Lesson Explainer: Structure of Muscles Biology • Third Year of Secondary School

In this explainer, we will learn how to describe the macroscopic and microscopic structure of skeletal fibers.

The adult male’s body contains around 650 skeletal muscles, making up about of the adult male’s body mass. Muscles are structures in the body that aid movement by contracting and relaxing. They help food move through our digestive systems after a meal, allow our legs to run a race, and make the pupils in our eyes become smaller when we look into a bright light. Without the ceaseless action of our muscles, even our hearts would stop beating!

There are three main different types of muscle, which carry out different functions: skeletal muscle, smooth muscle, and cardiac muscle. While skeletal muscles are under conscious control and so are called voluntary muscles, smooth muscles and cardiac muscles are controlled subconsciously, so we cannot “decide” to make them stop working. Figure 1 shows some examples of where these different types of muscle function and how they differ in their structure.

Some muscles, such as those in our limbs (arms and legs), can be controlled voluntarily to allow movement of certain parts of the body. These are called skeletal muscles, as they are attached to the bones of our skeletons by tendons. They are sometimes called striated muscles due to their striped appearance that you can see in Figure 1. Some skeletal muscles are described as antagonistic as they work in a pair where when one muscle contracts, the other relaxes, allowing a coordinated movement such as in the limbs. In addition to allowing our bodies to move, skeletal muscle also helps maintain one’s posture.

Smooth muscle, also known as involuntary muscle, is not under voluntary control but is found in many different organs. For example, there is smooth muscle tissue in the walls of hollow organs like the human body’s esophagus, stomach, and intestines to help move food through the digestive system. There is also smooth muscle tissue in artery walls, which contracts to help push blood through these blood vessels continuously without us having to think about it.

Cardiac muscle is a type of involuntary muscle only found in the heart. The cells of cardiac muscle are described as myogenic, which means that they do not need nervous stimulation to contract. This allows one’s heart to beat continuously and tirelessly in a regular rhythm to pump blood around the human body.

Let’s look at the macroscopic structure of skeletal muscle and the functions of its components. Macroscopic refers to structures that are visible to the naked eye, without needing to use a microscope.

Each muscle is considered an individual organ. Each of these muscles contains different tissues: skeletal muscle tissue, nervous tissue, blood tissue, and connective tissue. Skeletal muscle consists of many bundles of muscle fibers, which are sometimes called fascicles. These muscle fiber bundles contain connective tissues, cells, and organelles. Depending on its size, one muscle may be made up of up to thousands of individual muscle fibers!

Each bundle of fibers is surrounded by a protective layer of connective tissue called the perimysium, which you can see in Figure 2. The prefix peri-, like perimeter, means it is surrounding something, while the “my” in the middle of the word refers to the muscle. The perimysium helps the cells to withstand the pressure of muscle contraction. This connective tissue layer also provides a place for the blood and nervous tissue to connect to the individual muscle fibers. Blood brings oxygen, glucose, and other nutrients to the muscle cells to allow them to respire and release energy, grow, and repair themselves. The nerves stimulate contraction in the muscle.

Let’s have a closer look at these individual muscle fibers to observe their microscopic structure.

Each skeletal muscle fiber is one very long cylindrical muscle cell enclosed within a plasma membrane called the sarcolemma, as you can see in Figure 2. The prefix sarco- comes from the Greek word for flesh and is often used to describe components of muscles. The suffix -lemma comes from the Greek word for sheath, as it forms a protective membrane around each fiber. The sarcolemma is sometimes called the myolemma, which contains the prefix myo- (you may recall that this refers to muscles).

The muscle fibers have several adaptations that make them effective for their function.

Muscle fibers are much longer than other cells as they were formed by many individual muscle cells fusing together when you were only an embryo. This makes the muscles strong, as any junctions between cells add a point of weakness. This is why a suit of armor is most effective when it is formed from one continuous sheet. One muscle fiber therefore typically has many nuclei. Figure 3 shows a more magnified image of a part of a muscle fiber.

The cytoplasm within a muscle fiber is called the sarcoplasm. In most animal cells, the main role of the endoplasmic reticulum is to act as a site of protein synthesis, modification, and transport. Muscle fibers contain a specialized endoplasmic reticulum called the sarcoplasmic reticulum that extends throughout the muscle fiber. The sarcoplasmic reticulum contains calcium ions, which are needed to initiate muscle contraction.

Muscle cells require a large amount of energy when they contract. Muscle fibers therefore also contain many mitochondria to carry out cellular respiration to provide the ATP needed for contraction. You can see one of these many mitochondria in Figure 3.

Parts of the sarcolemma surrounding the muscle fiber folds inward. This forms structures called transverse tubules, or T tubules, which are shown in Figure 3. This means that an impulse arriving from a nerve can spread along the whole cell’s sarcoplasm so that the myofibrils in the cell can contract simultaneously.

Each muscle fiber contains long organelles called myofibrils, which are made of protein filaments. There can be between 1‎ ‎000 and 2‎ ‎000 myofibrils in one muscle fiber, arranged in parallel to each other and to the muscle fiber along its interior as you can see in Figure 3. Myofibrils are specialized for contraction.

You can think of the structure of muscles as a rope. Ropes are made of individual strings, and strings are made of lots of threads. These join together to provide a rope with their combined strength. Similarly, muscles are made of individual muscle fibers, and the muscle fibers are made of lots of myofibrils. These provide the muscle with their combined strength.

Myofibrils are made up of many repeating units called sarcomeres that you can see in Figure 4. For size reference, you can see the outline of sarcomeres within the myofibril in Figure 3. Myofibrils have repeating patterns of these sarcomeres that are made up of two myofilaments called actin and myosin.

Actin is a thin filament made up of two strands of protein twisted together, as you can see in Figure 4 represented by the red horizontal structures.

Myosin is thicker than actin and therefore appears darker in color. Myosin is a long rod-shaped filament with globular heads that project outward. The myosin filaments are represented in Figure 4 as the blue horizontal structures.

Myofibrils have alternating bands that appear lighter and darker due to the composition of actin and myosin within them in each sarcomere. This makes them look stripy, or striated.

The I band, also known as isotropic bands, as you can see in Figure 4, consists only of thin actin filaments. For this reason, they appear considerably lighter in color than the rest of the sarcomere and are sometimes called light bands. Within the I band is a region called the Z line, which appears slightly darker in micrograph images due to a high concentration of actin and other proteins. The two Z lines mark the boundary of the sarcomere.

The A band, also known as anisotropic bands, contains the thicker myosin filaments as you can see in Figure 4. This makes them appear considerably darker in color than the I bands, and for this reason, they are sometimes called the dark bands. The outer edges of the A band are darkest, as actin and myosin overlap. The inner region of the A band, called the H zone, is not as dark as only myosin is present. When the muscle contracts, the H zone will shorten, causing the whole sarcomere to also become shorter. The line in the middle of the H zone is called the M band.

Example 3: Identifying Regions in the Sarcomere

The diagram provided shows the basic structure of a sarcomere. Which letter indicates the I band?

Answer

The diagram shows us an image of a sarcomere that is made up of two myofilaments called actin and myosin. Actin is a thin filament, made up of two strands of protein twisted together, shown here in red. Myosin, which is a long rod-shaped filament with globular heads that project outward, is thicker than actin. Here, myosin is shown as the blue horizontal structures.

The I band consists only of thin actin filaments shown in this diagram in red. Within the I band is a region called the Z line. The space between two Z lines marks the boundary of the sarcomere.

The A band contains the thicker myosin filaments shown in this diagram in blue. The outer edges of the A band are where actin and myosin overlap. The inner region of the A band, called the H zone, only contains myosin. When the muscle contracts, the H zone will shorten, causing the whole sarcomere to also become shorter. The line in the middle of the H zone is called the M band.

So, let’s label our diagram with the information we have gained.

Therefore, the letter indicating the I band is Y.

Example 4: Identifying the Protein Composition of Regions of a Sarcomere

The diagram shows a labeled structure of a sarcomere.

1. Which part contains only actin?
2. Which part contains only myosin?
3. Which part contains actin and myosin?

Answer

The diagram shows us an image of a sarcomere that is made up of two myofilaments called actin and myosin.

Part 1

Actin is a thin filament made up of two strands of protein twisted together. Myosin is a long rod-shaped fiber with globular heads that project outward and is thicker than actin.

The I band consists only of thin actin filaments shown in this diagram in red. Within the I band is a region called the Z line. The space between two Z lines marks the boundary of the sarcomere.

Therefore, the part that contains only actin is the I band.

Part 2

The A band contains the thicker myosin filaments shown in this diagram in blue. The inner region of the A band, called the H zone, only contains myosin. When the muscle contracts, the H zone will shorten, causing the whole sarcomere to also become shorter.

Therefore, the part that contains only myosin is the H zone.

Part 3

The A band spans the entire length of the thick myosin filaments. The outer edges of the A band are where actin and myosin overlap.

Therefore, the region of the sarcomere that contains both actin and myosin is the A band.

The alternating patterns of actin and myosin in the sarcomere give skeletal muscle a striated, or striped, appearance as you can see in the micrograph image of striated muscle below. You can see the darker A bands containing thick myosin and lighter I bands containing thin actin. The darker Z line stands out in the middle of the I band. You can see the many nuclei surrounding the muscle fibers stained dark blue.

This banding and the resulting stripy appearance are present in both skeletal and cardiac muscle, which is why both muscle types are sometimes referred to as striated muscles. Smooth muscle does not contain these bands and is therefore unstriated. Figure 6 shows the difference in appearance between striated and unstriated muscle.

Let’s recap some of the key points we have covered in this explainer.