This article discusses four different types of muscle fibers, including aerobic slow twitch, anaerobic fast-twitch and, for the purpose of gaining additional insight provided by comparative study, the aerobic cardiac muscle fiber and the aerobic fast-twitch fiber. However, only the aerobic slow-twitch fiber and the anaerobic fast-twitch fiber are found in primate (including human) skeletal muscle.
"Aerobic" means "with oxygen", and in discussing body metabolism, it also means "with mitochondria". The mitochondrial structure acts as a substrate to bring reactants together and to catalyze reactions; it helps control and neutralize the radicals which occur in oxidative reactions; and the mitochondrial structure also produces enzymes which further catalyze reactions so they occur at body temperature.
Three of the four types of muscle fibers are aerobic. In each of these three types of aerobic fiber, the mitochondria are different from those in other two types.
1. The largest mitochondria are found in the aerobic cardiac muscle fibers; they are 3 times the size of those found in the aerobic slow-twitch fibers (and are therefore 9 times the size of mitochondria in aerobic fast-twitch fibers discussed below). Cardiac mitochondria have the added capability of oxidizing lactic acid back into pyruvic acid and pyruvate back into glucose. (The only other organ which contains these largest mitochondria is the liver.). These mitochondria are purplish in color; the presence of large numbers of these mitochondria gives the heart and liver tissues their purplish color.
2. The intermediately-sized oxygenative mitochondria in the small, aerobic slow-twitch fibers are typically referred as a 'powerhouse' of the cell. In photomicrographs of stained aerobic and anaerobic muscle cells abutted against each other, but with an oxygen-containing capillary in the corridor between them, the mitochondria in the slow-twitch fibers are seen bunched like moths around a flame near the cell wall abuting the capillary, while the anaerobic fiber shows no such activity. These intermediately-sized mitochondria are brownish red in color.
3. The aerobic fast-twitch fiber is not found in primates and is really no longer a muscle, but is instead a bag full of tiny mitochondria with a few contractile fibers remaining. The little mitochondria in this fiber are 1/3rd the size of those in the aerobic slow-twitch fiber, and are not able to oxidise fatty acids or ketones as can the larger mitochondria, but can oxidize only the components of glucose. These smallest mitochondra appear bright red in color like the myogloben which accompanies them.
The anaerobic fast-twitch muscle fiber contains mitochondrial fragments that produce the enzymes needed to reduce glucose to pyruvate and pyruvate to lactate.
Comparison of aerobic and anaerobic fibers might lead to calling the aerobic 'ectomorphic' and the anaerobic 'endomorphic'. This is because, as stated above, the mitochondria in the aerobic slow-twitch fibers are at the cell's periphery, but in the fast-twitch anaerobic fiber, organells and structures including mitochondria are spread throughout its interior.
The mitochondria in the aerobic slow-twitch fibers are naturally at the periphery, because the oxygen they need can only come from outside the cell. The fatty acid stores are also placed near the mitochondria, because that is where they will be metabolized. The myoglobin needs to be near the periphery and the mitochondria. Myoglobin has the same red color as hemoglobin and results in these aerobic fibers being referred to as red muscle fibers. The anaerobic fibers have no need for myoglobin since they have no aerobic mitochondria and as such are referred to as pale muscle fiber.
Aerobic fibers use large adenosine molecules as energy transporters, with adenosine monophosphate (AMP) moving out to the mitochondria to be recharged to ATP, then lumbering back to the interior to activate calcium ion release. The mitochondria of the aerobic fibers must also serve the oxidative needs of the anaerobic fibers next door, and are therefore busy oxidizing pyruvate carried in by the fast-twitch fiber's creatine, as well as its own fatty acids. (This is why the mix of the two different fibers cannot vary much beyond 50/50 numerically, even though anaerobic fibers are three to six times larger than aerobic fibers and so the final mix is almost 3 to 1 volume-wise).
A fat molecule produces almost eight times the energy of a pyruvate molecule, but the reductive mitochondria in the fast-twitch fibers can metabolize pyruvate nine times faster than fat. In the anaerobic fast-twitch fibers, the large adenosine molecules are locked into the matrix of the sarcoplasmic reticular cisterns in the fiber's wall, next to the calcium ion mechanisms that must be activated by ATP. Likewise, the glycogeneral stores are located next to the adenosine, which the process of glycolysis must recharge.
The fast-twitch fibers' mitochondrial fragments (which produce the enzymes to catalyze reactions) are also located here. Energy transport is handled by small, fast creatine molecules, which can readily pass through the membranes of both its own fast-twitch fiber and of the slow-twitch fiber next door to reach the mitochondria in the aerobic fibers. Just as these aerobic mitochondria will selectively metabolize pyruvate ahead of fat, they will also phosphorylate creatine and glucose molecules ahead of AMP.
When a runner reaches a speed of about eight and a half miles per hour, the respiratory quotient rises to one, which indicates no fat metabolism is happening. Speeds above 8.5 mph are produced only by the anaerobic fast-twitch fibers, which can contract three times faster than slow-twitch fibers (25 milliseconds versus 75 milliseconds). The fast-twitch fibers can produce a speed in excess of 25 miles per hour, which is attained in the 100 and 200-meter dashes.
It should also be clear that non-primate animals that don't have a 50/50 mix of aerobic slow-twitch and anaerobic fast-twitch fibers are in need of aerobic fast-twitch fibers (essentially bags of mitochondria) capable of oxidizing larege amounts of pyruvate from the anaerobic fibers.
The flight muscles of a bird are of necessity mostly all fast-twitch fibers. A photomicrograph shows that out of a sample of 30 fibers, 18 are anaerobic fast-twitch, with the anaerobic fibers being five to nine times larger than the aerobic fibers. If you have ever cut raw chicken or turkey breast, you will probably have noticed the tiny bright red dots located throughout the pale fibers indicating these clumps of aerobic fast-twitch fibers.
The reverse situation exists in the soleus muscle of the cat, in that it is made up entirely of aerobic slow-twitch fibers. This allows the cat to move with incredibly smooth slow motion when in stealth mode. To provide the quick leap when pounce mode comes, the gastrocnemius is mostly fast-twitch fibers. A sample of 30 cat gastrocnemius fibers reveals seven aerobic slow-twitch fibers, 17 anaerobic fast-twitch fibers, and six aerobic fast-twitch fibers.
This heritage shows up in the human soleus being weighted slightly towards slow-twitch fibers and the human gastrocnemius being weighted slightly toward the fast-twitch, but still close to a 50/50 mix.
Glycolysis provides anaerobic energy by splitting glucose into pyruvate and hydrogen ions. These cannot be oxidized until they reach the mitochondria in the aerobic fibers. The concentration in the anaerobic fibers will rise until the hydrogen free radicals threaten to shut down the process, at which time enzymes trigger the combination of hydrogen with pyruvate to form lactate, which will level off at a concentration high enough to cause a gradient sufficient to drive the lactate into the bloodstream as fast as it is being produced.
This usually produces a 10:1 ratio in favor of lactate to pyruvate. Extremely fast activity can drive the lactate concentration high enough to shut down the process. The 10:1 ratio of lactate to pyruvate is a consequence of the slow clearing of venous blood from the fascicular arrangement of muscle fibers. The actual conversion ratio is one lactate molecule for each pyruvate molecule. The pyruvate travels across to the slow-twitch fiber to be oxidized to carbon dioxide and water. The carbon dioxide and water then become more concentrated, like the lactate waiting to be cleared from the cell.
Venous waste pickup is as important as arterial supply for muscle operation. If the venous drainage is choked down by hypertonic muscles undermining the rhythmic pumping, the arterial blood flow will divert through the shunts so that both supply and pickup will be compromised. The energy contained in the lactate is temporarily lost to the muscle cells when it is dumped into the bloodstream, but upon reaching the liver, four-fifths of the lactate is reconverted back to glucose and returned to the muscles.
When glucose enters the muscle cell, it is phosphorylated by the mitochondrial energy so that the glucose phosphate supplements the creatine phosphate in carrying anaerobic phosphate energy within the cell.
After a period of maximum exercise has depleted the oxygen and anaerobic energy stores of the muscles, only three minutes and two and a half liters of oxygen are required to recharge the creatine to creatine phosphate and AMP to ATP, and to reload the myoglobin with oxygen. However, it takes one hour and eight liters of oxygen for the liver to clear the accumulated lactate.
A Note on Muscle Metabolism
When considering the reactions involved in muscle contraction, oxygen, hydrogen, and electron transfers result in either freeing or capturing bond energy. Bond energy is the ‘money’ that drives the muscle machine. So, when studying the reactions, it pays to follow the money.
A reduction reaction may involve a decrease in oxygen, or an increase in hydrogen, but it always involves an increase in electrons. Bond energy must be increased to hold these new electrons, so this reaction requires ‘money’. Therefore, a reduction reaction reduces your ‘money’ supply.
An oxidation reaction may increase the oxygen, or decrease the hydrogen, but always decreases electrons. This frees the bond energy which held those electrons, so this reaction is a ‘money’ maker.
T creatine also visits the aerobic mitochondria of the slow twitch fiber to be reductively phosphorylated to creatine phosphate. This energy is provided by the aerobic, oxidative “burning” of the pyruvate produced by the fast twitch fiber.
The money trail ends with the oxidative dephosphorylation of ATP into ADP with the released ‘money’ spent on the reduction reaction of contraction.
In the fast twitch fiber, anaerobic mitochondria oxidatively lyse glucose into pyruvate in order to be able to reductively phosphorylate creatine into creatine phosphate, which then transports the energy to the above-mentioned ADP, which the creatine phosphate oxidatively dephosphorylates to be able to reduce the ADP back to ATP.
Thomas Griner
6 August 2005
