Muscle Flashcards
Thick filament of the sarcomere
Myosin II molecules assemble via their tail regions, with their heads projecting to the outside of the filament. Note the central bare zone, which is free of head domains.
Excitation-Contraction Coupling
It is different in skeletal, cardiac, and smooth muscle.
Intercalated Disks
Individual cardiac myocytes are connected by “intercalated discs”. These contain desmosomes and other adherent junctions which hold the cells firmly together. Gap junctions are also present, forming “electrical synapses” that allow calcium to pass from cell to cell as a single wave.
myosin II
Form of myosin which generates contractile force in muscle
A myosin II molecule is composed of two heavy chains (green) and four light chains (blue). The light chains are of two distinct types, and one copy of each type is present on each myosin head. The α helices of two heavy chain molecules wrap around each other to form a coiled-coil, driven by the association of regularly spaced hydrophobic amino acids. The coiled-coil arrangement makes an extended rod in solution, and this part of the molecule is called the tail. Each myosin head binds and hydrolyzes ATP during muscle contraction.
Activation of smooth muscle contraction
Smooth muscle contraction is also triggered by calcium, but in this case it is independent of troponin (which is not present in smooth muscle).
The cycle starts with a pulse of Ca2+ that enters the smooth muscle cell and activates calcium-induced calcium release from the SR. This release of calcium leads to binding of Ca2+ to calmodulin, which in turn activates myosin light chain kinase. MLCK phosphorylates the light chain of myosin, leading to a conformational change in the myosin head and the activation of myosin ATPase and force generation.
Cardiac myocytes
Contain more mitochondria than skeletal muscle cells to supply the heart with enough ATP for its continuous workload
Smooth muscle organization in tissues
In hollow and tubular organs, smooth muscle is often arranged in two (or more) layers that are perpendicular to each other. This is most evident in the GI tract, where the smooth muscle is arranged into an inner circular layer (closer to the epithelium) and an outer longitudinal layer (closer to the serosa/peritoneum).
The circular layer regulates the diameter of the tube (contraction will constrict it), allowing regulation of flow and emptying, while contraction of the longitudinal layer will push things along (peristalsis)
Epimysium
A dense connective tissue layer that encloses the entire muscle, and is continuous with the tendon that attaches the muscle to the bone.
Synchronization of muscle contraction
An action potential (AP) initiated at the neuromuscular junction is normally the first event in E-C coupling. The AP rapidly propagates over the sarcolemma and into the T-tubules. When the T-tubule is depolarized, the voltage sensors (⊕) in the DHPRs move and open the RyRs, thus permitting Ca2+ to flow out of the SR into the cytosol to trigger contraction.
Once the impulse for muscle contraction (i.e., the action potential) is no longer present, cytoplasmic calcium concentrations must return to baseline to prevent further contraction. This is accomplished through the action of the SERCA (sarco/endoplasmic reticulum Ca 2+-ATPase) pump, which is located in the membrane of the SR and actively transports calcium from the cytoplasm back into the SR, using energy generated from ATP hydrolysis.
Muscle striation
Muscle fiber cross section
Rigor mortis
Since ATP is required to release myosin from actin, after you die and your muscles run out of ATP, myosin filaments will remain bound to actin and locked in their contracted conformation.
Type-Differential muscle fiber staining
The cardiac conduction system
The sinoatrial node (SAN) is known as the heart’s pacemaker. It is located within the wall of the right atrium (RA) and normally generates electrical impulses that are carried by special conducting tissue to the atrioventricular node (AVN). Upon reaching the AVN, located between the atria and ventricles, the electrical impulse is relayed down conducting tissue (bundle of His) that branches into pathways that supply the right and left ventricles.
Purkinje fibers receive nerve impulses from the conduction system and disseminate the impulses within the myocardium itself, ensuring that the heart connects as a unit. They stain more intensely eosinophilic than myocytes because they contain relatively fewer mitochondria (as they are not contractile), and mitochondrial DNA imparts a relatively basophilic appearance.
Z lines
Flank the ends of each sarcomere. They are visible as single dark lines in the electron micrograph and present the attachment sites for actin (thin) filaments.
The actin filaments, where they don’t overlap with myosin, are visible as light bands. The darkest area (on either side of the center) is where the myosin (thick) filaments intercalate with the actin filaments. The lighter area in the center is the area with only myosin.
Actin-myosin contraction cycle
https://embryology.med.unsw.edu.au/embryology/index.php?title=File:Actin_myosin_crossbridge_3D_animation.gif
Cardiac Muscle
Nuclei are more centrally placed, rather than being pushed to the periphery of the fiber as in skeletal muscle.
The cells often branch rather than exclusively forming linear fibers (not well-visualized here).
Cells are connected by intercalated disks, shown by the green arrows in the image. These structures, which contain gap junctions and desmosomes, are necessary to allow the heart to beat as a unit.
sarcolemma
plasma membrane of muscle
Initiation of cardiac contraction
Cardiac cells express a different form of the ryanodine receptor than skeletal muscle, called RyR2. The RyR2 differs from the RyR1 expressed in muscle in that it does NOT directly interact with the DHPR.
When the membrane is depolarized, the DHPR will open will allow calcium to enter the cell. Calcium can then bind to specific binding sites on the RyR2, triggering it to open and release calcium from the sarcoplasmic reticulum into the sarcoplasm. This mechanism is called calcium-induced calcium release.
Cross-sectional cardiac muscle diagram
Smooth muscle transmission EM
The nucleus (N) and cytoplasm (C) are labeled. In the cytoplasm near the nucleus (N) are mitochondria, glycogen particles, and Golgi complexes.
Several dense bodies (arrows) can be seen in the cytoplasm and at the cell membrane. Thin filaments and intermediate filaments both attach to the dense bodies. These function as the anchoring points for smooth muscle contraction.
Skeletal muscle histomorphology
Cardiac excitation-contraction-termination
The more calcium becomes available, the more RyR2 receptors will open. Importantly, because cardiac myocytes are connected by gap junctions, calcium will not only be able to activate contraction in one cell, but the calcium wave will be transmitted through gap junctions from one cardiomyocyte to another. This mechanism allows the whole heart to contract in a synchronous fashion. This also explains why there is less need of the SR to be in direct proximity to the T-tubules.
Termination of cardiac muscle contraction is accomplished through actively pumping Ca2+ out of the cytoplasm – both into the SR (80%, via the SERCA pump) and into the extracellular space (20%, via pumps in the plasma membrane called PMCA and NCX)
Bicep/Tricep
Troponin and tropomyosin
Tropomyosin binds along the groove of the actin filament. The troponin complex is made up of three polypeptides (troponins T, I, and C) which are named for their tropomyosin, inhibitory, and calcium-binding activities, respectively.
In resting muscle, the troponin I-T complex pulls tropomyosin into a position that interferes with the binding of the myosin heads, preventing any force-generating contraction. When calcium levels increase, troponin C binds to calcium, and this causes troponin I to release its hold on actin. This alters the position of tropomyosin such that the myosin heads are able to bind and walk along the actin filaments.
Neuromuscular Junction Contraction Cascade
T-tubules: Transverse tubules, Invaginations which go deep into the muscle and surround each myofibril.
Smooth muscle contraction cartoon
Smooth muscle
Compared to skeletal and cardiac muscle, smooth muscle is relatively disorganized. The cells are elongated, with the nucleus centrally placed in the middle of the cell and the two ends diminishing to a small pointed end (“fusiform”).
Skeletal muscle from the side
Top right: smooth muscle
Bottom right: skeletal muscle
This is from the esophagus, one of the few places in the body where both may be seen in one section.
Dystrophin
Hypertrophic cardiomyopathy cardiac muscle
