Muscles are the largest soft tissues of the musculoskeletal system. Muscle is derived from the Latin word “musculus” meaning “little mouse”. The muscle cell, muscle fibre, contain protein filaments of actin and myosin that slide past one another, producing contractions that move body parts, including internal organs. Associated connective tissue binds muscle fibres into fascicles or bundles, and these associated connective tissues also convey nerve fibres and blood vessels (capillaries) to the muscle cells.
Muscle tissue has four main properties: Excitability (ability to respond to stimuli), Contractibility (ability to contract), Extensibility (ability of a muscle to be stretched without tearing) and Elasticity (ability to return to its normal shape). Through contraction, the muscular system performs the following important functions: production of force and movement, supporting of the body, changing of body posture, stability of joints, production of body heat (to maintain normal body temperature), as well as, provision of form to the body.
Although muscles produce heat energy, they also require energy to perform their functions. Muscles are predominantly powered by the oxidation of fats and carbohydrates, but anaerobic chemical reactions are also used. These chemical reactions produce adenosine triphosphate (ATP) molecules that are used up by myosin filaments during muscle contractions.
There are three types of muscles. They are the:
- Skeletal muscles, which move bones and other structures (e.g. the eyes)
- Cardiac muscles, which form most of the walls of the heart and adjacent great vessels, such as the aorta
- Smooth or Visceral muscles, which form part of the walls of most vessels and hollow organs, move substances through viscera such as the intestine, and control movement through blood vessels
Muscles are classified histologically into striated muscles and non-striated muscles based on the structural characteristic called “striations” which is due to the arrangement of the muscle fibre’s actin and myosin filaments. Based on this microscopic classification, skeletal and cardiac muscles are grouped as striated muscles, while the visceral muscle is non-striated.
The muscular system develops from the mesoderm layer (except for the muscles of the iris which develop from neuro-ectoderm, and the muscles of the esophagus which are believed to develop by transdifferentiation from smooth muscle) and consists of cardiac, smooth, and skeletal muscles. Myoblasts (embryonic muscle cells) are derived from mesenchyme (embryonic connective tissue). Muscle contraction, generated by actin and the motor protein, myosin, facilitates movement and drives physiologic processes including circulation, respiration, and digestion. Cardiac and smooth muscle tissues develop from local populations of mesenchymal cells (splanchnic mesoderm), while skeletal muscles develop from mesoderm within the somites.
Myogenesis begins within a somite when cells respond to growth factors and activate the expression of myogenic basic helix-loop-helix transcription factors (MyoD). These cells become committed muscle precursor cells, or myoblasts, which fuse to form multinucleated myotubes that consist of terminally differentiated muscle cells. Many of the molecular mechanisms that regulate embryonic muscle cell proliferation and differentiation are thought to be reactivated during adult muscle regeneration. For example, Wnt signaling can induce satellite cell proliferation and myoblast fusion.
Gross And Microscopic Anatomy
Skeletal muscle cells are multinucleated; in fibres that are several centimeters long, there are thousands of nuclei. Hence the larger the skeletal muscle cell, the more nuclei it contains. Skeletal muscles consist of non-branching fibres (unlike cardiac) bound together by loose areolar tissue containing the usual complement of cells such as fibroblasts and macrophages. The membranous envelope, or epimysium, is impervious to the spread of fluid such as pus.
Skeletal muscle is found in many sizes and various shapes. The small muscles of the eye may contain only a few hundred cells, while the vastus lateralis of the thigh may contain hundreds of thousands of muscle cells. The shape of muscle is dependent on its general architecture, which in turn helps to define the muscle’s function. Some muscles, such as the gluteal muscles, are quite thick; some, such as the Sartorius of the thigh, are long and relatively slender; and others, such as the extensors of the fingers, have very long tendons. These differences in muscle shape and architecture 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.
This pattern is also called sphincter. The fascicular pattern is circular when the fascicles are arranged in concentric rings. Muscles with this arrangement surround external body openings, which they close by contracting. The general term used for these kinds of muscles is “sphincter”. Examples include the orbicularis muscles surrounding the mouth and eyes.
A convergent muscle has a broad origin, and its fascicles converge 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.
In a parallel arrangement, the length of the fascicles run to the long axis of the muscle. Such muscles are either strap-like, for example the sartorius muscle of the thigh, or spindle shaped with an extended belly, like the biceps brachii muscle of the arm. However, some scientists classify spindle-shaped muscles into a separate class as fusiform.
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
Skeletal muscles are attached directly or indirectly through tendons to bones, cartilages, ligaments, fascia, or to some combination of these structures. Some are attached to organs (the eyeball, for example), to skin (such as fascial muscles), and to mucous membrane (e.g. the intrinsic tongue muscles). Skeletal muscles produce movements of the skeleton and other body parts. The major skeletal muscles are often called voluntary muscles because individuals can control many of them at will; however, some of their actions are automatic. For example, the diaphragm contracts automatically; a person controls it voluntarily, however, when taking a deep breath. Skeletal muscles are striated. Skeletal muscles attach to bones with their tendons. Some tendons form flat sheets called aponeuroses that anchor one muscle to another, for example, the oblique muscles of the anterolateral abdominal wall.
Cells of a cardiac muscle also have one nucleus each. The cardiac muscle consists of much broader, shorter cells that branch. Part of the boundary membranes of adjacent cardiac muscle cells make very elaborate interdigitations (branchings) with one another under microscopic examination. This broadness, and the interdigitations of the cardiac muscle increase its surface area for impulse conduction. The muscle cells are arranged in whorls and spirals; each chamber of the heart empties by mass contraction, not peristalsis. Unlike the smooth muscles, cardiac muscle fibres are striated (striped appearance) and are joined to one another, end to end by cell junctions formed by intercalated discs.
Cardiac muscle form the muscular wall of the heart (i.e. the myocardium of heart). Some cardiac muscles are also present in the walls of the aorta, pulmonary vein, and superior vena cava (SVC). Innervation, like that of the smooth muscle, is by the autonomic nervous system (ANS). Heart rate is regulated intrinsically by a pacemaker (the SA node) composed of special cardiac muscle fibres that are also influenced and innervated by the ANS.
The cell (fibre) of a smooth muscle has one nucleus like most body cells. Smooth muscle consists of narrow spindle-shaped cells usually lying parallel. In tubes that undergo peristalsis (anterograde directional movement), they are arranged in longitudinal and circular fashion, e.g., as in the alimentary canal and ureter. Contractile impulses are transmitted from one muscle cell to another at specialized sites called nexuses (or gap junctions), when adjacent cell membranes lie unusually close together.
Smooth muscles are found in middle layer (tunica media) of the wall of most blood vessels, and the muscular part of the wall of the digestive tract. They are also found in the eyeball, where it controls lens thickness and pupil size.
Innervation of the smooth muscle is by ANS; hence the smooth muscle is an involuntary muscle. In addition, because of the gap junctions between smooth muscle cells, many of the cells do not receive nerve fibres.