Something as simplistic as walking to class, or eating a banana is a much more involved process than the superficial and visual aspect, which has been becoming ever more clear throughout this chapter. This subchapter will expand on your knowledge from the previous subchapters to focus on how nerves are able to communicate with muscle fibers and create muscular contractions. Neuro-muscular communication is one of the many highly integrated components involved in the aforementioned examples. This subchapter will also introduce the characteristics and organization of muscle tissues to understand how structural differences in a muscle allow for different contractions to be formed.

Types of Muscle Tissue

The following image provides a  visual representation of the three different types of muscle tissue found in humans and an example of each. Types of muscle tissue are not a huge focus of the course but are important background information to keep in mind as you move through the unit. This image is at course level. However, do not focus on differentiating between the muscle types visually.

As can be seen in the above photo, there are three types of muscle tissue: smooth muscle, cardiac muscle, and skeletal muscle. Cardiac and skeletal muscles are both subtypes of striated muscle. Striated muscle is named due to its appearance under a microscope, but is beyond the scope of this course. The appearance and organization of the muscle fibers vary among the different muscle types, and they are found in different locations in the body. Cardiac muscle is found solely in the heart and is responsible for the involuntary contractions that pump blood around your body. Smooth muscle is also under involuntary control and is found in a wide array of organs, including the intestines, urinary bladder, and blood vessels. Skeletal muscle is the only muscle type under voluntary control and is attached to your bones across joints to produce movement during contraction.

The Organization of a Striated Muscle Fiber

The following image provides a visual for the structure of a muscle fiber and shows the location of sarcomeres within myofibrils. The different levels of organization within muscles may seem confusing, but this image helps because it focuses on the structure of the cell itself, and the sarcomere, its functional unit. This image is at course level. However, don’t worry about being able to label the muscle fiber. Rather, focus on the organization of myofibrils within muscle fibers and sarcomeres within myofibrils.

Muscle fibers are the cells that make up muscle tissue. Muscle fibers look a little different than the typical cell diagrams that you see during high school biology classes. Muscle fibers are elongated and cylindrical. The cell membrane of a muscle fiber is called the sarcolemma, and the cytoplasm is called the sarcoplasm. Muscle fibers also have a structure called the sarcoplasmic reticulum, which is similar to the endoplasmic reticulum and stores calcium ions (Ca²⁺). Hundreds to thousands of myofibrils are also found within one muscle fiber. The myofibrils are elongated threads that run parallel to each other and contain the sarcomeres. The sarcomeres are the contractile units of the muscle fibers and are arranged consecutively, separated by Z lines (Z discs).

Did you know the prefixes “myo-” and “sarco-” originate from the Greek words? “Mu”, means “muscle”, and “sark”, means “flesh”. When you hear these prefixes, you can be confident that the word is related to muscles (ex. “myofibrils”, “myofilaments”, “sarcomeres”, “sarcolemma”).

The Structure of a Sarcomere

The following image shows the organization of a sarcomere and zooms in on portions of the thick and thin filaments. As you look through this image, try and connect the various labeled components to their role in muscle contraction. As well, pay attention to the relationship between troponin, tropomyosin, and actin.

Sarcomeres are composed of thin (actin) filaments and thick (myosin) filaments. Actin is anchored to z-lines. Myosin is freely mobile and its heads interact with actin for contraction. The myosin heads cannot bind anywhere on actin but rather, must interact with the binding sites on actin. However, while the muscle is relaxed, the binding sites for myosin are covered by the troponin-tropomyosin complex. The steps taken to create a muscle contraction are discussed later in this subchapter.

Are you interested in learning more about the sarcomere? The following image shows an electron micrograph of a sarcomere. One sarcomere runs between the two Z-lines (the white Zs). The lighter areas directly beside the Zs represent the regions where only thin filaments (actin) are found. The region labeled “M” is the area where only thick filaments (myosin) are found. The region between the exclusively actin and myosin regions are where both types of filaments overlap. This electron micrograph helps to visualize where the actin and myosin get their names of “thin” and “thick” filaments, respectively. The individual myosin filaments can be seen because of their thickness, but individual actin filaments cannot be seen because they are so thin.

Skeletal Muscle Contraction

The following diagram outlines the process of muscle contraction, beginning with the arrival of an action potential at a neuromuscular junction, and ending with a shortening of the muscle. This image is useful because it provides a big picture explanation of the mechanisms of muscle contraction, without focusing too much time on the smaller details. This diagram is below course level as it is only a summary, and you will be required to know more of the biochemical details.

The process of muscle contraction (excitation-contraction coupling) is as follows:

1. An action potential arrives at a neuromuscular junction.
2. As a result of the action potential reaching the end of the axon and depolarizing it, acetylcholine (ACh) is released from the neuron and binds to receptors on the muscle fiber, opening sodium ion channels.
3. The influx of sodium ions causes an action potential along the sarcolemma.
4. The action potential travels down t-tubules along the sarcolemma.
5. This membrane depolarization causes the opening of calcium channels on the sarcoplasmic reticulum, leading to increased intracellular calcium concentrations.
6. Calcium binds to troponin, which moves tropomyosin from the myosin-binding sites on actin.
7. Actin and myosin can now interact.
8. Myosin binds ATP and hydrolyzes it to ADP and P_i. This causes a conformational change in the myosin heads.
9. The myosin heads pull on actin, causing the sarcomeres to shorten and resulting in muscle contraction.

The video below provides a review of the process of skeletal muscle contraction. This is a helpful video because it allows you to see the actin and myosin interaction, and therefore muscle contraction, in action.
0:00-1:20 Muscle physiology
1:20 Sliding filament theory
3:35 Calcium release
This video is slightly below course level, as it does not include as many details as you will learn during lecture. However, watching all the steps come together in action can provide a strong framework to understand muscle contraction.

Remember that all the myosin heads of a sarcomere act independently. In order for movement to occur, all the myosin heads, on all the sarcomeres, within all the myofibrils, within a muscle fiber, must contract at the same time, and the muscle will shorten. Muscles have tendons that terminate at joints. When the muscle contracts, the tendon pulls on the joint, changing the angle of the two bones connected by the joint, and producing movement.

Ever wonder how Rigor Mortis works? After death, the body is no longer producing ATP, therefore, muscle contraction cannot be undone. This causes permanent muscle contraction.

Skeletal Muscle Calcium Sources

The following image provides an overview of sources of calcium in skeletal muscle cells. This image is at course level. However, don’t worry about knowing the “terminal cisternae”. It simply refers to the enlarged area of the sarcoplasmic reticulum.

Calcium is the second messenger involved in muscle contraction. In order for actin and myosin interaction to occur, calcium must be present inside the muscle fiber. In skeletal muscle, the calcium ions are stored in the sarcoplasmic reticulum. When the sarcolemma is depolarized, the local current travels along the t-tubules and causes the voltage-dependent dihydropyridine receptor to change shape, yank on the ryanodine receptor and open a calcium channel on the surface of the sarcoplasmic reticulum. Intracellular calcium levels increase. Increased intracellular calcium increases the force of contraction and decreased intracellular calcium decreases the force of contraction.

Cardiac Muscle Calcium Sources

The diagram below shows the sources of calcium for cardiac muscle contraction. As well, the diagram provides a strong overview of the mechanisms by which calcium is recycled during cardiac muscle relaxation.

Cardiac muscle fibers receive calcium from two sources: the sarcoplasmic reticulum and intracellularly. The calcium channels on the sarcoplasmic reticulum are ligand-gated. The ligand that must bind to open these channels is calcium. This is referred to as calcium-induced calcium release (CICR) because calcium both binds the channel and passes through the channel. The calcium that binds the channel is called “trigger calcium” and comes from extracellular sources. However, there is no difference between the extracellular calcium and the calcium from the sarcoplasmic reticulum. They are identical. In order to decrease intracellular calcium levels for muscle relaxation to occur, there are three mechanisms that the cell utilizes:

1. 1. A calcium ATPase on the sarcoplasmic reticulum.
   2. A calcium ATPase on the cell membrane.
   3. A calcium/sodium exchanger on the cell membrane. This exchanger makes use of the sodium concentration gradient set up by the sodium/potassium ATPase also on the cell membrane. While it moves sodium with its concentration gradient, it is able to move calcium against its concentration gradient.

Smooth Muscle Contraction

The image below shows the interaction between dense bodies, thin filaments, and thick filaments in smooth muscle tissue. It provides a nice visual for how all these components work together to cause muscle contraction. You learn how one set of dense bodies, thin filaments, and thick filaments cause muscle contraction, but it can be tricky to see the big picture and think about how multiple sets work together, which is why this picture can be so useful.

The structure of smooth muscle is different than that of striated muscle. Smooth muscle fibers do not have sarcomeres. However, they do have thin and thick filaments. Where the thin filaments are anchored to the Z-line in striated muscle, they are instead anchored to dense bodies in smooth muscle. Many thin filaments can be anchored to one dense body. Similar to striated muscle, myosin pulls on actin during contraction. This causes the dense bodies to move closer together and causes shortening of the muscle fiber.

The diagram below provides a nice visual of the process of smooth muscle contraction. It is a helpful resource as it shows the process in a different way by using images, rather than just words. This image is slightly below course level as it does not go into as much detail as your notes, however, it is a nice overview to begin with.

The steps of smooth muscle contraction are listed below (as shown in figure 10):

1. Cytoplasmic levels of calcium (Ca²⁺) increase in smooth muscle cells from both extracellular and intracellular sources.
2. Ca²⁺ binds the inactive form of calmodulin, activating it.
3. The calmodulin-Ca²⁺ complex then binds myosin light chain kinase (MLCK).
4. The calmodulin-Ca²⁺-MLCK complex activates myosin via the addition of an ATP molecule.
5. Myosin cleaves the ATP to form ADP and P_i. This is the form of myosin that binds actin, causing muscle contraction.

Subchapter Quiz

The questions below can be used to assess your knowledge within this chapter. There are five multiple-choice questions that you should attempt without referring to your notes. The questions will provide you with responses to your answers to guide your studying but should not be used as your only resource.