Flashcards in Skeletal And Cardiac Deck (29)
Describe the CT surrounding muscle.
Epimysium- Dense irregular CT
Perimysium- Less dense, irregular CT
Endomysium- Loose CT with reticular fibers present
Describe the formation of a muscle fiber.
Mesenchymal stem cells become myoblasts, which fuse to form immature myotubes (with central nuclei), that become mature mytotubes when their SR and t-tubules form, and nuclei are displaced to periphery.
Describe the purpose of satellite cells.
They are persistent stem cells with a single nucleus, that exists between muscle fibers. Serve as regenerative cells.
Describe the microanatomy of a t-tubule.
They are invaginations of the sarcolemma that surround myofibrils at the A/I band junction, and contain voltage sensitive DHP receptors.
Where does release of calcium from the SR occur?
At the longitudinal SR, which also contains the calcium ATPase reuptake channels.
Describe the structure of cardiac muscle cells.
1. Branching cells with central nuclei, likely a single nucleus but occasionally are binucleate.
2. Intercalated disks exist between the sarcolemma and the last Z-line of the thin filament. These disks have a longitudinal component with gap junctions for electrical coupling, and a transverse component with fascia adherens and desmosomes for mechanical coupling.
3. More mitochondria are present than in skeletal muscle, but T-tubules and SR are less extensive, often only forming dyads.
Describe the structure of smooth muscle.
Are the smallest of all muscle cells, and are elongated with a single central nucleus.
Describe thin filament attachment in smooth muscle.
Dense bodies connect individual filaments, as well as one filament to the sarcolemma, via alpha actinin.
What is a special feature of smooth muscle, barring dense bodies?
The presence of calveoli invaginations and bubbles at the surface where calcium enters the cytoplasm.
Describe the connection of neighboring smooth muscles.
They are extremely interconnected via extensive gap junctions.
Describe the types of smooth muscle innervation.
Either can occur directly by autonomic axons at an individual fiber, or the excitation at one can be spread to others via gap junctions.
Describe secretions of smooth muscle.
Can secrete types 3 and 4 collagen anywhere, and type 1 collagen and elastin if near blood vessels.
Describe the arrangement of myosin.
In thick filaments, it is arranged tail to tail, creating a central bare zone. Individual myosins are linked via myomesin and C protein at the midpoint of A bands, forming the M line.
What is titrin?
A protein that runs the entire length of thick filaments, from M to Z lines, to link thick filaments as a whole.
Describe the general structure of a thin filament, individually.
F-actin filaments exist with their plus ends linked to a Z-line by alpha-actinin. Nebulin then runs the length of the filament to assist with this binding. Note the length of an individual filament is modulated by this nebulin, as well as capping protein tropomodulin. Finally these filaments are linked to the cell membrane and external lamina via dystrophin.
Describe the tropomyosin and troponin of cardiac and skeletal muscle cells.
Tropomyosin exists within grooves of F-actin, with troponin associated with it, containing TnT to bind troponin, TnC to bind calcium, and TnI to bind myosin binding domains of the filament.
What links neighboring Z-lines of myofibrils?
Deamin and vimentin IF and MT aggregations.
Describe the structure of an individual myosin.
A long tail section composed of light meromyosin exists, as well as a globular head containing heavy meromyosin, which are linked via an S2 hinge. Note the globular head is composed of an essential light chain for stability and a regulatory light chain for muscle function.
What determines the rate of crossbridge cycling?
The level of ATPase activity within the myosin head.
What are the steps of the sliding filament model?
1. ADP and Pi are bound to myosin, tropomyosin blocks myosin binding site on actin.
2. Calcium influx occurs and binds to TnC, causing tropomyosin movement and myosin binding site exposure.
3. Cross bridge forms and ADP/Pi are released, head turns from 90 degrees to 45 degrees at the hinge, relative to the tail.
4. Recoil of the hinge is the power stroke, causing sarcomere shortening.
5. Attached crossbridge remains in rigor state until ATP binds myosin.
6. ATP binds to release crossbridge, hydrolysis causes head movement back to 90 degrees.
What are the types of muscle contraction?
Isotonic- Muscles are free to move, cross bridges occur at actin further along the filament.
Isometric- Muscle not free to move, crossbridge cycling occurs at the same spot continuously.
Describe the ideal and less so sarcomere lengths.
Ideal=Lo=1.95 to 2.25 micrometers
Over 3.6 and no filament overlap is present, crossbridge can't form.
Plateau phase is steady decrease in crossbridge efficiency as thick:thick and thin:thin overlap begins and causes steric hindrance, and is 1.67-1.95. Then major dropoff to zero bridges forming is below 1.27.
What are the steps in a muscle contraction (not crossbridge cycle)?
1. AP depolarizes DHPRs at a specific t-tubule.
2. DHPRs undergo a conformational change that cause ryanodine receptors in SR to open and release calcium.
3. Rise in intracellular calcium induces crossbridge cycling.
4. Relaxation is achieved when calcium ATPase in SR functions to move calcium back into sarcoplasmic reticulum, where it is bound and retained by calsequestrin.
What are some structural differences between cardiac and skeletal muscle?
Cardiac has a less extensive SR, smaller terminal cisternae (and often only one, forming a dyad), and t-tubules are larger.
Describe the contraction of a cardiac muscle cell.
1. Calcium enters a cell via voltage sensitive DHPRs during the plateau phase of an action potential (thus some calcium comes from the extracellular space).
2. Entered calcium binds RyRs, and cause them to release calcium from the SR to the cytoplasm (this is a CICR, depends on amount that enters cell via DHPRs).
3. Most of the calcium is uptook via the SR again, but an Na/Ca exchanger also exists at the surface membrane as calcium decrease must be quicker.
What are CICR differences in cardiac muscle, depending on resting or active phase?
Resting: CICR=5 L/min
Active: CICR=20-25 L/min
What are three methods to modulate cardiac muscle contractions?
1. Increase the CICR via activation of B1 adrenergic receptors via phosphorylation, causing them to be open longer and increase calcium entry.
2. Increase calcium sensitization of the myofilaments.
3. Utilize PKA to phosphorylate TnI, thus decreasing TnC affinity for calcium, resulting in off-phase relaxation occurring quicker (needed with quicker contractions). Note this PKA also phosphorylates SR phospholamban, increasing calcium ATPase reuptake).
Describe temporal summation.
It is the maximum frequency at which muscles can contract, determined by the absolute refractory period duration. Note skeletal and smooth muscle absolute refractory period ends before peak force occurs, so they can be tetanized. Mechanical and electrical mechanisms are highly similar in cardiac muscle, so tetanus cannot occur.