Muscles are the active part of our motor system. They are the focal point of our weekly efforts at the gym. They define our body significantly and are therefore a characteristic of attractiveness. However, even more important is their function. Movements are the results of alternating contraction and relaxation of muscles that amount to approximately 30-40 % of the entire body weight. The primary function of muscles is the conversion of chemical energy into mechanical energy to do their work. Without the support of muscles, we would not be able to keep our back straight, let alone walk straight. In this article you will find a compact overview of the different types of muscles, their structure, how to control movement with their origin and approach and which auxiliary structures they use.

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Smooth muscle tissue is found around organs in the digestive, respiratory, reproductive tracts and the iris of the eye

Image: Partial view of “Smooth muscle tissue is found around organs in the digestive, respiratory, reproductive tracts and the iris of the eye” by OpenStax College. License: (CC BY 3.0)


Classification of muscles

The four specific features of muscle tissue are electric excitability, contraction capability, flexibility and elasticity.

Histologically, the muscles of the body can be differentiated into three types:

Smooth musculature as musculature of the intestines (muscles of inner hollow organs) Smooth musculature as muscles of the intestine (muscles of inner hollow organs)
Heart musculature or myocardium Heart musculature or myocardium
Striated or skeletal musculature as part of the active motor system Striated musculature or skeletal muscle as part of the active motor system

Image: Partial View of “the musculature of our body can be differentiated into 3 types” by Phil Schatz. License: CC BY 4.0

Smooth musculature

Smooth musculature is present where a certain tonus must be maintained without great energy input because it is innervated by the vegetative nervous system and does not tire. This type of musculature can be found especially in vessel walls or the intestinal wall, i.e., the gastrointestinal tract and many organs of the urogenital system. Furthermore, the smooth musculature is capable of spontaneous self-contraction.

There are 2 types of smooth muscle tissue:

  • visceral smooth muscle tissue (single-unit type)
  • smooth muscle tissue of the multi-unit type

The visceral smooth muscle tissue (single-unit type) is the more common type and is – just like the heart muscle – autorhythmic. This type of tissue can be found in tubular arrangements that form parts of the walls of small arteries and veins as well as the walls of hollow organs (i.e., stomach, uterus, etc.).

The individual fibers are connected to each other via gap junctions and form a net that enables muscle action potentials to spread. Muscle action potentials are stimulated by a neurotransmitter, a hormone or an autorhythmic signal of a fiber and transfer it to the neighboring fiber. They contract simultaneously as a unit.

In contrast to the single-unit type, the multi-unit type of smooth musculature consists of single fibers, each of which having their own motor neuron endings and only few gap junctions between neighboring fibers. This is why “only” one fiber in the walls of large arteries, the airways of the lungs or inside the muscles of the iris that regulate the pupil diameter, contracts.

Smooth muscle tissue is found around organs in the digestive, respiratory, reproductive tracts and the iris of the eye

Image: Partial view of “Smooth muscle tissue is found around organs in the digestive, respiratory and reproductive tracts and the iris of the eye” by OpenStax College. License: CC BY 3.0

A single smooth muscle fiber is approximately 30 to 200 micrometers (µm) long, the middle part being the thickest (3-8 µm) and the ends being pointed. A smooth muscle fiber only has one oval, centrally located nucleus and is usually spindle-shaped and not very branched. The sarcoplasm of smooth musculature consists of myosin filaments and actin filaments. However, contrary to the striated muscle they are not arranged in well-ordered sarcomeres. They do contain intermediate filaments, though.

Actin filaments are connected to dense bodies that are similar to the Z band in striated muscle fibers. Intermediate filaments are connected to dense bodies as well and extend from one dense body to the next.

Why is this type of musculature referred to as smooth?

As the numerous filaments do not have a regular overlapping pattern, smooth muscle fibers do not show any striation, which gives them their “smooth appearance.”

Relaxation and contraction

Image: "The dense bodies and intermediate filaments are networked through the sarcoplasm, which cause the muscle fiber to contract." by OpenStax College. License: CC BY 3.0

Image: “The dense bodies and intermediate filaments are networked through the sarcoplasm, which cause the muscle fiber to contract.” by OpenStax College. License: CC BY 3.0

How does a contraction happen?

The sliding filament mechanism, which also involves myosin and actin filaments, produces tension during contraction, which is transferred to the intermediate filaments. They, then, pull on the dense bodies, which are connected to the sarcolemma, resulting in a shortening of the muscle fiber. The smooth musculature acts like a corkscrew during contraction. It coils into a spiral during contraction and turns into the opposite direction during relaxation.

Smooth musculature can be shortened and stretched more than other types of musculature. Another difference is that the contraction of smooth muscles starts much more slowly and lasts longer than the contraction of a skeletal muscle. This is true because calcium needs more time to reach the smooth muscle filaments.

You should remember that smooth muscle:

  • Is found extensively in large areas of inner organs and blood vessels.
  • Has no striation.
  • Function: involuntary movements.
  • Reacts slowly and serves the constant performance without strength peaks.

Skeletal musculature/striated musculature

The musculature of the motor system is striated because most muscles have their origin in and their insertion at the skeleton. Exceptions are the visceral structures of the head and neck, i.e., the tongue, the pharynx and larynx as well as the upper esophagus, which are not connected to the skeleton but still consist of striated musculature. Skeletal muscles are well supplied with nerves and blood vessels, and generally, each nerve that enters the skeletal muscle tissue is accompanied by one artery and two veins.

One muscle cell is up to 15 cm in length with a fiber thickness of between ten to 100 micrometers. The muscle fiber is the contractile element of the skeletal musculature. They are combined together to form muscle bundles and muscles by an enveloping system of collagen and elastic fibers.

Each muscle fiber consists of hundreds of myofibrils that represent the contractile structure of the skeletal muscle. The mature skeletal muscle fiber is characterized by many nuclei (over 100) below the sarcolemma and striated micro fibrils. Each micro fibril is surrounded by the sarcoplasmic reticulum. Actin and myosin filaments are interlocked within a micro fibril, thereby forming a sarcomere.

Each single cell is completely separated from its neighboring cells and works by itself.

How does striation occur?

Due to the arrangement of actin and myosin filaments in rows and the concurrent interlocking, where one end of the actin filaments protrudes between the myosin filaments, the characteristic striation of skeletal muscles arises. Light and electron microscopy show the following characteristics of striated muscle:

  • A- Bands
    • microscopically dark bands
    • formed by thick myosin filaments, between which thin actin filaments reach the border of the H-zone, when the muscles are relaxed
  • I- Bands
    • light bands
    • consist of thin actin filaments
  • Z- Lines
    • dark horizontal line in the middle of the I-Bands
    • thin actin filaments are connected to each other via a horizontal lattice
  • H- Zone
    • light zone in the middle of the A-band
    • consists of thick filaments
  • M- Bands
    • fine, dark band in the middle of the H-zone
    • thick, cross-connected filaments can be differentiated.
view of a sarcomere

Image: “Sarcomere

Within the myofibrils, the same bands are located next to each other at approximately the same height, resulting in the typical striation. One sarcomere has a length of approximately two micrometers in its relaxed state and consists of the line sequence Z-I-A-H-M-H-A-I-Z.

What happens during contraction?

The shifting of actin filaments causes a muscle contraction. Thereby, one differentiates between two types of force development:

  • Isotonic contraction with muscle shortening
  • Isometric contraction without muscle shortening

Isotonic contraction:

In an isotonic contraction, the extent of the overlapping between thin and thick filaments changes because, depending on the extent of the contraction, the actin filaments are pulled more or less between the myosin filaments. Since the length of thick and thin filaments remains constant, the I- and H-bands become narrower and the sarcomeres shorten themselves.

Isometric contraction:

In an isometric contraction, the length of the sarcomeres and the width of the cross-striation remain constant. However, force development still takes place as the mobile myosin heads cyclically approach the actin filaments in the same place, time and again, and the rotatory motion of the myosin head causes tension, which is emitted outward.

Innervation:

All muscle fibers innervated by a nerve fiber form a motor unit. Each muscle fiber has at least one synapse or motor endplate, which is usually located in the middle of the fiber. A contraction only takes place as a result of a nerve impulse at the motor endplate via the neurotransmitter acetylcholine. A single nerve fiber can innervate one muscle fiber or many muscle fibers. The smaller a motor unit, the more precise the movement.

You should remember about striated musculature:

  • Active motor system, random movements.
  • Shifting of actin filaments results in muscle contraction.
  • Immediate reaction – short performance with high strength peaks.
  • Connection with bones through tendons and ligaments.

Heart musculature

The heart musculature is striated but differs greatly from the skeletal musculature because of numerous special features. The most important difference is that the heart musculature works completely autonomously and independently from nerve impulses because there is no motor endplate. The nerve fibers of the vegetative nervous system, which innervate parts of the heart, have solely altering character but are not necessary for the actual function of the heart.

Heart muscle cells are irregularly branched and approximately 100 micrometers (µm) in length. They are found exclusively in the heart and have the same actin and myosin arrangement as well as the same bands, zones and Z-bands as skeletal muscle fibers.

Intercalated discs (Latin: intercalare = to insert) connect the fibers with each other and emphasize the unique character of heart muscle fibers, which is especially why they differ from skeletal muscles. Microscopically, they are irregular, transversal, thick parts of the sarcolemma, which connect the ends of the heart muscle fibers with each other. The discs contain desmosomes that hold the fibers together as well as gap junctions that facilitate muscle action potentials.

Picture of muscle tissue

Image: “Muscle Tissue” by Mdunning13. 1) Skeletal muscle cells, 2) Smooth muscle, 3) Heart musculature. License: CC BY-SA 3.0

How does a contraction proceed?

Compared to skeletal muscle tissue, heart muscle tissue contracts approximately 10 to 15 times longer due to prolonged influx of calcium into the sarcoplasm. Under normal circumstances, contraction and relaxation take place about 75 times per minute. The contraction occurs due to the stimulation of its own autorhythmic fibers. Due to the continuous rhythmic activity, ATP (adenosine triphosphate = energy source) is produced especially by the aerobic cellular respiration in the heart muscle. This continuous rhythmic activity is the main physiological difference between heart and skeletal muscle fibers.

You should remember about heart muscle:

  • Special musculature of the heart.
  • Striated muscle tissue is connected with intercalated discs.
  • Works autonomously, the vegetative nervous system has only a modulating influence.

The structure of a muscle

Almost every muscle is attached to two different bones via its tendons, which are referred to as origin and insertion.

The origin is defined as the bone that does not move or moves less than the other bone. Consequently, the insertion is a bone with more freedom of movement. In the process of a movement, the fixed bone is called punctum fixum, toward which the punctum mobile moves.

The m. biceps brachii, for instance, has its origin at the bony shoulder girdle and the insertion at a bone of the arm. The proximal attachment site is usually referred to as the origin, the distal one as the insertion site.

In which ways the muscle can pull together

Image: “Muscle length changes” by Phil Schatz. License: CC BY 4.0

Auxiliary structures of a muscle

A muscle does not directly merge into the bone but rather, via tendons. Tendons are made of firm, collagenous connective tissue that can withstand very strong traction to some extent. A gradual transition from muscle to tendon or tendon to muscle, respectively, prevents a rupture in case of sudden, strong strain.

Bundles of muscle fibers called fascicles by perimysium covered. Muscle fibers covered by the endomysium.

Image: “Bundles of muscle fibers” by Phil Schatz. License: CC BY 4.0

A muscle fascia surrounds the muscle. Fasciae are lamellae made of firm connective tissue that surround single muscles or muscle groups. There are

  • Single fasciae
  • Groups of fasciae and
  • Superficial body fascia

Single fasciae serve as the shaping and location of a muscle as well as a guiding wrap in which the fibers are tightly packaged.

Groups of fasciae, on the other hand, surround muscles with the same function and separate (as septa intermuscularia) antagonistic muscle groups. They also form individual compartments together with the superficial fascia.

The superficial fascia covers all muscles of the trunk and the extremities and separates the subcutis from the musculature.

Synovial sheaths (Latin: vagina synovialis tendinis) are connective tissue-like guiding tubes of long tendons of extremities that surround the tendon where it rests on bones and pressure is exerted. They consists of the stratum fibrosum on the outside, which keeps the tendon in its place, as well as the stratum synoviale, which facilitates the gliding of the tendon inside the tube of connective tissue. Vaginae synoviales are particularly important for reducing friction. They are found in the long tendons in the hand and foot, for instance.

Bursa sacs are spaces in the connective tissue that contain synovia, just like joints. They are also part of the auxiliary structures. A bursa sac is formed between bones and the underside of a muscle if a muscle moves around a protruding bone. It distributes pressure and facilitates the reciprocal shifting of the structures.

The sesamoid bone should also be named as a special auxiliary structure. It starts off as a cartilaginous deposit in the tendon, which ossifies later on. By raising the insertion angle, the sesamoid can also act as hypomochlion (bone protrusion). It balances out high tension where synovial sheaths alone are insufficient. The patella is the largest sesamoid in the human body.

Typical muscle injuries

Aside from muscle stiffness, the pulled muscle is a very common and typical sports injury that can occur in every muscle group.

A pulled muscle occurs if a muscle ruptures due to strong impact, accompanied by bleeding and severe pain. Depending on the severity of the injury, even restricted mobility can occur. Pulled muscles belong among closed muscle injuries since only the muscle is affected and no external signs are visible. Principally, it is not an actual injury of the muscle but a hardening of the muscle since it is the muscle tonus that is impaired.

In contact sports, specifically, the m. quadriceps femoris on the front side of the leg (extensor of the leg) is often at risk. Basketball players, on the other hand, rather have difficulties with the upper arm or shoulder musculature.

Risk factors for the occurrence of a pulled muscle:

  • too much strain;
  • lack of exercise;
  • lack of warming up before sports;
  • fluid- and electrolyte deficiency;
  • illnesses like colds, for instance.

This injury is treated with the help of the RICE therapy.

R – Rest

I – Ice

C – Compression with a bandage

E – Elevation of the affected extremity

The duration of the recovery process of a pulled muscle can vary. Pain often subsides after a few days, but the entire healing process usually lasts for up to three weeks. The sooner a pulled muscle is diagnosed and treated, the faster the individual will be free of pain and symptoms.

Other muscle injuries/muscle diseases are:

  • Myalgia (muscle pain)
  • Myoma
  • Myelomalacia
  • Myositis
  • Myotonia
  • Volkmann contracture
  • Muscular dystrophy
  • Fibromyalgia
  • Myasthenia gravis
  • Abnormal skeletal muscle contractions

Popular exam question regarding muscle tissue types

Answers can be found below the references.

1. What characterizes smooth musculature?

  1. Found extensively in blood vessels with striation and slowly contracting fibers.
  2. Reacts fast and serves the continuous performance without strength peaks.
  3. Connected to the bone through tendons and ligaments.
  4. Reacts immediately with short performance and high strength peaks.
  5. Is innervated via the vegetative nervous system and is found extensively in inner organs.

2. What is the Most Important Difference Between Heart and Skeletal Musculature?

  1. Actin and myosin filaments are arranged differently to secure a stronger contraction.
  2. The musculature of the heart is completely autonomous and works independently of all nervous impulses.
  3. Fibers are connected via intercalated discs that emphasize the unique nature of skeletal muscle fibers.
  4. A heart muscle fiber is characterized by many nuclei below the sarcolemma (over 100).
  5. Heart muscle fibers contain desmosomes but do not need gap junctions that enable muscle action potentials.

3. What is not an Auxiliary Structure of Muscles?

  1. Sesamoid bone
  2. Synovial sheaths
  3. Elastic connective tissue
  4. Bursa sacs
  5. Muscle fascia
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