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The human body is composed of three major muscle tissues: skeletal, smooth, and cardiac muscle. The process of muscle formation is called myogenesis, which can be divided into several distinct phases. Myogenesis occurs during embryonic development and in postnatal life, and it involves the differentiation, cell-cell fusion, and myonuclei migration of myofibers. Single-unit smooth muscle, also known as visceral smooth muscle, is myogenic, meaning it can contract without input from a motor neuron. Multi-unit smooth muscle, on the other hand, is neurogenic and requires stimulation from an autonomic nervous system neuron to contract. Skeletal muscle, which is responsible for voluntary movements, also undergoes myogenesis, and its formation is influenced by Myogenic Regulatory Factors (MRFs) such as Myf5, MyoD, and myogenin. While smooth and skeletal muscles exhibit myogenic properties, it is unclear if cardiac muscle, which is found in the heart, possesses the same characteristics.
| Characteristics | Values |
|---|---|
| Types of muscles | Cardiac muscle, skeletal muscle, smooth muscle |
| Smooth muscle embryological origin | Usually of mesodermal origin |
| Smooth muscle within the aorta and pulmonary arteries origin | Ectomesenchyme of neural crest origin |
| Smooth muscle within the coronary arteries origin | Mesodermal origin |
| Single-unit smooth muscle | Myogenic, can contract regularly without input from a motor neuron |
| Multi-unit smooth muscle | Neurogenic, contraction must be initiated by an autonomic nervous system neuron |
| Skeletal muscle | One of the three major muscle tissues of the human body |
| Skeletal muscle formation | Myogenesis |
| Skeletal muscle maintenance | Requires activation of satellite cells that can differentiate into new fibres |
| Skeletal muscle function | Dominates all voluntary movements, plays a role in movement, respiration, body temperature maintenance, and organ protection |
| Myogenic Regulatory Factors (MRFs) | Myf5, MyoD, myogenin, MRF4 |
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What You'll Learn
Cardiac muscle is myogenic
The heart is a myogenic muscle, which means that it generates a heartbeat on its own without any external stimulation. The cardiac muscle is one of the three major categories of muscles in the human body, including smooth muscle and skeletal muscle. Cardiac muscle, or myocardium, is a type of striated muscle composed of cardiomyocytes, which are responsible for the contractility of the heart. These cardiomyocytes are long, branched fibres connected via intercalated discs that contain gap junctions and desmosomes.
The contractile forces of the cardiac muscle determine the cardiac output, which is the amount of blood pumped into circulation to meet the metabolic demands of the body. This is defined as the heart rate multiplied by the stroke volume. The contractile functions of the heart require ATP, which can be obtained from various substrates such as fatty acids, carbohydrates, proteins, and ketones. The cardiac muscle is the hardest-working muscle in the body, contracting and relaxing 60-100 times per minute from intrauterine life until death, which can be well above 120 beats per minute during periods of stress and physical activity.
The cardiac muscle is similar to skeletal muscle in that it is composed of sarcomeres, which are the functional units of cardiomyocyte contraction. However, unlike skeletal muscle, the cardiac muscle is under involuntary control. The cardiac muscle transverse (T) tubular system consists of invaginations of the cell surface, and the sarcoplasmic reticulum associated with the T tubules takes the form of dyads rather than triads. Additionally, the cardiac muscle in the wall of the left ventricle has a heavy workload, forcing oxygenated arterial blood throughout the body.
The electrical activity of the heart regulates the heart rate, and electrocardiography (ECG) can be used to monitor and investigate this electrical activity. The speed, rhythm, and regularity of the heartbeat are critical factors in maintaining cardiac health. The cardiac muscle must also be adequately oxygenated to function properly, usually through an adequate coronary artery supply. Overall, the cardiac muscle plays a vital role in pumping blood into circulation and supplying the metabolic demands of the entire body.
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Skeletal muscle is myogenic
The process of myogenesis can be divided into several distinct phases. During embryonic myogenesis, mesoderm-derived structures generate the first muscle fibres of the body. In subsequent waves, additional fibres are generated along these template fibres. The process will give rise to long-range migratory cells that retain an extensive mitotic capacity, allowing their proliferation at target sites.
The Myogenic Regulatory Factors (MRFs) Myf5, MyoD, myogenin and MRF4 are members of the basic helix-loop-helix family of transcription factors that control the determination and differentiation of skeletal muscle cells during embryogenesis and postnatal myogenesis. Myf5 is considered to be the earliest expressed regulatory factor gene in myogenesis. If Myf-5 and MyoD are both inactivated, there will be a complete absence of skeletal muscle.
Myogenin (also known as Myf4) is required for the fusion of myogenic precursor cells to either new or previously existing fibres. Deleting myogenin results in a near-complete loss of differentiated muscle fibres and a severe loss of skeletal muscle mass in the lateral/ventral body wall.
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Smooth muscle is myogenic
Muscle tissue exhibits myogenic properties, which means that it can contract and relax spontaneously without any neural input. This is true for all types of muscle, including cardiac, skeletal, and smooth muscles. Smooth muscles are found in various parts of the body, such as the walls of blood vessels, airways, and the digestive tract. They play a crucial role in maintaining blood pressure and regulating the body's internal environment.
The myogenic behaviour of smooth muscles is particularly important in the context of vascular smooth muscle cells. These cells are responsible for regulating blood flow and maintaining blood pressure. When these cells are stretched, they depolarize, leading to contraction. This process is known as the myogenic response and is essential for maintaining proper blood circulation.
The myogenic response in smooth muscles is influenced by various physiological factors. For example, studies have shown that tumour necrosis factor (TNF) expression in resistance artery smooth muscle cells enhances microvascular myogenic vasoconstriction and affects blood pressure. Additionally, in diabetic individuals, the myogenic response in skeletal muscle arterioles is enhanced due to increased activation of voltage-dependent Ca2+ channels and/or protein kinase C in the smooth muscle.
Furthermore, the myogenic tone in smooth muscles is regulated by neurogenic and hormonal factors. In diabetic men, for instance, there is an impaired neurogenic and endothelium-mediated relaxation of penile smooth muscle, leading to impotence. These regulatory mechanisms ensure that smooth muscles maintain their tone and contractile function, contributing to overall physiological homeostasis.
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Myogenesis in embryonic development
Myogenesis is a complex, multistep process that refers to the formation and development of muscular tissue from undifferentiated cells. It involves the generation and regeneration of skeletal muscle. During embryonic development, myogenesis is influenced by a broad spectrum of signalling molecules that activate cell surface receptors, inducing intracellular pathways that lead to the expression of myogenic identity in muscle progenitors.
During early embryogenesis, one of the three germ cell layers of the embryo, the 'mesoderm' or the middle layer, segments into somites. The somites then undergo segmental distribution, with the upper layer breaking down into myotomes, which release muscular progenitor cells (MPCs). These MPCs residing in the myotome start expressing Pax3/7, which gives rise to muscle precursor cells or 'embryonic myoblasts'. Subsequently, these embryonic myoblasts differentiate into myocytes, which then fuse in response to fibroblast growth factor (FGF) to form primary myotubes. These primary myotubes act as a substrate for recruiting additional embryonic myoblasts.
The process of myogenesis is highly regulated by multiple regulatory factors, including MyoD, Myf5, myogenin, and MRF-4, which are members of the basic helix-loop-helix family of transcription factors. MyoD, for instance, is involved in the production of muscle cells in both the lateral and medial regions of the somite. It is also required for cell cycle arrest and differentiation. Myf5, on the other hand, is necessary for myoblast proliferation. The absence of both Myf5 and MyoD results in a complete lack of skeletal muscle and myogenin expression, as seen in Myf5:MyoD double-null mice.
The Notch signalling pathway, including ligands such as Delta1, also plays a crucial role in regulating myogenic differentiation during embryonic development. Active Notch signalling suppresses MyoD in cooperation with other factors, and mutations in the Delta1 ligand can lead to excessive myogenic differentiation and a loss of muscle precursors.
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![[(Myogenesis)] [Author: Grace Pavlath] published on (July, 2011)](https://m.media-amazon.com/images/I/41LzVT-sa3L._AC_UY218_.jpg)
Myogenic regulatory factors
The dynamics of their temporal and spatial expression, as well as their biochemical properties, have allowed for the identification of a precise and hierarchical relationship between the four MRFs. This relationship establishes the myogenic lineage as well as the maintenance of the terminal myogenic phenotype. The application of genome-wide technologies has provided new insights into how MRFs function to activate muscle gene expression.
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Frequently asked questions
Myogenic muscles are those that can contract regularly without input from a motor neuron. Single-unit visceral smooth muscles are myogenic.
The ciliary muscles, iris dilator muscle, and iris sphincter muscle are types of myogenic smooth muscles.
No, not all muscles are myogenic. Single-unit smooth muscles are myogenic, but multi-unit smooth muscles are neurogenic. Additionally, skeletal muscles and cardiac muscles are not myogenic.
