| Talk about the muscle tissue | A specialized connective tissue that is specialized in the production of mechanical work or muscle contraction. They derive from the mesoderm they are made of many muscle cells (myocytes or muscle fibres) that contain in their cytoplasm contractile proteins forming the myofilaments grouped into myofibrils. there are 3 types of muscle tissues that differ in cell cytology, location, function, and triggering mode. |
| What makes the skeletal muscle different from others? | Usually associated with the skeleton, its fibers are the longest muscle fibers among the others, and throughout its whole length there are transverse striations (bands). Their contraction is voluntary and allows voluntary movements and posture maintenance under the control of the central nervous system. |
| What makes the cardiac muscle fiber different from others? | Exists only on the heart wall, highly differentiated from the skeletal striated ones where cells perform involuntary rhythmic contraction. |
| What makes smooth muscle fibers different from others? | Involved in the processes related to the maintenance of the internal environment. Cells are unstriated and involuntary, found in the walls of vessels and hollow visceral organs and the organs of the respiratory tract. |
| Talk about the skeletal muscle fiber. | AKA rhabdomyocyte, it is the functional unit of the skeletal muscle, the cells are long cylindrical regularly contoured cells with transverse striations.
Most of the cytoplasm is occupied by microfibrils, which constitute the myoplasm, the remaining of the cytoplasm is called sarcoplasm and is the space between the myofibrils containing various components and organelles.
It is unable to divide and constitutes a syncytium formed in the embryonic state by fusion of mononuclear precursor cells called myoblasts. |
| What are the constituents of the sarcoplasm? | -Several hundreds of ovoid nuclei aligned at the periphery of the cell, against the plasma membrane and oriented in the longitudinal direction of the cell.
-Numerous and large mitochondria grouped in the vicinity of the nuclei and aligned between myofibrils.
-Highly developed sarcoplasmic reticulum (SER) that forms a network participating in the sarcotubular system or T-system
-Cytoskeleton formed mainly of microtubules and IFs of desmin
-Lipid vacuoles and glycogen close to the mitochondria
-Myoglobin red pigments which fix oxygen and deliver it to the mitochondria.
-Plasma membrane is called sarcolemma and is doubled by basal lamina. |
| Talk about the organization of the skeletal muscle fibers. | They associate to form together skeletal striated muscles, surrounded by a conjunctive envelope.
Dense fibrous CT surrounding the striated muscle and individualizing it is called epimysium.
Epimysium enters the muscle and forms the perimysium surrounding a group of muscle cells leading to the partition of fascicles
Perimysium enters the fascicles and forms the endomysium surrounding each cell and separating it from other ones. |
| Talk about the ultrastructure of myofibrils. | They are formed by the assembly of protein myofilaments and are differentiated for contraction. In the myoplasm several hundreds of myofibrils are observed, which constitutes the majority of cell volume. They are cylindrical, parallel, elongated in the cell direction clamped against each other, and extend from one end to another. In longitudinal section, they present a transverse striation due to the alternation of bands (Disks)
-Dark, anisotropic (A band) of non-homogenous appearance
-Light isotropic (I band) of homogenous, divided in the middle by a Z line, the portion between two Z lines is called the sarcomere. |
| Talk about the sarcomere. | It results from the orderly and parallel arrangement of fine and thick myofilaments along the myofibrillar axis. It comprises an A band with two half I bands from either sides. It is the smallest structural and functional unit of the muscle cell called contraction unit. Myofibrils are a sequence of sarcomeres in a cell, sarcomeres of all myofibrils are located at the same level which explains the transverse striation of the muscle cell. |
| Talk about the bands forming the sarcomere. | The dark band A is formed of thick myosin filaments and thin actin filaments arranged in a parallel manner, while the I band is formed of only thin actin filament.
Band A comprises in its middle a clearer transverse zone called M zone (dark transverse line). Myosin molecule comprises a cylindrical rod or axis (tail) which terminates at one of its ends by 2 spherical heads that interact with actin through actin binding sites, and have ATPase activity necessary for hydrolysis of ATP. |
| Talk about the protein organization inside the bands. | each half-band A comprises the heads and tails of the thick filaments whereas the H zone consists solely of tails. M line includes myomesin which connects the thick filaments to each other
Each I band contains fine actin myofilaments with troponin and tropomyosin molecules, it is divided transversely into two parts by the very dense Z line. Fine filaments extend on both sides of Z line over the entire length of the I band and the Z line composed of alpha actinin is thus the attachment point of these fine filaments. |
| What do fine filaments consist of? | Globular monomers (G-actin) forming the F-actin filaments composed of two helically entwined polypeptide chains, G-actin is characterized by binding sites on which myosin heads bind during contraction. Tropomyosin consists of two alpha helical polypeptide chains located in the groove of F-actin, Troponin is composed of 3 globular subunits forming a complex:
-tropnin T (Tnt) binding to the tropomyosin
-troponin I (Tni) inhibiting the attachment of myosin heads to actin at rest
-Troponin C (Tnc) binding sites for calcium. |
| Talk about the elastic filament present in the rhabdomyocyte. | Very thin formed of large protein titin (connectin) the third most abundant protein (after actin and myosin) in skeletal muscle, links extremities of A band to Z line, contributes to the stabilization of the Z line position, and lays a role in restoring the sarcomere length resting during relaxation. |
| Talk about the smoot sarcoplasmic reticulum. | Sarcolemma has many transversal invaginations (T tubules) arranged around the junction zone between A and I bands of each myofibril. The T tubule network is called the transverse system. Smooth sarcoplasmic reticulum is located around each myofibril and runs parallel to it. It consists of tubules and sacs that periodically meet to form the terminal cisternae at each A-I bands junction. SER contains a high Ca++ concentration which release in the cytosol is responsible for the muscle contraction. SER cisternae are adjacent to either side of a T tubule. The 2 parallel terminal cisternae and the intervening T tubule form a complex referred to as the traid. At this level depolarization wave is transmitted from the T tubule to the sarcoplasmic reticulum. |
| Talk about muscle resting.. | AKA filament sliding method, it is marked by the shortening of muscle cell resulting from the sliding of the fine filaments on the thick filaments.
When a muscle is at rest, Muscular cells are relaxed: thin and thick filaments overlap only for a small part of their length, Ca++ ions are in the terminal cisternae, ATP linked to myosin union bridges and actin linked tropomyosin and troponin complex interposes between myosin heads and actin binding sites, preventing their binding. |
| Talk about muscle contraction. | When a muscle is stimulated, acetylcholine is released and depolarizes the sarcolemma, electrical impulse is conducted through the T tubules to the sarcoplasmic reticulum, which then releases Ca++ ions and then binds to the troponin causing conformational change in the complex, and the tropomyosin then is displaced to the interior of the F actin helix groove. Myosin heads will then attach to the exposed actin filaments binding sites, bridges are formed and sliding process begins, Myosin heads attach and detach several times to produce tension and pull the actin filaments and the Z line toward the center of the sarcomere, towards the H zone, the length of the I band is shortened leading to a decrease in the size of the sarcomere while A band is the same size.
So contraction occurs simultaneously in all sarcomeres of all myofibril, muscle cells shorten and so does the muscle. |
| Talk about the heterogeneity of the rhabdomyocytes | -Red fibers with slow contraction: smallest diameter, rich in myoglobin mitochondria and oxidative enzymes of aerobic pathways, easily produce ATP and slowly degrade it, contract slowly and resistant to fatigue
-White fibers with fast contraction: Large pale cells with a small amount of myoglobin and mitochondria, and high glycogen content, producing ATP anaerobically and degrade it rapidly making fast irresistant to fatigue contractions
-Red fibers with fast contractions: Red cells with intermediate size containing many myoglobins and mitochondria, produce ATP through oxidative phosphorylation and degrade it rapidly making fast contractions and resistant to fatigue but lesser than red with slow.
They all exist in the skeletal muscle but in different compositions |
| Talk about the vascularization of skeletal muscles. | CT provides entry and pathways of blood vessels and nerves serving the muscle.
-Blood vessels enter the muscle following the connective septa of the perimysium, they divide into many branches within the connective tissue and join the thin endomysium layer with a dense network of long and sinuous capillaries extending between muscle cells.
-Lymphatic vessels: limited to thickest connective septa |
| Talk about the innervation of skeletal muscles. | recieve one or more nerves that ramify increasingly into branches within the connective tissue, each skeletal muscle is innervated by at least one nerve ending which contracts the surface at the level of the motor end plate or neuromuscular junction and governs its activity.
Motor end plate is the region of the plasma membrane of the muscle fiber containing neurotrasmitters receptors and which is directly opposite to the axon terminals of a motor neuron.
Motor unit is nerve fiber and all nerves innervating it. |
