Lecture 11: Muscle physiology 2 Flashcards
What is the organisation of muscle from biggest unit to smallest including layers of connective tissue in between
The whole muscle is wrapped in epimysium. underneath this are bundles of muscle called fascicles wrapped in perimysium.
In the fascicles are the muscle cells: myofibre wrapped in endomysium. This covers the sarcolemma of the myofibre which inside contains myofibrils are divided into contractile units called sarcomeres and contain myofilament
What are the defining features of a myofibre
- Striated due to arrangement of thick and thin filaments
- multinucleated with nuclei on periphery
- can be very large
- lack cellular connections to other myofibres
What is the T-tubular membrane system structure and function
Invaginations of the surface sarcolemma (continuous with the extra cellular space) which go in and around myofibrils. They help coordinate contraction through conducting APs throughout the entirety of the cell
What is the Sarcoplasmic reticulum structure and function
An intracellular membrane bound system which stores Ca2+ used for cross bridge cycling. It helps with feedback control, release and then reuptake of Ca2+ to coordinate contraction
What is a ‘triad’
A t-tubule at the junction of A and I bands, with the two terminal enlargements of Sarcoplasmic reticulum either side.
What forms the boundaries for a sarcomere and what are the 3 bands
Outer boundary is z line/discs with thin filaments attached, with M line in the middle which has the thick filaments only- attached to z discs by titin.
A band is where the thick and thin overlap.
I is thin filaments only on the outer edges
H is thick filaments only in the middle
What is the thick filament structure
Made of myosin with the tail facing the m line and 2 myosin heads
What is the thin filament structure
2 strands of actin twisted into a helix. Tropomyosin in the groove with its Troponin complex, nebulin to align the filaments.
What is the structure of the Troponin complex and its function
Made of Troponin tropomyosin which positions it on top of tropomyosin, T calcium which contains Ca2+ binding sites and T inhibitor which binds actin and inhibits the myosin head from binding to the actin binding site
List the 5 steps of cross bridge cycling
- Cytosolic Ca2+ increases to uM range
- Ca2+ binds to Troponin complex on Troponin C thorugh 2 high and 2 low affinity binding sites which releases inhibition by Tropnin I
- Troponin Ca2+ complex pulls tropomyosin away from the myosin binding site on actin
- Myosin (in high energy conformation due to previous hydrolysis of ATP) is able to bind strongly to actin and completes power stroke
- Actin filament moves
What is the role of ATP in cross bridge cycle
ATP hydrolysis provides the energy for cross bridge movement and ATP binding to myosin after its completed the stroke breaks the link between actin and myosin, allowing the cycle to continue
What are the two conditions for force generation by cross bridge cycling
Ca2+ conc remains high and ATP is present
What are DHPRs: Dihydropyridine Receptors
Voltage gated Ca2+ channel in the T-tubule membrane opposite the SR that changes conformation with the depolarisation of the membrane. Interacts the Ryr to bring about Ca2+ release from the SR
What are Ryanodine Receptors (RyR1)
Ca2+ release channels of the SR on terminal cisternae, usually inhibited by cytosolic Mg2+ which is overcome by voltage sensor activation by DHPR
How does the time for AP compare to time for contraction
AP in muscle is only for 2 ms and almost straight after AP in the neuron, but this occurs in the latent period for the muscle contraction with the contraction phase and relaxation phase going for 10-100 ms
