Embryology - Myogenesis
The formation of muscle tissues through the differentiation of progenitor myoblasts into myocytes during the development of an embryo is known as Myogenesis. The myoblasts are the progenitor cells of the muscle tissue. During embryonic development, the myoblasts either divide mitotically to give rise to more myoblasts or differentiate into myocytes or muscle cells. The decision as to either proliferate or differentiate is still unclear but in vitro studies showed that the presence of sufficient growth factors in a culture medium would result in the cell division of the myoblasts. In contrast, less growth factors in the medium resulted in the differentiation of the myoblasts. The stages of myogenesis are the following: delamination, migration, proliferation, determination, differentiation, specific Muscle Formation, and satellite cells. 1 In summary, the myoblasts begin to differentiate into myocytes by leaving the cell cycle and began expressing genes associated with the next stages. The myoblasts next align to one another and fuse. Genes that are essential during the fusion of myoblasts are myocyte enhancer factor-2 (Mef2) and twist transcription factor.
Satellite cells or muscle stem cells (MSC) are small multipotent cells with very little cytoplasm. MSC are precursors to skeletal muscle cells with the ability to give rise to more MSC or differentiated skeletal muscle cells.
The Three Types of Muscles in Humans
Attach to bones and have the main function of contracting to facilitate movement of our skeletons. Known also as striated muscles due to their appearance. The cause of this 'stripy' appearance is the bands of Actin and Myosin which form the Sarcomere, found within the Myofibrils.
Skeletal muscles are voluntary muscles because we have direct control over them through our nervous system. Contractions can vary to produce powerful, fast movements or small precision actions. Skeletal muscles are able to stretch or contract and still return to their original shape.
Found only in the walls of the heart. Similar to skeletal muscles in that it is striated and multi nucleated, and with smooth muscles in that its contractions are not under controlled by the autonomic nervous system. However, even without a nervous input contraction can occur due to cells called pacemaker cells. Cardiac muscle is highly resistant to fatigue due to the presence of a large number of mitochondria, myoglobin and a good blood supply allowing continuous aerobic metabolism.
An involuntary muscle controlled by the autonomic nervous system. The cells do not have the stripy appearance of Skeletal muscle due to lack of sarcomeres and they contain only a single nucleus. Smooth muscle is found in the walls of hollow organs such as the Stomach, Oesophagus, Bronchi and in the walls of blood vessels. This muscle type is stimulated by involuntary neurogenic impulses and has slow, rhythmical contractions used in controlling internal organs, for example, moving food along the Oesophagus or constricting blood vessels during Vasoconstriction.
The types of muscles all utilise myosin and actin filaments to generate force that leads to cell contraction. In skeletal and cardiac muscle, actin and myosin filaments are organised into sarcomeres that function as the fundamental unit of contraction. Skeletal muscle cells are elongated, multi-nucleated cells that range in length from millimetres to tens of centimetres and span the entire length of a muscle. Cardiac muscle cells are similar to skeletal muscle cells but are shorter and are attached to each other via specialised junctions called intercalated disks. Smooth muscle cells contain a single nucleus and lack sarcomeres. They specialise in slow, powerful contractions and are under involuntary control.
Anatomy of Skeletal Muscle
Skeletal muscles are sheathed by a tough layer of connective tissue called the epimysium. The epimysium contains many fascicles. Enclosing each fascicle is a layer called the perimysium which contains many muscle fibres Enclosing each muscle fibre is a layer of connective tissue called the endomysium.
The epimysium anchors muscle tissue to tendons at each end, where the epimysium becomes thicker and collagenous. It also protects muscles from friction against other muscles and bones.Connective tissue is present in all muscles as fascia.
The below video gives a good brief illustration of this
The sarcolemma is the cell membrane of a striated muscle cell. It forms a physical barrier against the external environment and also mediates signals between the exterior and the muscle cell. The sarcoplasm is the specialized cytoplasm of a muscle cell that contains the usual subcellular elements along with the Golgi apparatus, abundant myofibrils, a modified endoplasmic reticulum known as the sarcoplasmic reticulum (SR), myoglobin and mitochondria. Transverse (T)-tubules invaginate the sarcolemma and form a network around the myofibrils, storing and providing the Ca2+ that is required for muscle contraction. Myofibrils are contractile units (within the muscle cell) that consist of an ordered arrangement of longitudinal myofilaments (thin actin filaments and thick myosin filaments). The characteristic 'striations' of skeletal and cardiac muscle readily observable by light microscopy are the thin filaments (light) the thick filaments (dark). The Z-line defines the lateral boundary of each sarcomere. Contraction of the sarcomere occurs when the Z-lines move closer together, making the myofibrils contract, and therefore the whole muscle cell and then the entire muscle contracts. The interaction of myosin and actin is responsible for muscle contraction.
Within a muscle, the muscle fibres are functionally organised as motor units. A motor unit consists of a single motor neuron and all of the muscle fibres it innervates. The size of the unit can involve only a few fibres for fine movement to huge numbers for gross movement such as what occurs in walking. eg the eyes require rapid, precise movements but little strength; in consequence, extraocular muscle motor units are extremely small (with an innervation ratio of only 3!) and have a very high proportion of muscle fibres capable of contracting with maximal velocity. In contrast, the gastrocnemius, a muscle that comprises both small and larger units, has an innervation ratio of 1000–2000 muscle fibres per motor neuron, and can generate forces needed for sudden changes in body position.
The force generated by muscles to lift a pen is much less than the force required to lift a car wheel. The force of contraction produced by a muscle is increased in two ways: multiple motor unit summation, which involves increasing the number of muscle fibres contracting, and multiple-wave summation, which involves increasing the force of contraction of the muscle fibres.
Motor units also differ in the types of muscle fibres that they innervate. In most skeletal muscles, the small motor units innervate small “red” muscle fibres that contract slowly and generate relatively small forces. They are rich in myoglobin, mitochondria, and capillary beds, such small red fibres are resistant to fatigue. These small units are called slow (S) motor units and are especially important for activities that require sustained muscular contraction eg maintenance of an upright posture. Larger α motor neurons innervate larger, pale muscle fibres that generate more force. They have sparse mitochondria and are easily fatigued. These units are called fast fatigable (FF) motor units and are especially important for brief exertions that require large forces, such as running or jumping. A third class of motor units has properties that lie between those of the other two. These fast fatigue-resistant (FR) motor units are of intermediate size and are not quite as fast as FF units. As the name implies, they are substantially more resistant to fatigue, and generate about twice the force of a slow
The somatic nervous system controls all voluntary muscular systems within the body, and the process of voluntary reflex arcs. The basic route is as follows:
- Precentral gyrus ( primary motor cortex): the origin of nerve signals initiating movement.
- Corticospinal tract (upper motor neuron): Mediator of message from brain to skeletal muscles.
- Peripheral nerve (lower motor neuron): the messenger cell that carries the command to contract muscles.
- Neuromuscular junction: the messenger axon cell tells muscle cells to contract at this intersection
Physical training alters the appearance of skeletal muscles and can produce changes in muscle performance.The reverse ie a lack of use can result in decreased performance and muscle appearance. Although muscle cells can change in size, new cells are not formed when muscles grow. Instead, structural proteins are added to muscle fibers in a process called hypertrophy, so cell diameter increases. Conversely when structural proteins are lost and muscle mass decreases atrophy is said to occur.
Cellular components of muscles can also undergo changes in response to changes in muscle use. The changes in muscle differ depending on the type of exercise undertaken.
- Endurance exercise causes an increase in cellular mitochondria, myoglobin, and capillary networks in slow twitch fibers. Endurance athletes have a high level of slow twitch fibers relative to the other fiber types.
- Resistance exercise causes hypertrophy. Power-producing muscles have a higher number of fast twitch fibers than of slow fibers. .
The below video is a great talk on muscular adaptions to exercise.
The image shows a schematic representation of the in vivo status of thin-filament packing density and spacing in half a sarcomere from a normal pre-flight muscle and in half a sarcomere from an atrophic muscle after a 17-day space flight in humans.
There are three types of muscle atrophy: physiologic, pathologic, and neurogenic.
- Physiologic atrophy is caused by not using the muscles enough. This type of atrophy can often be reversed with exercise and better nutrition. People most at risk are: those with health problems that limit movement, or decreased activity levels; are bedridden; cannot move their limbs because of stroke or other brain disease; are in a place that lacks gravity, such as during space flights. Muscle atrophy due to age is called sarcopenia and occurs as muscle fibres die and are replaced by connective and adipose tissue. It occurs with increasing age, and is a major component in the development of frailty.
- Pathologic atrophy is seen with starvation, and diseases such as Cushing disease (because of taking too much medicines called corticosteroids), congestive heart disease and liver disease.
- Neurogenic atrophy is the most severe type of muscle atrophy. It can be from an injury to, or disease of a nerve that connects to the muscle. This type of muscle atrophy tends to occur more suddenly than physiologic atrophy. Examples are: Amyotrophic lateral sclerosis (ALS, or Lou Gehrig disease); Spinal Musclular Atrophy; damage to a single nerve, such as axillary nerve; Spinal cord injury; Guillain-Barre syndrome; nerve damage caused by injury, diabetes, toxins, or alcohol; Polio.
Physical Inactivity and Atrophy
Physical inactivity causes reduction in muscle mass , which is due to increased protein degradation or reduced protein synthesis in muscles. This directly impacts on a persons quality of life and is a main risk factor for chronic diseases. It is well known that physical exercise is important to maintain and promote the synthesis of muscle protein and in the activation of signalling pathways serving the muscle. Physical exercise promotion is essential in the physically inactive for the whole raft of health and lifestyle benefits that come with it.
Physiotherapists treat a whole range of muscular conditions eg muscle injury; muscle rehabilitation; sports enhancement of muscle; disuse atrophy; physical exercise promotion and education ...basically all the disorders of muscles you can think of!
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