Our first module, will focus on skeletal muscle.
How it contracts, and the fuels that it utilizes during exercise.
We're not going to have an extensive coverage of how skeletal muscle
functions. If you're uncertain about the basics of
skeletal muscle physiology, I suggest you go back and review that.
In any standard textbook of human physiology.
But skeletal muscle is characterized by it's specialized structure, and at
various levels, microscopic levels, you can see the, the well organized structure
of skeletal muscle. Ultimately the actin and the myosin
filaments, within skeletal muscle interact to generate force and that's
under the control of the nervous system. You should review the basic steps and
sequence of events involved in muscle contraction.
Starts in the motor cortex with activation of the motor centers, that
control various muscles of the body. There's descending motor activation
through the various motor pathways and finally excitation, the alpha-motoneurons
that go to specific muscles. Transmission of the electrical impulse
across the neuromuscular junction is mediated by the transmitter
acetylcholine, and ultimately, the excitation of the motor nerve is
transferred to excitation of the muscle membrane.
And the extension of the muscle membrane into the muscle, the T-tubule.
The fundamental event in muscle contraction we refer to as excitation
contraction coupling. Whereby depolarization of the t-tubule
and activation of the calcium-release channels from the sarcoplasmic reli
sarcoplasmic reticulum, results in calcium release into the Sarcoplasm.
Calcium then binds with Troponin C, allows the interaction of actin and
myosin, and we get the generation of force.
ATP or adenosine triphosphate, the high energy molecule that's utilized at
various steps in muscle contraction is very important, and we'll spend some time
looking at the energetics of muscle, and the fuels for exercise in our next
module. This figure summarizes those process,
processes in in graphical form. And you can see the various steps
involved beyond the neuromuscular junction, in the skeletal muscle.
We'll come back to this when we look at muscle fatigue in our fifth module, and
you can refer back to this figure, to see where parts of this process may be
affected during fatigue. So again, just to reiterate some of the
key steps. We have excitation of the muscle membrane
and the T-tubule, release of calcium from the cytoplasmic reticulum, and during
recovery, the resequestration of calcium back into the cytoplasmic reticulum.
The binding of calcium to the thin filament's complex and the interaction of
actin and myosin and the hydrolysis of ATP.
There are a number of processes within muscle contraction that are dependent
upon ATP. Notably, the myosin ATPase is important
in the cross-bridge cycle. But other important sources, or other
important sites of ATP utilization include the sodium potassium ATPase,
which is very important in maintaining the excitability of the sarcolemma.
Particularly in returning potassium to the muscle, after each action potential.
Another important ATPase is the calcium ATPase on the cytoplasmic reticulum, and
during relaxation when calcium is pumped back into the cytoplasmic reticulum, this
enzyme is very active. Studies in rodents with selective
inhibitors of the myosin ATPase have been used to reduce the force output of muscle
but during activation, there's still activity of the sodium-potassium-ATPase
and the calcium-ATPase. Suggest that the ATP utilization, due to
the myosin ATPase of the force production is about 50 to 60% of the total ATP
turnover. But all of these processes are critically
dependent upon ATP. We turn our attention to how ATP is
utilized during exercise, we need to just reflect on how, in exercise science, we
express exercise intensity and that's normally done relative to the maximal
oxygen uptake. We'll come back to that concept in our
third module on oxygen uptake. But for the moment, if you think of VO2
max, as a signpost, or a, a marker of exercise intensity.
Anything up to 100% of VO2 max is largely aerobic, with a small contribution from
the so called anaerobic energy stores. If you exercise at an intensity higher
than VO2 max, then clearly the other energy systems, the non aerobic energy
systems, are contributing to that power generation.
The muscle ATP concentration is in the order of five to six millimole per kilo
of wet muscle. If you're exercising at 75% of your VO2
max, reasonably moderate intense exercise, if you relied solely on the ATP
within your muscle, you'd only be able to go for about 15 seconds.
If you increased the exercise intensity to a higher level, say 140% of VO2 max,
you'd go for about six seconds. And then if you went as hard as you
possibly could, for a very short period, relying only on the ATP in your muscle,
you'd last less than two seconds. So clearly, other metabolic pathways in
skeletal muscle are activated, to generate ATP, to maintain muscle
contraction. These energy systems traditionally have
been divided into two types. The first, refers to substrate level
phosphorylation where the ATP is regenerated by the interactions and the
exchange of phosphate during series of reactions directly at the substrate
level. The first of these, is using creatine
phosphate, which is another high energy phosphate compound stored in skeletal
muscle. The other pathway, is the so called
anaerobic glycolysis, where glucose units, either from blood glucose or
muscle glycogen, are broken down in a series of steps, to pyruvate ,which can
then either be oxidized or converted to lactate.
And at high rates of energy expenditure, the conversion of pyruvate to lactate is
the dominant pathway. The other metabolic pathway is oxidative
phosphorylation. And this, in this pro, pathway the
metabolism of carbohydrate and fat generate a, a electron donors which then
interact with molecular oxygen. To, in, in the electric transport chain,
to establish a, an electromotive gradient which drives the re-synthesis of ATP.
This figure summarizes those energy systems, the important processes I
mentioned earlier where ATP is utilized in muscle contraction.
And then the other pathways, that regenerated ATP from ADP during exercise
of varying intensity and various duration.
In our next module, on fuels and exercise, we'll focus on these various
energy systems in more detail. For the moment, however, creatine
phosphate and the degredation of carbohydrate to lactate the so called
anaerobic pathways, and then in terms of oxidative metabolism, the utilization of
carbohydrate lipid and to a very small extent, protein.
It's worth just reflecting on an overview of skeletal muscle energy metabolism, and
we'll come back to this as I said, in the second module on fuels.
Here is the utilization of ATP in the cross-bridge cycle.
Creatine phosphate serves as a buffer for, ATP levels during very intense
exercise. The other metabolic pathways involve
carbohydrate. Where it's degraded through glycolysis,
and then either converted to lactate or into the mitochondria, where the Krebs
Cycle and the electron transport chain can generate ATP.
The other important fuel during exercise is fat, and this fat can either come from
the adipose tissue, triglyceride stores, and the plasma fatty acids, or the
intermuscular tryglycerides. These are broken down, the fatty acyl
CoAs, transfer into the mitochondria across the mitochondrial membrane.
There's a process known as beta oxidation.
Where these fatty acids can be degraded into the Kreb cycle, and in much the same
way, for carbohydrate generate ATP through the electron transport chain.
As I said, we'll come back to this topic in, mm, much more detail in our second
module on fuels for exercise.