Skeletal Muscles

∞ generated and posted on 2020.01.27 ∞

Skeletal muscles are bundles of myocytes which collectively have the effect of exerting tension on tendons and for the most part thereby on bones as well.

Skeletal muscles – which consist of individual muscles fibers or myocytes, i.e., muscle cells – are the means by which animals move specific parts of their bodies relative to both other parts of their bodies and the world about them.

This page contains the following terms: Muscle, Myocyte, Myoblast, Myofibroblast, Skeletal muscle, Myofibril, Myofiber, Endomysium, Perimysium, Epimysium, Muscle contraction, Troponin, Isotonic, Isometric, Sarcoplasmic reticulum, Endoplasmic reticulum, Neuromuscular junction, Transverse tubules, Sarcolemma, Myofibril, Sarcomeres, Actin, Tropomyosin, Myosin, Sliding filament model, Power stroke

Muscle

Animal contractile tissue.

Muscles typically are classified into three types called smooth muscle, skeletal muscle, and cardiac muscle. The skeletal muscle and cardiac muscles have a striated appearance, that is, they are striped when viewed though a microscope, whereas smooth muscle lacks striation. Muscles consist of multiple individual cells that otherwise are known as muscle fibers or myocytes. These cells are multinucleated and form though the fusion of precursor cells known as myoblasts.

Muscles function by contracting, which has the effect of pulling together tissues to which the muscles are attached, with attachment as via tendons. With skeletal muscles, in vertebrate animals such as ourselves, these tissues consist particularly of bones. Contraction of skeletal muscles thus involves a shortening of muscle cells, which has the effect of both thickening muscles and causing especially specific bones to be pulled towards each other, which in turn results in either movement or a stabilization of specific portions of bodies.

Note that muscles consist of both muscle tissue and connective tissue as well as, to a more limited degree, epithelial tissue and nervous tissue.

The above video provides a nice overview of muscle physiology, especially that of skeletal muscle, provided from a muscle building perspective.

Myocyte	The cells that make up muscle tissue.
	Myocytes are also known as muscle cells, muscle fibers, or myofibers. It is within myocytes that myofibrils are found. It is immediately external to myocytes that endomysium is found. It is surrounding bundles of myocytes that perimysium is found. It is on the surface of myocytes that neuromuscular junctions are found.

Myoblast	Muscle fiber precursor cells.
	Myoblasts fuse together in the course of development to generate myocytes, i.e., muscle fibers.

Myofibroblast

Cell types that possess contractile tissue, connective tissue , wound healing, and paracrine functions.

Myofibroblasts thus have both fibroblast- and muscle cell-like properties, i.e., so as to provide fibers and/or ground substance as well as contractile properties, the latter being smooth muscle like. They are involved in wound healing and their persistence in tissues can be associated with disease.

As intermediates between contractile tissue and connective tissue, they play roles in regulating some soft-tissue shapes. Alternatively, they can represent smooth muscle cells that have partially differentiated into connective tissue cells. They can also be involved in local chemical signaling (paracrine functions).

Links to terms of possible interest: Skeletal muscles, Biceps brachii, Biceps femoris, Brachioradialis, Deltoid, External oblique, Gastrocnemius, Gluteus medius, Gluteus maximus, Gracilis, Latissimus dorsi, Masseter, Orbicularis oculi, Masseter, Rectus abdominis, Sartorius, Semimembranosus, Semitendinosus, Soleus, Sternocleidomastoid, Temporalis, Tibialis anterior, Trapezius, Triceps brachii, Vastus medialis

The above video describes how it is that muscles function.

The above video demonstrates a cute way to model in class the micro-anatomy of a skeletal muscle fiber.

Skeletal muscle

Animal contractile tissue that is under voluntary control.

Skeletal muscle, more precisely stated, is under control of what is known as the somatic nervous system (which in humans corresponds to voluntary control). Skeletal muscle is also striated—heart muscle, i.e., cardiac muscle, is also striated, but not under voluntary control. Contrast smooth muscle which is neither under voluntary control nor striated.

Skeletal muscle is made up of multinucleated muscle cells called muscle fibers (a.k.a., myocytes) and these are embedded within a sheath of connective tissue. Multiple muscle fibers bundled together make up individual muscles, which in turn, for skeletal muscle, are typically connected to bone via connective tissue outgrowths called tendons.

Links to terms of possible interest: Connective tissue, Epimysium, Motor neuron, Multinucleated, Myocyte, Myofiber, Myofibril, Myonucleus, Myotendinous junction, Perimysium, Sarcolemma, Satellite cell, Skeletal muscle

Myofibril	Long bundles of actin and myosin proteins as found within muscle cells.
	Myofibrils consist of actin and myosin proteins as well as an elasticity-conveying proteins called titin. The sarcomere is a component of myofibrils and multiple myofibrils are arrayed in parallel within the cytoplasm of muscle cells.

Myofibers	Cells that make up muscle tissue.
	Myofibers are also known as myocytes. It is within myofibers that myofibrils are found. It is immediately external to myofibers that endomysium is found. It is surrounding bundles of myofibers that perimysium is found. It is on the surface of myofibers that neuromuscular junctions are found.

Endomysium	Nerve and capillary containing connective tissue that encases individual muscle fibers.
	The endomysium, or endomysia as the plural, collectively are further encased (or ensheathed) within as well as continuous with the perimysium. The endomysium consists of areolar connective tissue, which is a kind of loose connective tissue.

Perimysium	Connective tissue in which bundles of muscle fibers are embedded.
	These bundles of muscle fibers are known as muscle fascicles.

Epimysium	Connective tissue that covers entire muscles.
	The epimysium, or epimysia as the plural, is dense connective tissue that is continuous with the dense connective tissue of tendons.

The above video provides the very basics of what skeletal muscles are all about. Note that there is no narration.

Here is essentially the same thing as immediately above but in slightly more detail.

Muscle contraction

Myocyte shortening due to action of actin and myosin.

Muscle contraction is initiated via a stimulus such as provided by a neuron (as is the case with skeletal muscle) or instead via the action of hormones (as seen with smooth muscle) and is implemented by the sliding of actin and myosin forcibly past each other (as occurs at the expense of ATP).

In between is conduction of a nerve-like impulse across the muscle fiber (myocyte) plasma membrane, further conduction into the body of the muscle fibers through tunnels known as T tubules, stimulation of specialized endoplasmic reticulum known as sarcoplasmic reticulum, and then release of Ca²⁺ (calcium ions) into the muscle fiber cytoplasm.

The calcium ions interact with a protein called troponin as found within a troponin-tropomyosin complex, which in the absence of calcium ions interferes with myosin binding to actin. Calcium ion binding to the troponin-tropomyosin complex thus allows muscle contraction to proceed. Calcium ion, however, is rapidly taken back up into the sarcoplasmic reticulum, and thus away from troponin-tropomyosin complexes, resulting in a lack of continuing muscle contraction unless further muscle stimulation occurs.

ATP plays multiple roles in this process, particularly stimulating the release of myosin from actin, then charging myosin for further binding, and then pushing against actin (in a process known as the sliding-filament mechanism). When a muscle cell dies, calcium ions are leaked into the muscle cell cytoplasm, allowing myosin binding to actin. A lack of ATP, since dead cells cannot generate ATP, prevents subsequent release, resulting in a locking together of these two proteins that we call rigor mortis.

Troponin	Protein to which calcium ions bind stimulating muscle contraction.
	Troponin interacts with the protein tropomyosin which in turn is found between the proteins actin and myosin in resting muscle. The binding of troponin to myosin causes tropomyosin to move from its position between actin and myosin, allowing those proteins to interact, which in turn powers muscle contraction.

Links to terms of possible interest: Isometric muscle contraction, Isotonic muscle contraction, Muscle contraction

Isotonic	Muscle tissue contraction that does not involve change in muscle tension despite muscle change in length.
	In an isotonic exercise you are moving something but at a constant rate. Note that generally this movement/muscle contraction cannot go on forever since muscles can only contract so far. Weight lifting for body building, when done well, involves isotonic contractions. Contrast with isometric.

Isometric	Muscle tissue contraction that does not involve change in muscle tissue length.
	Isometrics are a comparatively unusual form of exercise in which the force exerted by a muscle exactly counteracts the force exerted by an object. Picture yourself in an arm wrestling match in which neither individual is winning. Contrast with isotonic.

The above video is a chalk talk walking through muscle contraction starting with the motor neuron and ending with reference to the sliding filament model.

Sarcoplasmic reticulum

Specialized membranes found within muscle cells that upon stimulation release calcium ions into the muscle cell cytoplasm.

Sarcoplasmic reticulum (SR) is found in close association with actin-myosin filaments within muscle cells and are stimulated via the impulses that travel down from the muscle cell plasma membrane through what are known as transverse tubules.

The released calcium ions interact with troponin-tropomyosin complexes which, in the absence of calcium ions, inhibit muscle contraction. Following calcium ion release, these ions are then taken back up into the sarcoplasmic reticulum at the expense of energy, i.e., ATP. They then become available for release upon further stimulation.

The sarcoplasmic reticulum furthermore can be viewed as a specialized form of what otherwise are the commonplace organelles found within eukaryotic cells called endoplasmic reticulum.

Links to terms of possible interest: Ca²⁺, Muscle contraction, Muscle fiber, Myofibril, Sarcoplasmic reticulum, T tubule, Transverse tubule

Endoplasmic reticulum

Series of endomembrane cisternae and tubes found within eukaryotic cells involved in various metabolism functions such as the biosynthesis of lipids and endomembrane-associated proteins.

Endoplasmic reticulum generally comes in two forms, smooth endoplasmic reticulum and rough endoplasmic reticulum. Rough endoplasmic reticulum is so called because of its association with ribosomes, where are organelles that are involved in protein synthesis within cells. Cisternae are enclosed volumes, in this case small, membrane-enclosed sacs that are associated with numerous metabolic processes within cells.

Excellent video which emphasizes the various pre-contraction aspects of muscle contraction, with some central emphasis on the role of sarcoplasmic reticulum.

Neuromuscular junction	Point of connection between muscle cells and the neurons that control them.
	Neuromuscular junctions are the points at which nervous system control of muscle functioning (contraction) is effected. They are contact points between individual muscle cells and individual neurons. Their role is to initiate action potentials in stimulated myocytes. These action potentials are then propagated first along the sarcolemma and then deeply into these cells along transverse tubules.

Transverse tubules

Elongated plasma membrane indentations found in muscle cells along which impulses travel to sarcoplasmic reticulum.

The plasma membrane of muscle fibers is described also as the sarcolemma and the transverse tubules (T tubules) represent invaginations into the sarcolemma.

The process of nerve impulse conduction is one of depolarization across the plasma membrane, which in turn involves the movement of sodium ions (Na⁺) and potassium ions (K⁺) across that membrane. What the T tubules allow is a continuation of this depolarization event deep into the muscle fibers so that it can occur closely adjacent to the sarcoplasmic reticulum.

This spatial/physical proximity allows the rapid stimulation of calcium ion release from the sarcoplasmic reticulum once the sarcolemma is depolarized. This can occur because that depolarization in part occurs in close physical proximity to the sarcoplasmic reticulum due to the T tubule penetration deep into the muscle cell.

Links to terms of possible interest: Action potential, Ca²⁺, Muscle contraction, Muscle fiber, Muscle triad, Myofibril, Sarcolemma, Sarcoplasmic reticulum, T tubule, Terminal cisterna, Thick filament, Thin filament, Transverse tubule

Quick, somewhat comprehensive overview of the cell biology of muscle contraction including consideration (0:20 to 0:51) of the role of T tubules in the process.

Sarcolemma

Plasma membrane of myocytes.

In muscle cells (myocytes) the plasma membrane invaginates thereby increasing its surface area as well as contact with the relative interior of cells. Sarcolemma is also an excitable membrane, able to conduct membrane action potentials from neuromuscular junctions, where these action potentials are initiated, to the vicinity of sarcoplasmic reticulum, from which calcium ions are released into the interior of myocytes.

Myofibril

Long bundles of actin and myosin proteins as found within muscle cells.

Myofibrils form in the course of what is known as myogenesis, which is the formation of muscle fibers. Much of this process of myogenesis occurs during embryogenesis, i.e., before we are born. Myogenesis involves the fusion of progenitor muscle cells called myoblasts and this results in the generation of large, multinucleated cells that we call muscle fibers. It is within these muscle fibers that multiple, individual myofibrils are found and it is during myogenesis that the myofibrils form.

The myofibrils begin to form within the myoblasts prior to the formation of muscle fibers. They spontaneously form once the proteins that make them up are available within cells. Within the muscle fiber the myofibrils are intimately associated with and indeed surrounded by sarcoplasmic reticulum, which supplies the calcium ions that must reach the myofibrils for muscle contraction to occur.

Links to terms of possible interest: Actin, Connective tissue, Deep fascia, Epimysium, Muscle fascicle (Fasciculus), Muscle belly, Myocyte, Myofiber, Myofibril, Myonucleus, Myosin, Perimysium, Sarcolemma, Sarcoplasm, Satellite cell, Striated muscle, Tendon

Sarcomeres

Repeated striations found along myofibrils that give skeletal and cardiac muscle striped appearances at microscopic scales.

The sarcomeres actually are units of bundled myosin that are separated from other such units so that in the course of muscle contraction the myosin units can pull themselves towards each other (that is, along associated actin filaments). This pulling in addition to shortening the distance between bundles of myosin, also shortens myofibrils, which in turn shortens muscle fibers (i.e., muscle cells/myocytes), thereby shortening muscles.

The sarcomeres are visible particularly in the more structurally organized skeletal muscle and cardiac muscle, which as a consequence is described as striated muscle. Smooth muscle, by contrast, is less well structurally organized and therefore is not visibly striated.

Links to terms of possible interest: Actin, H band, H zone, I band, M line, Muscle cell, Muscle contraction, Muscle fiber, Myofibril, Myosin, Sarcomere, Striation, Thick filament, Thin filament, Z line

The above video does a pretty good job showing explicitly what sarcomere consists of, keeping in mind that what you are looking at are multi-protein-scale structures.

The above video provides an overview of what sarcomeres are all about. It's not perfect but it's also not a bad place to start in learning about the different parts that make up a muscle cell.

Actin

Protein constituent of microfilaments that plays a key role in muscle contraction.

Actin forms into long chains within eukaryotic cells that make up – along with microtubules and intermediate fibers – what is known as the cell's cytoskeleton. The cytoskeleton is involved in maintaining cell shape, anchoring both the plasma membrane and organelles that are found within the cytoplasm, and, if the cell is so capable, effecting cell movement.

Actin itself is involved in cytokinesis, organelle movement, etc. As such, actin is a ubiquitous protein and one that apparently has been coopted by animals to serve as one basis of the contractile ability of muscle cells.

In muscle contraction, the protein myosin pushes against the more rigid actin filaments, pulling itself along these actin filaments (or thin filaments, which otherwise are known as microfilaments). In the process, both actin filaments and myosin molecules are drawn towards each other, resulting in a shortening of myofibrils and thus a shortening of muscle fibers themselves.

Links to terms of possible interest: Actin, Ca²⁺, Myosin, Sarcomere, Tropomyosin, Troponin

Tropomyosin	Actin-associated proteins that serve to regulate actin functioning.
	Tropomyosins are associated with actin as found in both muscles and in other cell types. Within muscles they serve to interfere with actin-myosin interaction, thus preventing muscle contraction.

The above video is at best tangential to our interest in actin functioning within sarcomeres, but the video is short and there are relatively cool graphics.

Myosin

Protein that plays a key role in muscle contraction by pushing against actin fibers.

Myosin-like proteins, as with actin, are fairly commonly found within eukaryotic cells. Their general function is to provide the mechanical action of pushing and associated movement.

This pushing occurs against actin molecules, and in animals this propensity has been coopted to provide muscle contraction, which can be viewed as within-cell mechanical action that is translated into whole-cell movement (specifically, contraction), which in turn is transformed into muscle contraction (and which has the result of pulling two bones towards each other).

The pushing mechanism is powered by ATP, which effects the separation of myosin and actin from each other and then imparts on myosin a shape change that can be triggered, upon subsequent myosin association with actin, into what is known as a power stroke.

Links to terms of possible interest: Myofilament band, Myosin, Sarcomere, Thick filament

Sliding filament model

Description of the molecular functioning of sarcomeres.

The sliding filament model is a description of the myofibril contraction as involves the sliding of myosin fibers relative to actin fibers.

This is mediated by a combination of regulation of myosin access to actin (as via the actions of the proteins troponin and calcium ions) and the pushing action of myosin when myosin is able to gain access to the actin filaments.

Note that ATP hydrolysis is required for detachment of myosin from actin as well as, subsequently, the myosin-mediated power stroke when myosin can again gain access to actin.

Links to terms of possible interest: Actin filament, ADP, ATP, Cross bridge, M line, Muscle contraction, Myofilament, Myosin, Pi, Powerstroke, Thick filament, Thin filament

The above video is a brief but professionally presented look at the how myosin interacts with actin to effect sarcomere shortening.

Power stroke

Molecular process of myosin pushing against actin.

The power stroke is powered by the conformational change in the shape of the protein myosin as it releases the phosphate donated from ATP during the previous step of myosin release from actin.

The paddle shaped ends of the myosin proteins push against the actin microfilaments, resulting in the shortening of sarcomeres and associated myofibrils, and thus the shortening of myocytes and associated muscles.