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Muscles and muscle tissue

Recommended video: Skeletal muscle tissue [12:25]
This type of tissue is found in skeletal muscles and is responsible for the voluntary movements of bones.

Muscle is defined as a tissue primarily composed of specialized cells/fibers which are capable of contracting in order to effect movement. This can relate to movement of the body or body parts with our external environment, however did you know that almost all movement of blood, food and other substances within the body is often the result of muscle contraction?

Depending on the type, the primary function of muscle is to move the bones of the skeleton. However, muscles also enable the heart to beat and can be found in the walls of hollow organs, such as the intestines, uterus and stomach.

In this article, we will explore the many functions of muscle in the human body as well as its basic structure, types and classifications.

Key facts about muscle tissue
Types Striated muscle (skeletal, visceral striated, cardiac)
Non striated (smooth) muscle
Skeletal muscle Striated muscle; formed of long, multinucleate, unbranched myocytes
Attached at one or either ends to a bony attachment point
Cardiac muscle Striated muscle; formed of short, uninucleate, branching myocytes which connected at intercalated discs
Specialized muscle of the heart → myocardium
Smooth muscle Non striated muscle; formed of short, uninucleate, spindle shaped myocytes
Located in the walls of internal organs, blood vessels etc.
Connective tissues Endomysium: loose connective tissue surrounding muscle cells/fibers
Perimysium:
Fibrous sheath which divides muscle tissue into fascicles
Epimysium:
Fibrous sheath which surrounds entire skeletal muscles
Contents
  1. Overview
  2. Structure of muscle tissue
  3. Skeletal muscle
    1. Sarcomere
    2. Connective tissues of muscle
    3. Gross anatomy
    4. Visceral striated muscle
  4. Cardiac muscle
    1. Pacemaker cells and innervation
  5. Smooth muscle
    1. Smooth-muscle-like cells
  6. Clinical correlations
    1. Dermatomyositis and polymyositis
    2. Muscular dystrophy
    3. Muscle atrophy
    4. Sleep twitches
  7. Sources
  8. Related articles
+ Show all

Overview

Muscle tissue has four main properties:

  • Excitability: an ability to respond to stimuli
  • Contractibility: an ability to contract
  • Extensibility: an ability to be stretched without tearing
  • Elasticity: an ability to return to its normal shape

Through these properties, the muscular system as a whole performs several important functions. These include the production of force and movement, support of body stature and position, stability of joints, production of body heat (to maintain normal body temperature), as well as, provision of form to the body.

Structure of muscle tissue

Regardless of its morphology or type, muscle tissue is composed of specialized cells known as muscle cells or myocytes (myo- [muscle, Greek = mys]), commonly referred to as muscle fibers (all of these terms are interchangeable); this is due to their extensive length and appearance.

Myocytes are characterized by protein filaments known as actin and myosin that slide past one another, producing contractions that move body parts, including internal organs. Interestingly, these proteins are not exclusive to muscle cells; actin and myosin are commonly found as cytoskeletal elements in many cell types and are involved in cellular functions relating to the changing of cell shape (e.g. cell movement, phagocytosis etc.). Myocytes however, are characterized by a particular abundance of these proteins within their cytoplasm, so much so that they occupy most of their interior. Furthermore, in the case of myocytes, actin and myosin filaments are generally oriented along a single axis, thereby eliciting movement in a linear fashion.

At its most basic level, muscle tissue is classified as either striated or non-striated/smooth based on the presence or absence of ‘striations’ (i.e. stripes/furrows) seen at a microscopic level; these are formed due to a particular arrangement of actin and myosin filaments within the myocyte (discussed below).

Striated muscle can be further subdivided into three classifications based on its location and morphology: 

  • skeletal muscle
  • visceral striated muscle
  • cardiac muscle

Skeletal muscle

Skeletal muscle is the most common type of muscle tissue found in the body and consists of highly elongated, multinucleate, non branching cells which are arranged in a parallel manner. Skeletal myocytes often measure several centimeters, or tens of centimeters in length, with the number of nuclei contained within being proportional to their length. They are biologically classified as ‘syncytia’; cells which are formed by fusion of several smaller, mononuclear cells. In the case of skeletal muscle, the cells which merge to form myocytes are known as myoblasts.

Skeletal muscle is often referred to as ‘voluntary’ muscle due to the fact that we think of its contraction being under conscious control; this is a misconception however, as skeletal muscle is involved in various movements which are under a subconscious level (e.g. breathing).

Sarcomere

The cytoplasm of each skeletal muscle fibers/myocytes (referred to as sarcoplasm (sarco-, commonly used to denote muscle-related terms [flesh, Greek = sarx])) is largely occupied by subunits known as myofibrils which extend the length of the cell. Myofibrils are essentially chained structures composed of repeating units of contractile units known as sarcomeres. These are primarily composed of two protein myofilaments, thin actin filaments and thick myosin filaments and are responsible for muscular contraction.
The arrangement of actin and myosin filaments within each sarcomere is very regular and contributes to the formation of distinct striations (i.e. stripes) of light A bands and dark I bands when viewed under light microscope.

Structure of a sarcomere in skeletal muscle

Myosin is visible as the A band of the sarcomere. Actin filaments are anchored to a structure known as the Z disc (or Z line) located at either end of the sarcomere; they are present across the entire length of the I band (region of actin filaments only, no myosin) and a portion of the A band (region where actin and myosin filaments overlap). The A band is especially important when considering the dynamics of muscle contraction, as it is the location where filament movement occurs. Actin filaments do not extend completely into the A bands, leaving a central region, known as the H zone, which appears slightly lighter than the rest of the A band due to the fact that it does not contain both myofilaments. The center of the H zone has a vertical line, known as the M line, which connects myosin filaments to each other.

During contraction, sarcomere length shortens due to ‘walking’ of the myosin filaments along the actin towards each z disc, pulling them centrally; this action in turn reduces the size of the H zone. This mechanism for contraction is known as ‘sliding filament theory’.

Hypertrophy (growth) of adult skeletal muscle occurs due growth of existing muscle fibers:

  • growth in the girth of muscle fibers is thought to take place through ‘splitting’ of existing myofibrils as a result of stress placed on sarcomeres during physical activity, thereby increasing the mass of the muscle as whole.
  • growth in the length of muscle fibers occurs as a result of new sarcomeres being added to the end of existing myofibrils.
Key terms related to sarcomeres
Actin Thin filaments of the sarcomere
Myosin Thick filaments of the sarcomere
I band
Light band of the sarcomere; contains only actin (thin) filaments (I band= isotropic)
(memory aid: I is the lIght band)
Z disc
a cross-striation which bisects the I band, marking the beginning and end of one sarcomere; serves as an anchoring point actin filaments
(Z = Zwischenscheiben / ’in between’ discs)
A band Dark band of the sarcomere; contains both actin (thin) and myosin (thick) filaments (A band = anisotropic band)
(memory aid: A is the dArk band)
H zone Lighter zone of the A band which lacks actin filaments; contains only myosin
(H zone = Hellezone / ‘light zone)
M line A fine line in the center of the H zone/A band which connects myosin filaments to one another
(M line = Mittelscheibe / ‘middle disc’)

Connective tissues of muscle

Skeletal muscle fibers are bound together by loose areolar tissue, known as endomysium (endo- = within) which contains the usual complement of cells such as fibroblasts and macrophages, as well as small nerve and vascular branches.

Bundles of striated muscle fibers which tend to work together to perform a specific function are enclosed by a thicker layer of connective tissue known as perimysium (peri- = around), to form muscle fascicles. These fascicles, in turn, are grouped together by a final outer covering of dense connective tissue, known as epimysium(epi- = outside), which forms the muscle [organ] as a whole. The endomysial, perimysial and epimysial coverings merge where muscles attach to adjacent structures forming tendons, fasciae or aponeuroses.

Gross anatomy

Skeletal muscle is found in many sizes and various shapes. The small muscles of the eye may contain only a few hundred myocytes/muscle fibers, while the vastus lateralis muscle of the thigh may contain hundreds of thousands of myocytes.

The shape or form of muscle is generally dependent on its fascicular architecture and fiber length which also helps to define the muscle’s function e.g. some muscles, such as the gluteal muscles, have numerous thick, short fascicles, while others such as the sartorius muscle have a lesser amount of long and relatively slender fascicles. These differences in muscle shape and fiber arrangement permit skeletal muscle to function effectively over a relatively wide range of tasks.

Fascicles or bundles (group of muscle fibres) of skeletal muscles can be arranged into four basic structural pattern, circular, parallel, convergent, and pennate. This difference in fascicular arrangement also accounts for the different shapes and functional capabilities of various skeletal muscles.

Circular

Circular muscles (also known as skeletal sphincter muscles) have a fascicular pattern where the fascicles are arranged in concentric rings. Muscles with this arrangement surround external body openings, which they close by contracting. Muscles with this fascicular arrangement may be termed as orbicular muscles (Latin: orbiculus = small disc), such as the orbicularis oculi (which covers the eye) and orbicularis oris (which surrounds the mouth).

Convergent

A convergent (a.k.a. triangular) muscle has a broad origin with fascicles converging toward a single tendon of insertion. Such a muscle is triangular or fan shaped. One example is the pectoralis major muscle of the anterior thorax.

Parallel

In a parallel arrangement, the length of the fascicles run to the long axis of the muscle. There are three types of parallel muscles:

Pennate

In a pennate pattern, the fascicles are short and they attach obliquely to a central tendon that runs the length of the muscle. Pennate muscles are of three forms:

  • Unipennate, in which the fascicles insert into only one side of the tendon, as in the extensor digitorum longus muscle of the leg;
  • Bipennate, in which the fascicles insert into the tendon from opposite sides. The tendon is central giving the muscle a resemblance of a feather. The rectus femoris of the thigh is bipennate;
  • Multipennate, which looks like many feathers side by side, with all their quills inserted into one large tendon. The deltoid muscle, which forms the roundness of the shoulder is multipennate.

Want to quickly master the names of all major muscles in the body? Build the foundations of your muscular system knowledge with our free muscles quiz guide.

Visceral striated muscle

Visceral striated muscle is structurally identical to skeletal muscle (i.e. looks the same in microscopic preparations). However, it is limited to a number of soft tissue structures, namely the tongue, pharynx and upper third of the esophagus.

Test your knowledge on the histology of the skeletal muscle with this quiz.

Cardiac muscle

Like skeletal muscle, cardiac muscle is striated and hence is composed of similar contractile proteins which are also structurally arranged into sarcomeres (discussed above). It is here however, where most similarities between these muscle tissue types ends. Cardiac muscle cells/fibers (cardiomyocytes) are much shorter and broader compared to those found in skeletal muscle and are branched at their ends. They are generally uninucleate (i.e. have one nucleus each), however sometimes may be binucleate. The nucleus is centrally located compared to those seen peripherally in skeletal myocytes, with the myofibrils passing on either side, leaving a clear zone of perinuclear sarcoplasm around the nucleus.

Striations in cardiac muscle are not as defined as those seen in skeletal muscle as they are slightly obscured by relatively large amounts of mitochondria and other organelles present in the cell (reflecting the higher metabolic demands of this tissue compared with skeletal muscle). Cardiomyocytes are connected at their ends by specialized junctional complexes known as intercalated discs; these serve to functionally couple all cardiomyocytes, thus allowing rapid propagation of signals for contraction across the heart muscle tissue. In light microscopy, they are identified as dark staining, irregularly arranged short lines which cross the cardiac muscle tissue, perpendicular to the fiber direction.

Cardiomyocytes are surrounded by fine, loose connective tissue, similar to endomysium seen in skeletal muscle albeit less organized. Condensations of perimysial-like dense connective tissue can be observed dividing groups of cardiac muscle cells/fibers into fascicles which, unlike that seen in skeletal muscle, whirl and spiral in a multidirectional manner (except in the case of the papillary muscles). As a result, cut sections of cardiac muscle tissue will usually present various orientations of muscle fibers adjacent to one another.

Pacemaker cells and innervation

Cardiac pacemaker cells (a.k.a. ‘stimulating’ cardiomyocytes, cardiac conducting cells) are highly specialized/modified cardiomyocytes which are capable of generating and carrying contractile signals across the myocardium. These are arranged into nodes (sinuatrial and atrioventricular), atrioventricular ‘bundles’ (of His) and subendocardiac conducting networks (commonly referred to as Purkinje fibers) to form the conducting system of the heart.

These networks are regulated by both parasympathetic and sympathetic divisions of the autonomic systems which send branches to the nodes mentioned above; sympathetic input increases heartbeat, while parasympathetic signals slow it down. Impulses carried to the heart by these autonomic nerve endings do not initiate contractile impulses but rather modify heart rate by their influence over cardiac pacemaker cells.

Learn more about the conducting system of the heart in this study unit. 

Smooth muscle

Smooth muscle is most commonly found in the walls of tubular structures (e.g. vessels, gut, ducts, bronchi etc.) as well as hollow organs (e.g. urinary bladder, uterus) and principally functions to modify the diameter/size of these structures in order to propel/expel the contents within (or alternatively to contain contents within an organ, in the case of sphincters). When compared to skeletal/cardiac muscle, smooth muscle is morphologically and functionally much more diverse and is subject to subconscious/involuntary control. Therefore, its arrangement varies from organ to organ; nevertheless there are several common attributes which will be discussed below.

Smooth muscle cells are generally uninucleate and are much smaller and shorter than those seen in skeletal muscle. They are spindle-shaped with long tapered ends and are usually packed together with their long axes parallel to neighboring cells in an ‘interdigitating’ manner. Each cell is enveloped by a basement membrane and other connective fibers which bridge the spaces between adjacent cells; condensations of these extracellular structures, known as dense plaques, provide a region of attachment for the smooth muscle cells. Two adjacent dense plaques allow for cell–to-cell attachment, providing mechanical stability to the tissue. The components of the extracellular matrix are produced by the smooth muscle cells themselves, rather than fibroblasts as seen in skeletal muscle.

Actin and myosin filaments in smooth muscle

One of the primary differences between smooth muscle and skeletal/cardiac muscle cells is the fact that the contractile proteins (actin/myosin) are not organized into sarcomeres; therefore they lack striations as seen in other muscle tissue types. Instead, actin (thin) and myosin (thick) filaments are scattered across the sarcoplasm of the cell. Actin filaments are attached to condensations of cytoskeletal intermediate filaments known as dense bodies (which therefore are functionally equivalent to Z-discs seen in skeletal muscle) as well as dense plaques mentioned above. Myosin filaments are located between actin filaments. During contraction they cause actin filaments to slide past each other, causing the cell to shorten mainly along its long axis.

Cell-to-cell communication is largely facilitated via gap junctions which are located close to openings in the basement membrane, allowing the regulation and propagation of contractile signals across the smooth muscle tissue.

Learn more about smooth muscle in the following study unit. 

Smooth-muscle-like cells

It’s also worth noting there are several other contractile cells which resemble smooth muscle cells (either morphologically and/or functionally) however are not classed as such. Examples include:

  • pericytes (found in the capillary walls)
  • myoid/peritubular cells (testis),
  • myoepithelial cells (often found in glandular epithelium) and
  • myofibroblasts (skin, wound healing).

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