MUSCLE FIBER: TYPES, CHARACTERISTICS AND FUNCTIONS - BIOLOGY - 2025

The muscle fiber or myocyte is the type of cell that makes up muscle tissue. In the human body, there are three types of muscle cells that are part of the cardiac, skeletal, and smooth muscles.

Cardiac and skeletal myocytes are sometimes called muscle fibers because of their elongated, fibrous shape. The cells of the heart muscle (cardiomyocytes) are the muscle fibers that comprise the myocardium, the middle muscular layer of the heart.

Skeletal muscle cells make up the muscle tissues that are connected to bones and are important for locomotion. Smooth muscle cells are responsible for involuntary movement, such as contractions that occur in the intestines to propel food through the digestive system (peristalsis).

Types of myocytes, characteristics and their functions

- Skeletal muscle myocytes

Skeletal muscle cells are long, cylindrical, and striated. They are said to be multinucleated, which means they have more than one nucleus. This is because they are formed from the fusion of embryonic myoblasts. Each nucleus regulates the metabolic requirements of the sarcoplasm around it.

Skeletal muscle cells require high amounts of energy, which is why they contain many mitochondria in order to generate enough ATP.

Skeletal muscle cells, form the muscle that animals use for movement, and are compartmentalized in different muscle tissues around the body, for example the biceps. Skeletal muscles are attached to bones by tendons.

The anatomy of muscle cells differs from that of other cells in the body, so biologists have applied specific terminology to different parts of these cells. Thus, the cell membrane of a muscle cell is known as a sarcolemma, and the cytoplasm is called a sarcoplasm.

Sarcoplasm contains myoglobin, an oxygen storage protein, as well as glycogen in the form of granules that provides it with an energy supply.

The sarcoplasm also contains many tubular protein structures called myofibrils, which are made up of myofilaments.

Types of myofilaments

There are 3 types of myofilaments; thick, thin and elastic. Thick myofilaments are made of myosin, a type of motor protein, while thin myofilaments are made of actin, another type of protein used by cells to form muscle structure.

Elastic myofilaments are made up of an elastic form of anchoring protein known as titin. Together, these myofilaments work to create muscle contractions by allowing the "heads" of the myosin protein to slide along the actin filaments.

The basic unit of striated (striped) muscle is the sarcomere, composed of actin (light bands) and myosin (dark bands) filaments.

- Cardiac myocytes (cardiomyocytes)

Cardiomyocytes are short, narrow, and fairly rectangular in shape. They are about 0.02mm wide and 0.1mm long.

Cardiomyocytes contain many sarcosomes (mitochondria), which provide the energy required for contraction. Unlike skeletal muscle cells, cardiomyocytes normally contain only one nucleus.

In general, cardiomyocytes contain the same cellular organelles as skeletal muscle cells, although they contain more sarcosomes. Cardiomyocytes are large and muscular, and are structurally connected by intercalated discs that have gap junctions for cell diffusion and communication.

The discs appear as dark bands between cells and are a unique aspect of cardiomyocytes. They are the result of the membranes of the adjacent myocytes being very close together, forming a kind of glue between the cells.

This allows the transmission of contractile force between cells as electrical depolarization spreads from one cell to another.

The key role of cardiomyocytes is to generate enough contractile force for the heart to beat effectively. They contract together in unison, causing enough pressure to propel blood throughout the body.

Satellite cells

Cardiomyocytes cannot divide effectively, which means that if heart cells are lost, they cannot be replaced. The result of this is that each individual cell must work harder to produce the same result.

In response to the body's possible need for increased cardiac output, cardiomyocytes can grow, this process is known as hypertrophy.

If the cells are still unable to produce the amount of contractile force the body requires, heart failure will result. However, there are so-called satellite cells (nurse cells) that are present in the heart muscle.

These are myogenic cells that act to replace damaged muscle, although their number is limited. Satellite cells are also present in skeletal muscle cells.

- Smooth myocytes

Smooth muscle

Smooth muscle cells are spindle-shaped and contain a single central nucleus. They range in size from 10 to 600 μm (microns) in length, and are the smallest type of muscle cell. They are elastic and therefore important in the expansion of organs such as the kidneys, lungs, and vagina.

The myofibrils of smooth muscle cells are not aligned as in cardiac and skeletal muscle, which means that they are not striated, which is why they are called "smooth."

These smooth myocytes are arranged together in sheets, allowing them to contract simultaneously. They have underdeveloped sarcoplasmic reticulum and do not contain T tubules, due to the restricted size of the cells. However, they do contain other normal cell organelles, such as sarcosomes, but in lower amounts.

Smooth muscle cells are responsible for involuntary contractions and are found in the walls of blood vessels and hollow organs, such as the gastrointestinal tract, uterus, and bladder.

They are also present in the eye and contract, changing the shape of the lens causing the eye to focus. Smooth muscle is also responsible for the peristaltic contraction waves of the digestive system.

As with cardiac and skeletal muscle cells, smooth muscle cells contract as a result of depolarization of the sarcolemma (a process that causes the release of calcium ions).

In smooth muscle cells, this is facilitated by gap junctions. The gap junctions are tunnels that allow the transmission of impulses between them, so that depolarization can spread and allow myocytes to contract in unison.

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