Muscle Tissue

Among its main characteristics are: excitability, contractility, extensibility and elasticity.

Muscles represent 40% of body mass. Therefore, in many animals, muscle tissue is the most abundant.

Muscle tissue cells are elongated and are called muscle fibers or myocytes. They are rich in two proteins: actin and myosin.

In the study of muscle tissue, its structural elements are given different names. Understand each of them:

Cell = Muscle Fiber;
Plasma Membrane = Sarcolemma;
Cytoplasm = Sarcoplasm;
Smooth Endoplasmic Reticulum = Sarcoplasmic Reticulum

Functions of Muscle Tissue

• Body movement
• Stabilization and posture
• Regulation of organ volume
• Heat production

Muscle tissue is classified into three types: skeletal striated , cardiac striated , and smooth or nonstriated .

Each tissue is formed by muscle fibers that have particular morphological and functional characteristics, as we will see below:

Striated Skeletal Muscle Tissue

The term skeletal is due to its location, as it is connected to the skeleton.

Striated skeletal muscle tissue has voluntary and rapid contraction.

Each muscle fiber contains several myofibrils , filaments of proteins (actin, myosin, and others).

The organization of these elements causes transverse striations to be observed under a light microscope, which gave the tissue the name striated.

Striated skeletal muscle fibers are shaped like long cylinders, which can be as long as the muscle to which they belong. They are multinucleated and the nuclei are located on the periphery of the fiber, close to the cell membrane.

Longitudinal section of skeletal fibers, where it is possible to observe their striations

Muscle fiber and contraction

Muscle contraction allows locomotion and other body movements.

Muscle fibers contract due to the shortening of myofibrils, cytoplasmic filaments rich in actin and myosin proteins, arranged along their length.

These filaments can be observed under an optical microscope. In this microscope, the presence of transverse striations can be observed due to the alternation of light bands (Band I, actin myofilaments) and dark bands (Band A, myosin myofilaments).

This structure is called a sarcomere , which represents the functional unit of muscle contraction.

A muscle cell has between tens and hundreds of sarcomeres arranged in the myofibril. Each sarcomere is delimited by two transverse discs, called Z lines.

In short, muscle contraction refers to the sliding of actin over myosin.

This is because actin and myosin form organized filaments that allow them to slide over each other, shortening the myofibrils and leading to muscle contraction.

In the cytoplasm of the muscle fiber, it is possible to find several mitochondria, which provide the energy necessary for muscle contraction and glycogen granules.

Muscle fibers are held together by connective tissue. This tissue allows the contraction force generated by each individual fiber to act on the entire muscle.

Furthermore, connective tissue nourishes and oxygenates muscle cells and transmits the force generated during contraction to neighboring tissues.

To learn more, also read: Muscular System and Muscles of the Human Body .

Cardiac Striated Muscle Tissue

It is the main tissue of the heart.

This tissue has involuntary, vigorous and rhythmic contraction.

It consists of elongated and branched cells, equipped with one nucleus or two central nuclei.

They present transverse striations, following the organization pattern of actin and myosin filaments. However, they do not group into myofibrils.

It differs from skeletal striated muscle tissue in that its striations are shorter and not as evident.

Cardiac muscle tissue in longitudinal section. Striations are less apparent

Cardiac fibers are surrounded by a sheath of protein filaments, the endomysium. There is no perimysium or epimysium.

The cells are joined together at their ends by specialized structures: the intercalated discs. These junctions allow adhesion between the fibers and the passage of ions or small molecules from one cell to another.

Almost half of the cell volume is occupied by mitochondria, which reflects the dependence on aerobic metabolism and the continuous need for ATP.

Connective tissue fills the spaces between cells and your blood capillaries provide oxygen and nutrients.

The heartbeat is controlled by a set of modified cardiac muscle cells, called the cardiac pacemaker or sinoatrial node . Approximately every second, an electrical signal propagates through the cardiac muscle, causing contraction.

Smooth or Non-Striated Muscle Tissue

Its main characteristic is the absence of striations.

Present in visceral organs (stomach, intestine, bladder, uterus, gland ducts and blood vessel walls).

It forms the wall of many organs and is responsible for internal movements such as the movement of food through the digestive tract.

This tissue has involuntary and slow contraction.

The cells are uninucleate, elongated and with sharp ends.

Unlike striated skeletal and cardiac tissue, smooth muscle tissue does not have striations. This is because the actin and myosin filaments do not organize themselves in the regular pattern presented by striated cells.

Smooth Muscle Tissue and the absence of striations

The cells are joined together by gap junctions and tight junctions.

In smooth muscle tissue, neither perimysium nor epimysium is found.