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Learn Cartilages, Bones and Muscles
1. Which are the organs that are part of the musculoskeletal system?
The main organs and tissues that are part of the musculoskeletal system in humans are the cartilages, the bones and the muscles.
Musculoskeletal System - Image Diversity: musculoskeletal system
2. What are the functions of the musculoskeletal system?
The musculoskeletal system has the functions of supporting and protecting organs, maintenance of the body spatial conformation, motion of organs, limbs and bodily portions and nutrient storage (glycogen in muscles, calcium and phosphorus in bones).
3. Which type of tissue are the cartilaginous and the osseous tissue?
The cartilaginous and the osseous tissues are considered connective tissues since they are tissues in which the cells are relatively distant from others with a great amount of extracellular matrix in the interstitial space.
4. What are the cells that form the cartilaginous tissue?
The main cells of the cartilages are the chondrocytes, originated from the chondroblasts that secrete the intersticial matrix. There are also chondroclasts, cells with many lisosomes and responsible for the digestion and remodelation of the cartilaginous matrix.
Musculoskeletal System - Image Diversity: cartilaginous tissue
5. What is the constitution of the cartilaginous matrix?
The cartilaginous matrix is made of collagen fibers, mainly collagen type II, and of proteoglycans, proteins associated to glycosaminoglycans, chiefly hyaluronic acid. The proteoglycans provide the typical rigidity of the cartilages.
6. What are some functions of the cartilages in the human body?
Cartilages are 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. In joints there are cartilages that cover the bones providing a smooth surface to reduce the friction of the joint movement. In the formation of bones the cartilages act as a mold and they are gradually substituted by the osseous tissue.
Musculoskeletal System - Image Diversity: human cartilages
7. What are the three main cell types that form the osseous tissue? What are their functions?
The three main cell types of the osseous tissue are the osteoblasts, the osteocytes and the osteoclasts.
Osteoblasts are known as bone-forming cells since they are the cells that secrete the proteinaceous part of the bone matrix (collagen, glycoproteins and proteoglycans). The 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 the bone matrix. Osteocytes have the function of supporting the tissue.
Osteoclasts are the giant multinucleate cells that remodelate the osseous tissue. They are originated from monocytes and they contain many lisosomes. Osteoblasts secrete enzymes that digest the osseous matrix creating canals throughout the tissue.
Musculoskeletal System - Image Diversity: osseous tissue osteoblasts osteocytes osteoclasts
8. What is the bone matrix? What are its main components?
Bone matrix is the content that fills the intercellular space of the osseous tissue. The bone matrix is made of mineral substances (about 5%), mainly phosphorus and calcium salts, and organic substances (95%), mainly collagen, glycoproteins and proteoglycans.
Musculoskeletal System - Image Diversity: bone matrix
9. What are the Haversian canals and the Volkmann’s canals of the bones? Is the osseous tissue vascularized?
The Haversian canals are longitudinal canals present in the osseous tissue within which blood vessels and nerves pass. The osseous tissue distributes itself in a concentric manner around these canals. The Volkmann’s canals are communications between the Harvesian canals.
The osseous tissue is highly vascularized in its interior.
Musculoskeletal System - Image Diversity: bone anatomy
10. What are the functions of the osseous tissue?
The main functions of the 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 body weight; to serve as a site for mineral storage, mainly of calcium and phosphorus; to form protective structures for important organs like the brain, the spinal cord, the heart and the lungs; to work as a lever and support for the muscles, providing movement; to contain the bone marrow where hematopoiesis occurs.
11. What are the flat bones and the long bones?
The main bones of the body may be classified as flat or long bones (there are bones not classified into 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.
Musculoskeletal System - Image Diversity: flat bones long bones
12. What are the types of muscle tissues? What are the morphological features that differentiate those types?
There are three types of muscle tissue: the skeletal striated muscle tissue, the cardiac striated muscle tissue and the smooth muscle tissue.
The striated muscles present under microscopic view transversal stripes and their fibers (cells) are multinucleate (in the skeletal) or may have more than one nucleus (in the cardiac). The smooth muscle does not present transversal stripes and it has spindle-shaped fibers each with only one nucleus.
Musculoskeletal System - Image Diversity: muscle tissue
13. Which is the type of muscle tissue that moves the bones?
The bones are moved by the skeletal striated muscles. These muscles are voluntary (controlled by volition).
Musculoskeletal System - Image Diversity: skeletal striated muscle tissue
14. Which is the type of muscle tissue that contracts and relaxes the heart chambers?
The myocardium of the heart is made of cardiac striated muscle tissue.
Musculoskeletal System - Image Diversity: cardiac striated muscle tissue
15. Which is the type of muscle tissue that performs the peristaltic movements of the intestines?
The smooth muscle tissue is responsible for the peristaltic movements of the intestines. The smooth muscles are not controlled by volition.
Musculoskeletal System - Image Diversity: smooth muscle tissue
16. Which is the type of muscle tissue that helps to push the food down through the esophagus?
The esophageal wall in its superior portion is made of skeletal striated muscle. The inferior portion is made of smooth muscle. In the intermediate portion there are skeletal striated and smooth muscles. All of these muscles are important to push the food down towards the stomach.
17. How is the striped pattern of the striated muscle cells formed?
The functional units of the muscle fibers are the sarcomeres. Within the sarcomeres blocks of actin and myosin molecules are posed in organized manner. The sarcomeres align in sequence forming myofibrils that are longitudinally placed in the cytoplasm of the muscle fibers (cells). The grouping of consecutive blocks of actin and myosin in parallel filaments creates the striped pattern of the striated muscle tissue seen under the microscope.
Musculoskeletal System - Image Diversity: muscle cell
18. What are sarcomeres?
Sarcomeres are the contractile units of the muscle tissue formed of alternating actin blocks (thin filaments) and myosin blocks (thick filaments). Several sarcomeres placed in linear sequence form a myofibril. Therefore one muscle fiber (cell) has many myofibrils made of sacomeres.
The compartments where myofibrils are inserted are delimited by an excitable membrane known as sarcolemma. The sarcolemma is the plasma membrane of the muscle cell.
Musculoskeletal System - Image Diversity: sarcomere
19. What are the main proteins that constitute the sarcomere? What is the function of those molecules in the muscle cells?
In the sarcomere there are organized actin and myosin blocks. Troponin and tropomyosin also appear associated to actin.
The actin molecules when activated by calcium ions liberated in the proximities of the sarcomere are pulled by myosin molecules. This interaction between actin and myosin shortens the myofibrils originating the phenomenon of muscle contraction.
20. What are the positions of actin and myosin molecules in the sarcomere before and during the muscle contraction?
Schematically actin filaments attached perpendicularly to both sarcomere extremities (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 by their extremities. During contraction actin filaments slide along the myosin filaments and the sarcomeres shorten.
Musculoskeletal System - Image Diversity: actin-myosin interaction
21. How do calcium ions participate in muscle contraction? Why do both muscle contraction and muscle relaxation spend energy?
In the muscle cells calcium ions are stored within the sarcoplasmic reticulum. When a motor neuron emits stimulus for the muscle contraction neurotransmitters called acetylcholine are released in the neuromuscular junction and the sarcolemma is excited. The excitation is conduced to the sarcoplasmic reticulum that then realeases calcium ions into the sarcomeres.
In the sarcomeres the calcium ions bind to troponin molecules associated to actin activating myosin binding sites of actin. The myosin, then able to bind to actin, pulls this protein and the sarcomere shortens. The summation of simultaneous contraction of sarcomeres and myofibrils constitutes the muscle contraction. During muscle relaxation the calcium ions return back to the sarcoplasmic reticulum.
For myosin to bind to actin, and thus for the contraction to occur, hydrolysis of one ATP molecule is necessary. During relaxation the return of calcium ions to the sarcoplasmic reticulum is an active process that spends ATP too. So both muscle contraction and relaxation are energy-spending processes.
Musculoskeletal System - Image Diversity: muscle contraction
22. What is myoglobin? What is the function of this molecule in the muscle tissue?
Myoglobin is a pigment similar to hemoglobin and present in muscle fibers. Myoglobin has a great affinity for oxygen. It keeps oxygen bound and releases the gas under strenuous muscle work. So myoglobin acts as an oxygen reserve for the muscle cell.
23. How does phosphocreatine act in the muscle contraction and relaxation?
Phosphocreatine is the main means of energy storage of the muscle cells.
During relaxed periods ATP molecules made by the aerobic cellular respiration transfer highly energized phosphate groups to creatine forming phosphocreatine. In exercise periods phosphocreatine and ADP resynthesize ATP to dispose energy for the muscle contraction.
24. What happens when the oxygen supply is insufficient to maintain aerobic cellular respiration during muscle exercise?
If oxygen from hemoglobin or myoglobin is not enough for the energy supply of the muscle cell the cell then begins to do lactic fermentation in an attempt to compensate the deficiency.
The lactic fermentation releases lactic acid and this substance causes muscle fatigue and predisposes the muscles to cramps.
25. What is the neurotransmitter of the neuromuscular junction? How does the nervous system trigger muscle contraction?
The nervous cells that trigger the muscle contraction are the motor neurons. The neurotransmitter of the motor neurons is acetylcholine. When a motor neuron is excited the 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 axonal extremity the depolarization allows the entrance of calcium ions into the axon (note that calcium also has a relevant role here). The calcium ions stimulate the neuron to release acetylcholine in the synapse.
Acetylcholine then binds to special receptors in the outer surface of the sarcolemma, the permeability of this membrane is altered and an action potential is created. The depolarization is then conduced along the sarcolemma to the sarcoplasmic reticulum that thus releases calcium ions for the sarcomere contraction.
Musculoskeletal System - Image Diversity: neuromuscular junction
26. To increase the strength of the muscle work is the muscle contraction intensely increased?
An increase in the strength of the muscle work is not achieved by increase in the intensity of the stimulation of each muscle fiber. The muscle fiber obeys an all-or-nothing rule, i.e., its contraction strength is only one and cannot be increased.
When the body needs to increase the strength of the muscle work a phenomenon known as spatial summation occurs: new muscle fibers are recruited in addition to the fibers already in action. So the strength of the muscle contraction increases only when the number of active muscle cells increases.
27. What is the difference between spatial summation and temporal summation of muscle fibers? What is tetany?
Spatial summation is the recruiting of new muscle fibers to increase the muscle strength. Temporal summation occurs when a muscle fiber is continuously stimulated to contract without being able to conclude relaxation.
The permanence of a muscle fiber under a continuous state of contraction by temporal summation is known as tetany (e.g., the clinical condition of patients contaminated by the toxin of the tetanus bacteria). Tetany ends when all available energy for contraction is spent or when the stimulus ceases.
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