Muscle Flashcards
Describe the ultra structural appearance of skeletal muscle and state which bands contain actin, myosin or both
Skeletal - striated
MHAZI - M line is in the H band which is in the A band. Z line is in the I band.
Actin - thin filament
Myosin - thick filament
Classify the three histological forms of muscle
Skeletal
Smooth
Cardiac
Outline the mechanisms of the sliding filament model of muscle contraction
Actin, tropomyosin and troponin molecules form the actin filaments
Myosin molecules make up myosin filaments
Calcium binds to troponin –> conformational change so tropomyosin moves –> binding site exposed
Stage 1 - Attachment - myosin head is tightly bound to actin molecule
Stage 2 - Release - ATP binds to myosin head causing it to uncouple from actin
Stage 3 - Bending - hydrolysis of ATP causes myosin head to bend and advance
Stage 4 - Force Generation - myosin head binds and causes power stroke (myosin returns to former position)
Stage 5 - Reattachment - myosin head binds tightly again and cycle repeats
Describe the mechanism of innervation of muscle and excitation contraction coupling
- Action potential arrives –> voltage gated calcium ion channels open –> calcium flows from extracellular fluid into presynaptic neurone’s intracellular fluid
- Influx of calcium –> neurotransmitter vesicles to move and fuse with membrane –> release of ACh into synaptic cleft
- ACh diffuses across cleft –>
binds to receptors on motor end plate - Post synaptic receptors open –> potassium out, sodium in (depolarisation) –> spreads to T tubules
- Voltage sensor proteins of T tubule membranes change conformation
- Gated calcium ion channels of terminal cisternae activate –> calcium ions released from cisternae –> sarcoplasm
- Calcium ions bind to troponin –> contraction cycle –> calcium ions are returned to terminal cisternae
Explain the mechanical continuity of muscle fibres, muscle sheath, tendons and bone
Muscle fibre (endomysium) --> fascicle (perimysium) --> epimysium Endo - inside, peri - around, epi - on/above
Explain the hierarchical composition of a typical muscle, outlining the principal components at molecular, organellar, cellular, histological and regional anatomical levels
Fascicles –> fibres (cells) –> myofibrils –> myofilaments (actin, myosin)
Describe the limited natures of repair possible in a mature muscle
Skeletal - cells cannot divide tissue can regenerate by mitotic activity of satellite cells (hyperplasia follows muscle injury), satellite cells fuse with existing cells to increase mass (hypertrophy), gross damage is repaired by connective tissue (scar), if blood or nerve supply is interrupted, muscle cells degenerate and are replaced by fibrous tissues
Smooth - cells maintain mitotic activity and can form new cells e.g. in pregnant uterus (hypertrophy and hyperplasia)
Cardiac - incapable of regeneration, following damage fibroblasts invade, divide and lay down scar tissue
Describe the histology of heart and smooth muscle, relating structure to function
Cardiac:
Striated
Centrally positioned nuclei
Branching - intercalated discs for electrical and mechanical coupling to other cells
Gap junctions - electrical coupling
Adherens type junctions - anchor cells, provide anchorage for actin
T tubules are in line with Z bands
Smooth:
Not striated
Spindle shaped with central nucleus
Contraction is slower, more sustained and requires less ATP
Capable of being stretched
Responds to stimuli in form of nerve signals, hormones, drugs, blood gases
Thick and thin filaments are arranged diagonally - contracts in a twisting way
Outline the structure and function of the Purkinje fibres of the heart
Purkinje fibres (modified monocytes) transmit action potentials to the ventricles from the AVN. They are large cells with abundant glycogen, sparse myofilaments, extensive gap junction sites They conduct action potentials rapidly compared to regular cardiac muscle - ventricles contract in a synchronous manner.
