Introduction

The muscle cell, also known as the myocyte is the smallest subunit of all muscular tissues and organs throughout the body. It is here in the myocyte, where the physiological steps of muscle contraction and where the pathophysiology of numerous muscular diseases takes place. There are three types of muscle cells in the human body: skeletal, smooth, and cardiac muscle. The common function of each specialized myocyte is the contraction of their various organs, some essential for life. Therefore, dysregulation of these crucial functions can lead to significant morbidity and mortality. This article examines the role of the muscle myocyte in various systems, the physiology of myocyte contraction and the pathophysiology of diseases involving the myocyte.[1][2]

Issues of Concern

• Describe the specialized structures within myocytes and the physiological mechanism of contraction.
• A brief discussion of myogenesis.
• Discuss the function of myocytes in their organ systems.
• Highlight some of the diseases involved in myocyte dysfunction.

Cellular Level

The muscle myocyte is a cell that has differentiated for the specialized function of contraction. Although cardiac, skeletal, and smooth muscle cells share much common functionality, they do not all share identical features, anatomical structures, or mechanisms of contraction.

Skeletal Muscle Myocyte

Skeletal muscle myoblasts (progenitor myocytes) differentiate and fuse into multinucleated muscle fibers called myofibrils that behave as a unit. These myofibrils, in turn, are composed of overlapping thick and thin filaments (myofilaments) that are arranged longitudinally into sarcomeres. Thus, shortening or contraction of skeletal muscle fibers is a result of sarcomere shortening. Thick filaments are composed of myosin, which is a protein polypeptide. Each myosin molecule has two globular heads which are involved in the contraction through binding thin filaments. Thin filaments include actin (contains a binding site for myosin heads), tropomyosin and troponin (has three subunits: troponin T, troponin I and troponin C). These sarcomere structures give skeletal muscle its striated appearance and are readily visible on electron microscopy.[3][2]

Skeletal myocytes also contain structures called T tubules which are extensions of the myocyte plasma membrane. They are open to the extracellular space and function to carry depolarizing potentials to the intracellular space, allowing coordinated contractions. Also, T tubules contain dihydropyridine receptors which are essential for contraction after myocyte excitation. The sarcoplasmic reticulum (SR) is a fundamental structure in the skeletal muscle myocyte. It is the site of calcium (Ca2+) storage and regulation in the myocyte. The SR contains terminal cisternae which mechanically couple with T tubules and voltage-sensitive ryanodine receptors which are channels that release Ca2+. The SR also has a Ca2+ -ATPase channel that pumps Ca2+ back into the SR from the intracellular space after excitation. In skeletal muscle, one T tubule binds two terminal cisternae in a triad arrangement.[2]

The most popular model for understanding muscle contraction is the sliding filament model, which describes sarcomere shortening by recurrent myosin/actin interactions. During each interaction, the myosin heads work to bring adjacent actin free ends closer to the center of the sarcomere. In the resting skeletal muscle myocyte, tropomyosin blocks the myosin binding sites on actin.[4]

1. When a significant motor end plate depolarizing potential overcomes the threshold of the skeletal myocyte, the cell fires an action potential.
2. This depolarizing potential is propagated to T tubules, causing a conformational change in the dihydropyridine receptors.
3. The mechanically coupled terminal cisternae also undergo a conformational change, inducing Ca2+release from the SR and increased intracellular Ca2+concentration[Ca2+].
4. Ca2+ binds to troponin C, causing a conformational change in troponin that moves tropomyosin from the myosin binding site on actin. Note: in the presence of Ca2+, troponin C remains in this configuration, leaving the myosin binding site on actin available for myosin interactions.
5. At first, no adenosine triphosphate (ATP) is bound to myosin causing myosin to bind to actin (permanent lack of ATP causes permanent myosin and actin interaction and is the mechanism behind rigor mortis).
6. ATP then binds myosin and myosin disassociated with actin.
7. Hydrolysis of this bound ATP to ADP plus inorganic phosphate induces a change in myosin to the “cocked position.” Note: only ADP remains attached to myosin.
8. The myosin head then interacts with actin at another binding site.
9. ADP is released from myosin causing another change that results in the “power stroke.” At this time, myosin is bound to actin and will remain bound without more ATP.
10. Another ATP molecule binds myosin. The cycle continues, shortening the sarcomere as myosin slides along the actin.

Relaxation occurs when there is a decrease in excitatory motor end plate potentials, a decrease in action potentials and repolarization of the myocyte. Ca2+ gets sequestered back into the SR by Ca2+ -ATPase pumps, decreasing intracellular [Ca2+]. These myocytes also contain Na+/Ca2+ exchangers on the cell surface that use the Na+ electrochemical gradient to exchange Na+ into the cell in exchange for Ca2+ out of the cell. Ca2+ disassociates from troponin C, and tropomyosin blocks the myosin binding sites on actin once again.[2]

Some key points to note about the sarcomere on electron microscopy are as follows. The sarcomere appears between Z lines, and contraction approximates these Z lines. Contraction results in shortening of both the H (only thick filaments) and I (only thin filaments) bands. The A (length of a thick filament with overlying thin filament) band in the sarcomere always stays the same length.[4]

The following are noteworthy points about muscle contractions. Skeletal muscle is under voluntary control, except for reflexes and the diaphragm during involuntary respiration. Lower motor neurons innervate these myocytes from the spinal cord and respond to the neurotransmitter acetylcholine (ACh). When ACh binds its receptors on the myocyte, sodium (Na+) is allowed into the cell, causing the depolarization mentioned above. Maximum muscle tension occurs when there is an optimal overlap of thick and thin filaments. That is, all myosin heads can interact with actin. If the muscle fiber is stretched too far, thick and thin filament interaction decreases. Furthermore, if the muscle shortens too considerably, the large myosin heads crowd out each other, decreasing myofilament interactions. Lastly, the maximal velocity that a muscle fiber may contract will decrease as the load on the muscle increases.[5][6][7]

Smooth Muscle Myocyte

 Similar to skeletal muscle, smooth muscle cells also contain thick and thin filaments. However, unlike skeletal muscle, these myofilaments are not organized into longitudinal sarcomeres, and they do not contain troponin. The lack of sarcomeres and thus, lack of striations, gives smooth muscle its name. Smooth muscle myocytes fuse to form three types of muscle. Those myocytes that contract as separate units are named multi-unit smooth muscle. They are in the iris of the eye or the vas deferens. Multi-unit smooth muscle is usually highly innervated and under autonomic control. Smooth muscle cells that contract together are named single-unit smooth muscle. They are more common and can are present in the gastrointestinal tract, bladder, and uterus.[8][9]

In contrast to multi-unit smooth muscle, single-unit smooth muscle cells highly communicate for coordinated contractions. These cells are under autonomic control and modulation by hormones or neurotransmitters. Lastly, smooth muscle myocytes can differentiate into vascular smooth muscle. These cells are also responsible for blood pressure regulation.[8]

The mechanism of smooth muscle contraction is different from the mechanism described above for skeletal muscle. However, much like a skeletal muscle cell, an increase in intracellular [Ca2+] is the critical factor involved in muscle contraction. Multiple mechanisms may cause an increase in intracellular [Ca2+] in the smooth muscle myocyte. Depolarization of the myocyte after ACh binds its receptors on the cell surface, which subsequently opens voltage-gated L type Ca2+ channels. The opening of Ca2+ channels on the myocyte membrane secondary to hormone or neurotransmitter binding its receptor (ligand-gated). Hormones or neurotransmitters induce the release of Ca2+ from the sarcoplasmic reticulum (SR) through inositol 1,4,5-triphosphate (IP3) gated Ca2+ channels. Regardless of the mechanism of increased [Ca2+], the downstream mechanism remains the same.[2]

1. Ca2+ binds a molecule called calmodulin.
2. Ca2+-calmodulin complex subsequently activates an enzyme called myosin light-chain kinase. (Kinases serve the purpose of phosphorylation).
3. Then, myosin light-chain kinase phosphorylates (adds a phosphate group) to myosin.
4. Phosphorylated myosin binds to actin and begins contraction through the cross bridging cycle mentioned above under skeletal muscle. Note: The mechanism of using ATP for muscle contraction in skeletal muscle is the same in smooth muscle.
5. A decrease in intracellular [Ca2+] and increased activity of an enzyme called myosin light-chain phosphatase (removes phosphate from myosin) produces relaxation.

Intracellular [Ca2+] decreases by being pumped back into the SR by ATPase pumps or by Na+/Ca2+ exchangers on the cell surface.[2]

Note: Hormones that produce smooth muscle contraction or relaxation, do so by modulating intracellular [Ca2+] or myosin light-chain phosphatase. For example, nitric oxide provides relaxation by increasing myosin light-chain phosphatase activity.[10]

 Cardiac Muscle Myocyte

 The physiology of the cardiac myocyte is more intricate than skeletal or smooth muscle, although it shares some similarities. The cardiac myocyte contains sarcomeres like skeletal muscle, thus, is striated. The mechanism of muscle myocyte shortening is the same as skeletal muscle mentioned above. Cardiac myocytes have unique structures that are vital for the proper functioning of the heart. Intercalated disks which are present at the periphery of the cell, maintain adhesion between myocytes. Gap junctions, which are present at intercalated discs, allow electrical communication between cells. The rapid spread of the depolarizing potential between adjacent cells helps with coordinated contractions, which is vital for survival. Cardiac myocytes also contain T tubules. However, unlike skeletal muscle, one T tubule binds one terminal cisterna in a dyad arrangement. The sarcoplasmic reticulum is present in cardiac myocytes and also serves the function of Ca2+ storage.[11]

The action potential in a cardiac myocyte is unique. It consists of a resting phase called Phase 4, which is maintained by cell permeability to potassium (K+) out of the cell. Phase 4 is followed by Phase 0, which is characterized by a rapid upstroke/depolarization due to the opening of voltage-gated Na+ channels and Na+ influx into the cell. Phase 1 is the initial repolarization caused by the closing of Na+ channels and the opening of voltage-gated K+ channels. Phase 2 is called the plateau phase. In Phase 2, Ca2+ enters the cells through voltage-gated Ca2+ channels, while K+ continues to leave the cell. This balance of inward and outward cations maintains the plateau phase. In Phase 3, the Ca2+ channels close and the rapid efflux through open K+ channels results in repolarization of the cell.[12]

The cardiac myocyte can receive its stimulus from cardiac pacemaker cells in the SA or AV node, the bundle of His, bundle branches, or Purkinje cells.

1. The action potential from these cells (generally SA node and AV node), spreads along the cardiac myocyte membrane into T-tubules.
2. Ca2+ enters the cell during Phase 2 through L type Ca2+ channels.
3. Ca2+ entry induces Ca2+ release from SR, otherwise called Ca2+-induced Ca2+ release.
4. Ca2+ concentrations increase, and it can bind Troponin C and cause myosin/actin cross-bridge cycling mentioned above in the skeletal myocyte section.
5. Relaxation occurs when the Ca2+ is taken up into the SR by Ca2+-ATPase pumps or by Na+/Ca2+ exchangers on the cell membrane.[2][13]

Cardiac muscle tension and the ability to contract is directly proportional to the intracellular Ca2+ concentration. Thus, factors that increase intracellular Ca2+ cause an increase in the contraction force. For example, with increased heart rate, Ca2+ begins to pool in the myocyte and results in stronger cardiac contractions as well. Preload is a term for the end-diastolic volume (classically related to right atrial pressure). When more blood volume fills a heart chamber, the myocytes stretch and results in more forceful contractions. This phenomenon is called the Frank-Starling relationship and may be related to a stretch-induced increase in Ca2+ concentration. This relationship matches venous return to the heart and cardiac output from the heart. Afterload is the pressure against which the myocytes must contract. The maximal velocity of contraction decreases with increased afterload.[12][14]

Development

During embryogenesis and fetal development, the skeletal muscle myocyte derives from the mesodermal germ cell layer in a process called myogenesis. The process divides into three steps.

1. Multi-potential mesodermal cells become myoblasts.
2. Myoblasts exit from the cell cycle.
3. They differentiate into skeletal muscle fibers.

Numerous factors called myogenic regulatory factors (MRFs) in the myocyte play a vital role in this intricate process. Some key MRFs include Myf-5, MyoD, myogenin, and MRF4. They work at specific points in the process of myogenesis to influence gene transcription and proper myocyte functioning. Transcription factors and signal molecules from the nearby notochord, neural tube, surface ectoderm, and lateral plate mesoderm, also induce differentiation of the skeletal myocyte.[15][16]

The embryonic heart is the first organ to become functional, and the developing heart shows four chambers by day thirty-two. Cardiogenic mesoderm forms cardiac myocytes as part of the process called cardiogenesis. Through an intricate network of interactions by growth factors and transcriptional regulators (Wnt, BMP, Nkx2.5, FGF, Gata4), cardiogenic mesoderm forms large parts of the ventricles, atria, and outflow tracts. The cardiogenic mesoderm also contributes to the conduction system, endocardium, and the aortic and pulmonary cushions.[17]

The development and differentiation of smooth muscle myocytes are dependent on the various organ. For example, visceral smooth muscle development is different from the development of vascular smooth muscle. Thus, there have been smooth muscle precursors derived from predominantly form mesoderm, but neuroectoderm and endoderm are also contributory. Similar to skeletal and smooth muscle myocytes, numerous factors influence the development and differentiation of smooth muscle myocyte.[18][19]

The myogenesis of skeletal, cardiac, and smooth muscle is very complex and deserve individual attention beyond the scope of this article.

Function

In cardiac muscle, myocytes respond to action potential generated by the SA and AV node by contraction of the atria and ventricles, respectively. Abnormal cardiac myocyte function after myocardial infarction, for example, can lead to life-threatening arrhythmias. The contraction of skeletal muscle fibers is necessary for movement. Importantly, the diaphragm is made of skeletal muscle and is responsible for respiration. Smooth muscle cell contraction is involved in various organs, for example, the walls of the gastrointestinal tract, where smooth muscle contracts to propel food forward in a process called peristalsis. Contraction of vascular smooth muscle is responsible for helping to regulate blood pressure.[13][2]

Pathophysiology

The pathophysiology of diseases that affect muscle myocytes is secondary to mutations in genes and dysfunction of myocyte proteins.

Duchenne muscular dystrophy (DMD) is an X-linked inherited disorder that affects the dystrophin protein and is the most prevalent neuromuscular disorder. The dystrophin protein is responsible for anchoring intracellular cytoskeletal elements, to the myocyte membrane during contraction. Dystrophin can be completely absent or non-functioning, depending on the type of inherited mutation. Absence of the protein leads to myocyte death. The effects typically present as skeletal muscle with weakness in the pelvic girdle muscles that progressively involve other muscles. Muscle biopsy may show muscle replaced by fat. DMD affects cardiac myocytes to cause a dilated cardiomyopathy, which is typically the cause of death. DMD has an early onset of age, and children may be wheelchair users by ten years old. Genetic testing will confirm the diagnosis. Treatment of this disorder generally consists of systemic corticosteroids (prednisone or prednisolone).[20][21]

Becker muscular dystrophy (BMD) is an X-linked disorder that affects that is a milder form of Duchenne muscular dystrophy. Mutations in BMD results in partially functioning dystrophin proteins. Onset is usually later that DMD and the symptoms are not as severe.[22]

Myotonic muscular dystrophy is an autosomal dominantly inherited disease that results in CTG trinucleotide repeat expansions in the DMPK gene. This gene codes for an enzyme called myotonin protein kinase. Deficiency of this enzyme leads in myotonia (sustained muscle contraction), muscle weakness, and wasting. Myotonic muscular dystrophy also affects the cardiac conduction system (resulting in arrhythmias), eyes (cataracts), endocrine system (testicular atrophy), and central nervous system. If myotonic muscular dystrophy is suspected, genetic testing can confirm the diagnosis. Treatment consists of managing secondary complications.[23]

Dermatomyositis and polymyositis are autoimmune disorders that involve the myocyte. Skeletal muscle is usually affected, and the disease affects proximal muscles in the shoulder and hips. Dermatomyositis has cutaneous manifestations (malar rash, Gottron papules or heliotrope rash). Dermatomyositis involves humoral mediated inflammation of the muscle perimysium (the sheath surrounding a bundle of muscle fibers). Polymyositis, which has no cutaneous manifestations involves cell-mediated inflammation of the endomysium (the sheath that surrounds a single myocyte). Dermatomyositis and polymyositis are commonly associated with malignancies and other autoimmune disorders. Treatment involves systemic steroid and immunosuppressant agents.[24]

Mitochondrial myopathies are a group of rare disorders that usually present with myopathy and other manifestations. These disorders have mitochondrial inheritance patterns, which means that only mothers can pass mitochondrial DNA onto their offspring. In other words, an affected female will give mutated mitochondrial DNA to all of her offspring, but an affected male (who got the disease from his mother) will pass the mutated mitochondrial DNA to none of his offspring. These disorders demonstrate heteroplasmy, which is variable expression within a population. Imagine two siblings inherit mutated mitochondrial DNA from their mother. Heteroplasmy means that one of the siblings may be affected while the other is not, or the degree of their disease expression may not be equal. Symptoms of these disorders include skeletal muscle myopathy and myalgia (pain), smooth muscle myopathy (dysphagia), and cardiomyopathy. There are many other symptoms associated with the various syndromes. Diagnostic tests include muscle biopsies which may reveal “ragged red fibers” (accumulation of pathologic mitochondria) and genetic testing. No definitive treatment has demonstrated significant efficacy in treating these disorders.[25]

Like any other type of cell in the body, myocytes can become neoplastic (benign or malignant). Skeletal myocytes undergo neoplastic change to become rhabdomyoma (benign) or rhabdomyosarcoma (malignant). Smooth muscle cells undergo neoplastic change to become leiomyoma (benign) or leiomyosarcoma (malignant).

Clinical Significance

Antiarrhythmic drugs serve to control the heart rate in arrhythmias or to convert the rhythm back to normal sinus rhythm. There are four classes of antiarrhythmic, and they each affect different channels in the heart. Class 1 (Na+ channel blockers) and Class 3 (K+ channel blockers) affect channels in the myocyte and exhibit an effect on the myocyte action potential. Class 2 and 4 antiarrhythmic drugs exhibit their effects on the SA and AV nodes. Class 1 (Na+ channel blockers) affect Phase 0 (upstroke) of the cardiac myocyte action potential and have variable effects on Phase 3. The goal of Class 1 antiarrhythmic is to control the rhythm of the myocardium. Class 3 antiarrhythmic drugs (K+ channel blockers) affect Phase 3 of the cardiac myocyte action potential. A severe adverse effect of class 3 antiarrhythmics is a polymorphic ventricular tachycardia called Torsades de pointes, due to QT prolongation. Class 3 antiarrhythmic drugs are also therapeutic agents for rhythm control.[26]

Malignant hyperthermia (MH) is a life-threatening condition caused by inhaled anesthetics (halothane, sevoflurane, desflurane, isoflurane) or the muscle relaxant succinylcholine. The inheritance pattern for this condition is typically autosomal dominant. The pathophysiology of malignant hyperthermia involves mutations in the voltage-sensitive ryanodine receptors. In susceptible individuals, inhaled anesthetics or succinylcholine causes increased Ca2+ release from the sarcoplasmic reticulum (as a result of ryanodine receptor mutation). The patient develops hyperthermia and severe muscle rigidity that can lead to rhabdomyolysis. Dantrolene, which binds the ryanodine receptor and prevents Ca2+ release from the SR, is used to treat MH.[27]

Digoxin/digitalis is a drug that is used to treat patients with severe heart failure. Digoxin has been shown to reduce symptoms and hospitalizations without lowering mortality. Digoxin is an ionotropic agent because it increased cardiac contractility. Its primary mechanism of action is through blocking Na+/K+ ATPase pumps (which pumps Na+ out and K+ in) on the myocyte surface. Blocking this pump leads to an increase in intracellular Na+. Due to increased intracellular Na+ and decreased extracellular Na+ concentration, there is a decrease in the activity of a Na+/Ca2+ exchanger on the cell surface, which usually exchanges Na+ into the cell for Ca2+ out of the cell. Thus, digoxin ultimately results in increased intracellular Ca2+ concentrations and increased contractility. Increased Ca2+ also lengthens Phase 2 of the cardiac myocyte action potential and slow the heart rate.[28]

Rhabdomyolysis is a potentially severe condition that results from injury to skeletal muscle. Some causes of rhabdomyolysis include trauma, toxins, drugs, infections, muscle ischemia, exercise, neuroleptic malignant syndrome, and malignant hyperthermia — injury to the myocyte release cellular components such as myoglobin, creatine kinase and lactate dehydrogenase. Rhabdomyolysis may present with myalgias, weakness. Myoglobin begins to spill into the urine (myoglobinuria), and patients may describe their urine as “tea-colored.” More severe complications of rhabdomyolysis include acute renal failure and disseminated intravascular coagulation. Diagnostic testing for rhabdomyolysis includes following the creatine kinase level, which is very sensitive. Treatment is mainly supportive and provides for fluids to preserve renal function.[29]