The main organs and tissues of the musculoskeletal system in humans are cartilage, bones and muscles.
The musculoskeletal system has the function of supporting and protecting organs, the maintenance of the structure of the body, helping in the movement of organs, limbs and parts of the body, and nutrient storage (glycogen in muscles, calcium and phosphorus in bones).
Cartilaginous and osseous tissues are considered connective tissues, since they are tissues in which the cells are relatively distant from others, with a large amount of extracellular matrix in the interstitial space.
The main cells of cartilage are chondrocytes, which are produced from the chondroblasts that secrete interstitial matrix. It also contains chondroclasts, which are cells with a large number of lysosomes and which are responsible for the digestion and remodeling of cartilaginous matrix.
Cartilaginous matrix is made of collagen fibers, mainly collagen type II, and of proteoglycans, proteins attached to glycosaminoglycans, chiefly hyaluronic acid. Proteoglycans are the reason for the typical rigidity of cartilage.
Cartilage is responsible for the structural support of the nose and ears. The trachea and the bronchi are also organs with cartilaginous structures that prevent the closing of these tubes. Joints contain cartilage that covers the bones, providing a smooth surface to reduce the friction of joint movement. In the formation of bones, cartilage acts as a mold and is gradually substituted by osseous tissue.
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The three main types of cells of osseous tissue are osteoblasts, osteocytes and osteoclasts.
Osteoblasts are considered bone-forming cells since they are the cells that secrete the proteinaceous part of the bone matrix (collagen, glycoproteins and proteoglycans). Bone matrix is the intercellular space where the mineral substances of the bones are deposited.
Osteocytes are differentiated mature osteoblasts formed after these cells are completely surrounded by bone matrix. Osteocytes have the function of supporting the tissue.
Osteoclasts are the giant multinucleate cells that remodel osseous tissue. They are produced from monocytes and contain a large number of lysosomes. Osteoblasts secrete enzymes that digest osseous matrix, creating canals throughout the tissue.
Bone matrix is the content that fills the intercellular space of osseous tissue. Bone matrix is made of mineral substances (about 5%), mainly phosphorus and calcium salts, as well as organic substances (95%), mainly collagen, glycoproteins and proteoglycans.
The Haversian canals are longitudinal canals present in osseous tissue within which blood vessels and nerves pass. Osseous tissue distributes itself in a concentric manner around these canals. The Volkmann’s canals are communications between the Harvesian canals.
Osseous tissue is highly vascularized in its interior.
The main functions of osseous tissue are: to provide structural rigidity to the body and to delineate the spatial positioning of the other tissues and organs; to support the weight of the body; to serve as a site for mineral storage, mainly of calcium and phosphorus; to form protective structures for important organs such as the brain, the spinal cord, the heart and the lungs; to work as a lever and support for the muscles, providing movement; and to contain the bone marrow where hematopoiesis occurs.
The main bones of the body can be classified as flat or long bones (some bones are not classified according to these categories). Examples of flat bones are the skull, the ribs, the hipbones, the scapulae and the sternum. Examples of long bones are the humerus, the radius, the ulna, the femur, the tibia and the fibula.
There are three types of muscle tissue: skeletal striated muscle tissue, cardiac striated muscle tissue and smooth muscle tissue.
Striated muscles present transversal stripes under microscopic view and their fibers (cells) are multinucleate (in skeletal) or may have more than one nucleus (in cardiac). Smooth muscle does not present transversal stripes and has spindle-shaped fibers, each of which has only one nucleus.
Bones are moved by the skeletal striated muscles. These muscles are voluntary (controlled by volition).
The myocardium is made of cardiac striated muscle tissue.
Smooth muscle tissue is responsible for the peristaltic movements of the intestines. Smooth muscles are not controlled by volition.
The esophageal wall in its superior portion is made of skeletal striated muscle. The inferior portion is made of smooth muscle. The intermediate portion contains both skeletal striated and smooth muscles. All of these muscles are important in pushing the food down towards the stomach.
The functional units of muscle fibers are sarcomeres. Within sarcomeres, blocks of actin and myosin molecules are placed in an organized manner. The sarcomeres align in sequence to form myofibrils, which are longitudinally placed in the cytoplasm of muscle fibers (cells). The grouping of consecutive blocks of actin and myosin in parallel filaments creates the striped pattern of striated muscle tissue seen under a microscope.
Sarcomeres are the contractile units of muscle tissue, formed of alternating actin blocks (thin filaments) and myosin blocks (thick filaments). Several sarcomeres placed in a linear sequence form a myofibril. Therefore, one muscle fiber (cell) has many myofibrils made of sarcomeres.
The compartments where myofibrils are inserted are delimited by an excitable membrane known as the sarcolemma. The sarcolemma is the plasma membrane of a muscle cell.
The sarcomere contains organized actin and myosin blocks. Troponin and tropomyosin also appear bound to actin.
When activated by calcium ions released in the proximities of the sarcomere, the actin molecules are pulled in by myosin molecules. This interaction between actin and myosin shortens the myofibrils, producing the phenomenon of muscle contraction.
Schematically, actin filaments attached perpendicularly to both ends of the sarcomere (longitudinal sides) make contact with myosin filaments positioned in the middle of the sarcomere and in parallel to the actin filaments.
Before the contraction, the sarcomeres are extended (relaxed) since the contact between actin and myosin filaments is only made at their ends. During contraction, actin filaments slide along the myosin filaments and the sarcomeres shorten.
In muscle cells, calcium ions are stored within the sarcoplasmic reticulum. When a motor neuron emits a stimulus for muscle contraction, neurotransmitters called acetylcholine are released at the neuromuscular junction and the sarcolemma is excited. The excitation is transmitted to the sarcoplasmic reticulum, which then releases calcium ions into the sarcomeres.
In the sarcomeres, the calcium ions bind to troponin molecules attached to actin, thus activating the myosin binding sites of actin. The myosin, then able to bind to actin, pulls this protein and the sarcomere shortens. The combined simultaneous contraction of sarcomeres and myofibrils constitutes a muscle contraction. During muscle relaxation, the calcium ions return to the sarcoplasmic reticulum.
For myosin to bind to actin, and therefore for the contraction to occur, the hydrolysis of one ATP molecule is necessary. During relaxation, the return of calcium ions to the sarcoplasmic reticulum is also an active process that consumes ATP. Therefore, both muscle contraction and relaxation are energy-consuming processes.
Myoglobin is a pigment similar to hemoglobin which is present in muscle fibers. Myoglobin has a large affinity to oxygen. It keeps oxygen bound and releases the gas under strenuous muscle work. Therefore, myoglobin acts as an oxygen reserve for muscle cells.
Phosphocreatine is the main means of energy storage of muscle cells.
During periods of relaxation, ATP molecules produced by aerobic cellular respiration transfer highly energized phosphate groups to creatine molecules, thus forming phosphocreatine. During periods of exercise, phosphocreatine and ADP resynthesize ATP to release energy for muscle contraction.
If oxygen from hemoglobin or myoglobin is not enough to supply energy to the muscle cells, the cell begins to use lactic fermentation in an attempt to compensate for that deficiency.
Lactic fermentation releases lactic acid and this substance causes muscle fatigue and predisposes the muscles to cramps.
The nervous cells that trigger muscle contraction are motor neurons. The neurotransmitter of the motor neurons is acetylcholine. When a motor neuron is excited, a depolarizing current flows along the membrane of its axon until reaching the synapse at the neuromuscular junction (the neural impulse passage zone between the axon extremity and the sarcolemma). Near the extremity of the axon, the depolarization allows calcium ions to enter the axon (note that calcium also has an important role here). The calcium ions stimulate the neuron into releasing acetylcholine in the synapse.
Acetylcholine then binds to special receptors on the outer surface of the sarcolemma; the permeability of this membrane is altered and an action potential is created. The depolarization is then transmitted along the sarcolemma to the sarcoplasmic reticulum, which then releases calcium ions which cause the sarcomere to contract.
An increase in the strength of a muscle is not achieved by an increase in the intensity of the stimulation of each muscle fiber. The muscle fiber obeys an all-or-nothing rule, meaning that its contraction strength is only one and cannot be increased.
When the body needs to increase the strength of a muscle, a phenomenon known as spatial summation occurs: new muscle fibers are recruited in addition to the fibers already in action. Therefore, the strength of the muscle contraction increases only when the number of active muscle cells increases.
Spatial summation is the recruiting of new muscle fibers to increase muscle strength. Temporal summation occurs when a muscle fiber is continuously stimulated to contract without being able to go through relaxation.
When a muscle fiber remains in a continuous state of contraction via temporal summation, it is known as tetany (this is the clinical condition of patients contaminated by the toxin of tetanus bacteria). Tetany ends when all available energy for contraction is spent or when the stimulus ceases.
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