Skeletal Muscle Contraction
Similar to the information presented on energy systems, the information on skeletal muscle contraction may seem very technical, but we will sift through some of the irrelevant details and stay focused on the important features of skeletal muscle contraction. Endurance athletes may gain a new appreciation for the intricacies and synchrony of this physiological process. Note: The terms muscle fiber and muscle cell are used interchangeably in this section.
Skeletal Muscle Anatomy
In describing the anatomy of skeletal muscle, we will move from the general to the specific. Having said that, a good way to think of the anatomical structure of skeletal muscle is to compare it to the cable of a suspension bridge. The cable of a suspension bridge has several internal bundles of smaller-diameter cable wrapped in an overlapping configuration that significantly enhances the strength and stability of the “mother cable.” The anatomical structure of skeletal muscle is similar: As we probe deeper into the muscle, the muscle fibers are progressively smaller in diameter and are bundled tightly together; the fibers are reinforced by various connective and overlapping anatomical structures that provide additional support.Figure 1.11 shows the characteristics of skeletal muscle that are similar to the cable of a suspension bridge. Notice how the muscle fibers get progressively smaller in diameter as you view the figure from upper right to lower left. Also notice the connective and supportive tubelike structures that surround each sequential layer of skeletal muscle. The main structure in the muscle fiber of skeletal muscle is the sarcomere, which lies in the myofibril. The myofibril is very important because it is the basic unit of all skeletal muscle contraction.
A more detailed version of the myofibril and surrounding structures is shown in figure 1.12. Focus on the following important structures that surround the sarcomere: T-tubule, tubules of the sarcoplasmic reticulum, and terminal cisternae of the sarcoplasmic reticulum. These structures are important because they are involved in the initial phase of skeletal muscle contraction, which is called the excitation phase (described in the next section). Figure 1.12 also shows the mitochondria, which you learned about earlier in this chapter.
Now let’s take a detailed look at where the real action of
skeletal muscle contraction takes place—the sarcomere. Briefly review the
various lines, bands, and zones of the sarcomere shown in figure 1.13 on page 18. The two
most important structures in skeletal muscle contraction are actin (thin
filament) and myosin (thick filament). The anatomical structure of myosin is
shown in the blowup frame on the bottom left. Myosin is made up of a tail
segment and two large heads. An important feature of myosin is that the heads
have the ability to move. To illustrate this feature, we can compare the
structure of myosin to the structure of a person’s lower arm: forearm (myosin
tail), wrist (joint between myosin tail and heads), and hand (myosin heads).
Just as the hand has the ability to flex, extend, and rotate around, so do the
myosin heads. This anatomical characteristic is very important in the process of
skeletal muscle contraction.
The anatomical structure of actin is shown in the blowup frame
on the middle left in figure 1.13. Actin is the
double strand of egg-shaped structures lined up in an end-to-end configuration.
A thinner protein strand called tropomyosin overlaps the outer surface of actin,
and troponin is attached to and positioned at regular intervals on tropomyosin.
The bottom half of figure 1.13 shows several actin
and myosin filaments in relation to one another. Troponin and tropomyosin are
regulatory proteins in a muscle cell, and actin and myosin are contractile
proteins. The regulatory proteins help ensure that uncontrolled muscle
contractions do not occur, and they prevent uncontrolled binding of actin and
myosin.
Phases of Skeletal Muscle Contraction
Now that you are familiar with the anatomical structures involved in skeletal muscle contraction, let’s look at the sequence of neural, biochemical, and physiological events that allow skeletal muscle contraction to take place. The process of skeletal muscle contraction occurs in three phases (and each of these phases involves several steps):
1. Excitation phase
2. Coupling phase
3. Contraction phase
The excitation phase of muscle contraction (see figure 1.14) refers to the neural impulse that serves to spark the sequence of biochemical and physiological steps that result in skeletal muscle contraction. The key steps in the excitation phase are as follows: 1. Motor nerves embedded in the muscle fire off electrical impulses called action potentials. These action potentials move through the muscle fiber like electricity traveling through a power line (figure 1.14a). 2. The action potential moves along the sarcolemma and down the T-tubules to the sarcoplasmic reticulum (figure 1.14b). 3. The action potential triggers the release of calcium (Ca2+) from the terminal cisternae of the sarcoplasmic reticulum (figure 1.14c).
The coupling phase of skeletal muscle contraction is also shown in figure 1.14 on page 19. Coupling refers to the interconnection of the contractile filaments, actin and myosin. Here are the key steps in the coupling phase:
1. Calcium binds to the troponin complex.
2. The troponin complex changes its shape and configuration, thereby allowing tropomyosin to recede into the space between the actin strands.
3. As tropomyosin recedes from the outer surface of actin, it no longer blocks the outer surface of actin from interfacing with myosin.
4. The binding sites on actin are now fully exposed. The myosin heads quickly attach (couple) to actin at the binding sites.
The particular contraction step regarding skeletal muscle contraction is actually found inside figure 1. 15. Contraction is the term for this routine regarding functions in which myosin basically “pulls” on actin, thereby illustrating the 2 contractile filaments nearer together in addition to resulting in physical contraction. This step is frequently often called this sliding off the road filament principle. Understand that this contraction step regarding skeletal muscle contraction will not likely occur except ATP occurs.
After the contraction phase, the skeletal muscle will return to the relaxed, noncontractile state when the motor nerves stop firing off action potentials, which in turn shuts off the release of calcium from the terminal cisternae of the sarcoplasmic reticulum. Without calcium present, tropomyosin and the troponin complex resume their noncontractile positions—that is, they serve to block myosin from attaching to actin. The process of skeletal muscle contraction is one of the most amazing aspects of human physiology. We hope that you now have a better understanding and appreciation for what takes place in skeletal muscles during endurance activities.
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