STRIATED MUSCLE: CHARACTERISTICS, FUNCTIONS AND CLASSIFICATION - ANATOMY AND PHYSIOLOGY - 2025

The striated muscle is a muscle tissue type consists of elongated cylindrical cells called muscle fibers, which corresponds to 90% of the total body muscle mass and 40-50% of the total body weight. These muscle fibers have a uniform diameter.

In addition, their length can be variable without reaching the full length of the muscle, if not on the contrary, they overlap arranged in fascicles separated from each other by connective tissue. Each fascicle is formed by the union of many muscle fibers.

In turn, each of these fibers is made up of hundreds or thousands of myofibrils, which are made up of multiple actin filaments (thin filaments) and myosin (thick filaments). When speaking of skeletal muscle, both the skeletal muscles and the cardiac muscles are covered.

However, cardiac muscle fibers, while striated, are so specific and particular that they are treated as a different type of muscle. An approximate 640 striated muscles are calculated in the human body and it receives this name because longitudinal striations can be clearly seen under the microscope.

These streaks correspond to bands A (actin and myosin) and bands I (actin only), which are arranged in an intermittent pattern. Each of these patterns is called the sarcomere, which is the fundamental contractile unit of skeletal muscle.

characteristics

Striated muscle tissue is a dark red tissue due to its rich vascularity. It is distributed throughout the entire body, involving the bone system and forming the heart.

In electron microscopy, the striations, which are responsible for their name, can be evidenced, and the skeletal striated muscles can be differentiated from the cardiac striated muscles by the arrangement of their nuclei.

These muscles have properties of tonicity, elasticity, contractility and excitability that are very different from smooth muscles, which gives them the ability to modify their shape and strength more than any other organ in the human body.

Tonicity

The tonicity of the striated muscle refers to the tension in which the muscle is when it is in a state of rest, and this tone is maintained involuntarily or unconsciously thanks to the autonomic nervous system, which allows the rotation in the contraction of the fibers always maintaining the muscle contracted without reaching fatigue.

In the absence of innervation, the muscle not only loses its properties of tonicity, contractility and excitability, but it also atrophies and degenerates through disuse.

Elasticity

Muscle elasticity is the ability of the muscle to stretch and return to its initial size, this characteristic can be worked through stretching exercises, which will gradually increase the length of the fibers that will always have the ability to return to their initial length.

Contractility

The contractile capacity of skeletal muscle is characterized by voluntary contraction and its speed, unlike smooth muscles that contract involuntarily and slowly.

Excitability

The property of excitability refers primarily to the ability of the cardiac striated muscle to transmit and propagate the action potential from one cell to another, as it functions at neuronal synapses.

Features

The main function of skeletal muscle is to mobilize the body in general, inserting itself into the bones by means of connective tissue structures called tendons and using them as a lever to, through contraction and relaxation, move the bones and joints.

In order to fulfill its functions, the vascularization and muscular innervation is one of the richest in the body, and the main or larger arteries usually access the muscle through the muscle belly.

Vascularization

The most important characteristic of muscular vascularization is the adaptability of the arteries and capillaries; in this way, when the muscle contracts, the arteries increase vascularity up to 500 times to provide the muscle with oxygen and prevent muscle fatigue.

In the same way, some muscles are responsible for maintaining the upright posture of the human body, they activate an almost imperceptible isometric contraction to maintain the posture in the field of gravity.

These muscles are known as slow twitch muscles, they have the ability to maintain sustained isometric contractions and, at the same time, antagonists.

For example, to keep the spine upright, the back muscles need the abdominal muscles to counteract the force exerted backwards.

Likewise, skeletal skeletal muscle fulfills the function of energy biotransformation, generating heat from chemical compounds used in its contraction and relaxation.

Classification

Striated muscle can be classified into two types according to its location:

Striated cardiac muscle

It is also called myocardium and, as its name indicates, it refers to the type of muscle that makes up the cardiac musculature. The fundamental unit of the myocardium is the myocyte and is considered the contractile cell of the heart.

Although they are elongated, the fibers of this type of muscle have the characteristic of having a single and large nucleus in their center, and their contractility escapes voluntariness, contracting involuntarily with each heart beat.

This automatic and unconscious contraction is regulated by the autonomic nervous system and its frequency can vary according to the state of rest or activity of the patient and the existence or not of pathologies.

The cells of the cardiac skeletal muscle are so specialized that not only can they contract, but they also have a certain capacity for automaticity that allows the propagation of action potentials for their contractility.

Skeletal striated muscle

As its name indicates, this type of muscle is responsible for the mobilization of the skeleton, joining the bone structures through insertions of connective tissue and collagen known as tendons, which when contracted allow skeletal mobility.

It is important to clarify that, despite receiving the name of skeletal muscle -because it is responsible for the movement in general of the human body-, some muscles are inserted into other muscles or even into the skin, such as some of the facial expression.

It is voluntary; that is, their contraction is regulated by the central nervous system, they can develop a rapid contraction and as an important characteristic they can suffer exhaustion after sustained contractions.

They are formed by a muscular belly, which is located in the central region of the muscle, and the fibers that make up each muscle vary according to the functional properties of each one; for example:

Muscles responsible for maintaining posture

Type I red fibers rich in myoglobin that are characterized by being slow-twitch and resistant to fatigue.

Muscles responsible for applying force

Type IIB white fibers rich in glycogen; that is, they are glycolytic in their contractile mechanism, they are fast-twitch, and they tire quickly.

Muscles that must apply forces for long periods

White type IIA oxidative-glycolytic fibers, they are fast contracting but resistant to fatigue, they are considered mixed between type I fibers and type IIB fibers.

References

1. Y. Shadrin. Striated Muscle Function, Regeneration, and Repair. YOU. National Library of Medicine. National Institutes of Health. Recovered from: ncbi.nlm.nih.gov
2. Austin Summer. Chapter 81. Anatomy and Physiology of Muscle and Nerve. Neurology and Clinical Neuroscience, 2007. Recovered from: sciencedirect.com
3. Guyton and Hall Treatise on Medical Physiology 12th Edition. Editorial Elsevier. Unit II. Membrane, nerve and muscle physiology. P. 44-98.
4. Beatriz Gal Iglesias. Bases of Physiology. 2nd Edition. Chapter 4. Physiology of muscle. Pages 58-76.
5. Fracisco Guede. Biomechanics of the muscle. University of the Americas. Recovered from: fcs.uner.edu.ar