm, there’s a high concentration of a red pigmented protein called myoglobin, which binds and stores oxygen.
Myoglobin is similar to hemoglobin, which is the oxygen-carrying protein in the blood, except that myoglobin binds oxygen even more strongly than hemoglobin.
