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Organization of Skeletal Muscle, Physiology of Muscle Contraction, Neuromuscular Junction

This is a system of muscles. Physical muscles are composed of muscle fibers, which are individual muscle cells.
The organization of skeletal muscle
This is a system of muscles. Physical muscles are composed of muscle fibers, which are individual muscle cells. Fibrous fascicles surround connective tissue and are composed of muscle fibers. Each fascicle is joined together to form one muscle cell. Muscle fibers are connected by various kinds of connective tissue. A thin fiber of connective tissue called endomysium surrounds each muscle fiber. In endomysium, elastic fiber collagen is found in areolar connective tissues. Embryonic membranes are thin, loose structures. They assist in connecting muscles together within fascicles and provide an environment that assists in the growth of muscle cells. It is through the endomysium that nutrients and oxygen are delivered to muscle fibers. Similarly, motor neurons in this layer control muscle fibers.

Perimysium, a connective tissue that surrounds fascicles, comes in two types in total. The perimysium is rich in collagen and elastin fibers. Each bundle of fascicles is surrounded by an epimysium at the end, and all together they make up the muscle. An epimysium is a fibrous, elastomeric material that surrounds bones and muscles, keeping them from rubbing against each other. A loose connective tissue surrounds and supports the epimysium - this tissue is found around and around the organs in the body.

Skeletal muscle fiber organization
It is the orientation of the fascicles within a muscle that determines the direction and strength of the contraction of that muscle. The human body is arranged in seven different ways by fascicles.

Parallel muscles make up the majority of muscles in the body. Fascicles are parallel to each other in parallel muscles, and they are arranged along the muscle's length, running parallel to the attachment points (usually tendons). The hamstring in the upper leg is composed of tendons, which are flexible collagen tissues that connect muscles and bones. In other words, the muscle can also generate force parallel to the parallel axis. Fusiform and non-fusiform parallel muscles are different types of parallel muscles.
  1. Parallel muscles in the fusiform group have a spindle-like shape. The midsection bulges out while the ends taper away. During contraction of parallel fusiform muscles, the bulge increases, as in the biceps brachii in the upper arm.
  2. Non-fusiform parallel muscles have a relatively constant diameter instead of bulging in the middle, such as the sartorius muscle that allows movement in the hip and thigh.
Pectoralis major in the chest is an example of convergent muscles that taper to a point at one end and have a broad attachment area at the other. Despite the broad range of attachment points, convergent muscles do not exert the same level of force on tendons as parallel muscles, but this broad distribution increases the flexibility of the muscle, which makes it able to change the direction it pulls. An additional type of muscle known as pennate muscles contains tendons that run their entire length.

Pennate muscles are another major muscle type that contains tendons that run the length of the muscle. The term pennate derives from the Latin word penna meaning feather, which refers to a leg extending at an angle. These short fascicles are arranged like the barbs of a feather in the pennate muscle. Unipennate muscles, bipennate muscles, and multipennate muscles comprise the three types of pennate muscles. Fascicules exist on only one side of a tendon in unipennate muscles, such as those in the forearm's extensor digitorum. Muscles with bipennate tendon structures, such as the rectus femoris in the leg, have fascicles on both sides. Fascicles surround tendon endings in multipennate muscles, such as those in the shoulder deltoids. Because pennate muscles are inclined, they can't move the tendon very far. Nonetheless, they are generally smaller, have fewer individual muscle fibers, and therefore have greater tension.

As a final note, circular muscles, such as the orbicularis Oris around the mouth, are arranged in a concentric circle. This muscle, which also goes by the name of sphincters, shrinks the opening when it contracts and opens it when it relaxes.

Skeletal muscle fibers
Cells in muscles with specialized structures that make them capable of movement are called muscle fibers. The muscles are packed with rod-shaped structures called myofibrils found in each fiber. Fibers of myofibrils contain threads of a protein called myofilaments, which are ultramicroscopic threads of protein. Alternating bands of light and dark are created by the myofibril's myofilaments. Actin and myosin are the proteins situated in the center of each sarcomere and form the bands and end of the myofibrils. Sarcomeres are made up of bands of actin and thin protein filaments called actin filaments. One sarcomere is formed by two bands of light and one of dark color. In muscle fibers, the sarcomeres are composed of thick and thin filaments that are regular and recurrent structures. Sarcomeres are arranged in repeated patterns that give skeletal muscle its striped appearance.

Sarcomeres have thick, cylindrical myofibrils arranged in the middle, called the A-band. In I-band regions, actin myofibrils are found at the ends of sarcomeres. Z-discs surround each sarcomere and bind actin filaments. The Z-disc consists of multiple stabilizing proteins and acts as a platform for actin molecules to anchor. In addition to actin, titin, or connectin, is used in binding the disc to the membrane. The H-zone, or myosin-only region, is located within the sarcomere's center. The actin and myosin proteins slide past each other when stimulated at the neuromuscular junction, pulling the Z-discs together. The actin and myosin proteins slide past each other when stimulated at the neuromuscular junction, pulling the Z-discs together. As a result, the H-zone and I-band shrink, and the muscle contracts. As a result, the H-zone and I-band shrink, and the muscle contracts.

Besides the specialized cellular structures found in muscle fibers, there are also other cellular structures. The sarcolemma is a membrane that surrounds muscle fibers. There are special features called transverse tubules or T-tubules on this membrane. Myofibril bundles are connected to the transverse tubule by a channel that runs through the sarcolemma. During the activation of the muscle, the signal traveling through the membrane quickly reaches all of the myofibrils located in the muscle fiber. The sarcoplasmic reticulum includes structures called terminal cisternae that line both sides of the transverse tubule. The sarcoplasmic reticulum is different from the smooth endoplasmic reticulum only in the sense that it is muscle-specific. It forms a network at the center of each myofibril, which contains a calcium ion reserve that is waiting for stimulation by a motor neuron to release calcium ions. The transverse tubules reside next to the terminal cisternae, waiting for an action potential to release calcium reserves from their reserves.

Myocytes, which are the progenitor cells for muscle, fuse during the formation of striated muscle fibers. Unlike stem cells, progenitor cells develop into specific cells with specific functions, such as muscle or bone cells. Progenitor cells contain multiple nuclei. The nuclei of these cells are flat and located just outside of the cell membrane to ensure myofibrils do not impede the action of cell membranes. Other organelles may also be found within muscle fibers. Muscle fibers contain multiple specialized cell parts, among them, mitochondria, also called acrosomes.

Physiology of muscle contraction
Sarcomeres must shorten when a muscle cell contracts. Sarcomeres, however, do not shorten as a result of thick or thin filaments. As a result, the sarcomere shortens rather than shrinks, and the filaments do not change length. A sliding filament theory of muscle contraction was proposed to explain the observed differences in the sarcomere bands at various levels of muscle contraction and relaxation. The mechanism of contraction involves myosin bonding with actin, producing cross-bridges that cause filaments to move.

During sarcomere shortening, some regions are shorter, whereas others remain the same. An arc of a sarcomere is found at the distance between two Z lines; when a muscle contracts, the arc of the sarcomere is shrunk. In the H zone, which lies in the center of the A zone, there only exist thick filaments that were compressed through the contraction. There are no thick filaments in the I band, and it is also constricted during contraction. While muscles contract, the A band does not become shorter. However, it moves closer to other A bands of different sarcomeres as close together as possible. The thin filaments are pulled toward the center by the thick filaments, which is achieved when the Z discs are close to the thick filaments. With the inward movement of thin filaments, thickness and overlap are increased and overlap zones are formed.

The act of shortening muscles is caused by the binding of myosin heads to actin and pulling the actin inward. The energy that drives this process is provided by ATP. Globular actin binds to the myosin protein through a binding site. An enzyme in myosin hydrolyses ATP to ADP to release inorganic phosphate and energy. At this site, enzymatic activity will also hydrolyze ATP to ADP. A release of actin occurs when myosin is bound to ATP, causing myosin and actin to separate. ADP and inorganic phosphate are generated after the newly bound ATP are converted. An enzyme called ATPase binds to myosin at the binding site. By hydrolyzing ATP, energy is released which causes the angle of the myosin head to be converted into a "cocked" position. It remains to be seen whether ADP and Pi will separate from the myosin head, which is ready to move forward.

When actin-binding sites are blocked, ATP is hydrolyzed, while myosin remains attached and unharmed. A cross-bridge occurs when the actin-binding sites are exposed, which results in the myosin heads spanning the distance between myosin and actin molecules. Upon release of Pi, myosin releases any stored energy achieved through the conformational change. During myosin contractions, the head of the myosin can be seen moving towards the M line, pulling along actin. In response to actin pulling, filaments move away from the M line by approximately 10 nm. The power stroke is distinguished by the fact that it is the step that produces force. The actin tends to be pulled toward the M-line, shortening the sarcomeres and causing the muscles to contract.

Myosin heads contain energy and are in a high-energy configuration when they are cocked. Myosin heads expend this energy during power strokes, which results in them going into a low-energy state after power stroke. ADP is released following a power stroke, but actin and myosin, which have been bound together during the power stroke, remain bound together. Myosin then attaches to ATP to start the cross-bridge cycle, allowing the contraction of muscles to continue.

Neuromuscular junction
Neurons communicate with each other via a synapse, a special structure that enables them to exchange information. Approximately 10,000 connections can be made and received between nerve cells per cell in the brain. A synapse can also be modified in strength. Throughout the nervous system, synapses play a significant role in organizing information.

The synapse that takes place between skeletal muscle cells and spinal motor neurons is almost always known. While synaptic transmission at the skeletal neuromuscular junction mirrors synaptic transmission within the brain, there are many fundamental variations between the two processes. Thus, by understanding this synapse, you can comprehend the others as well. At the neuromuscular junction, three chemical synapses are typical of those found in the nervous system. As a beginning, the presynaptic membrane and the postsynaptic membrane are distinct from one another. Synaptic clefts exist between these two cell types. By following the space one could infer that there must be some mechanism whereby information can flow between postsynaptic neurons and presynaptic neurons. A characteristic feature is the dense concentration of small spherical vesicles. Neurotransmitter substances are contained within synaptic vesicles. A high density of mitochondria can also be found at synapses. In addition, the postsynaptic membrane in many cases becomes thickened, which is at least partly because the postsynaptic membrane contains numerous specialized receptors that bind that neurotransmitter substance released by the presynaptic neuron.

Nerve impulses are transmitted from motor neurons to muscle fibers through the vertebrate NMJ in a 1:1 ratio, which makes the central nervous system responsible for controlling skeletal muscle contractions. Both presynaptic motor axons and postsynaptic skeletal muscle fibers are highly specialized at the NMJ so that action potentials reach the specific neuron pathways efficiently. Chemical synapses have similar structural characteristics to the NMJ: During motor nerve transmission, synaptic vesicles containing acetylcholine (ACh) are delivered to the postsynaptic muscle membrane through a gap of 50 to 80 nanometres. On each crest and in each trough of the postsynaptic membrane of muscle fibers are acetylcholine receptors (AChRs) at high density, and voltage-gated sodium channels at high density. Basal lamina (also known as basement membrane) contains extracellular matrix materials originating from muscle fibers that are tightly wrapped along the entire length of the muscle fibers. A molecule secreted by nerves and muscles is present in the basal lamina of the synaptic cleft and is different from that found outside the synapse. In peripheral nerve terminals, specialized glial cells called Schwann cells cover the ends of the nerves. Additionally, Schwann cells produce an attachment lamina that fuses with muscle fibers at the edges of the NMJ. Keratinocytes also form loose covers over NMJs, but they are not characterized quite well.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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