QuestionSBA
QuestionChoice
Action potentials propagate down transverse tubules to a triad structure consisting of a transverse tubule and two cisternal ends of sarcoplasmic reticulum. At these triads, modified dihydropyridine receptors which themselves cannot cause calcium flux alter their conformation when the membrane is depolarised, causing allosteric interaction with attached ryanodine receptors on the sarcoplasmic reticulum. This allows calcium to move down its electrochemical gradient from the sarcoplasmic reticulum into the sarcoplasm.
10001171
a
Action potentials activate dihydropyridine receptors coupled with sarcoplasmic reticulum ryanodine receptors, causing calcium release into the sarcoplasm
QuestionChoice
Muscle cells use ACh which binds nAChRs to achieve this initial depolarisation which stimulates neighbouring sodium channels to open, generating an action potential which spreads across the muscle cell.
10001172
b
Glutamate binds NMDA and AMPA receptors on skeletal muscle membranes, opening channels which allow sodium entry and depolarising the muscle cell
QuestionChoice
Calcium flux through dihydropyridine receptors occurs in cardiac muscle but not skeletal muscle. This is necessary due to the poorly developed transverse tubules, particularly in the atria which only have caveolae.

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10001175
e
Action potentials activate dihydropyridine receptors which subsequently allow calcium flux into the cytoplasm. This induces calcium-induced calcium release via ryanodine receptors on the sarcoplasmic reticulum
QuestionChoice
Caldesmon is used in smooth muscle for this role whilst skeletal and cardiac muscle use troponin. In smooth muscle, there is also modulation of caldesmon activity by phosphorylation by protein kinase C (downstream of PLC which generates DAG and IP3 from PIP3) and modulation by calcium-calmodulin complexes.
10001173
c
Upon calcium flux into the cytoplasm, caldesmon undergoes a conformational change, removing tropomyosin from actin-binding sites, and allowing myosin cross-bridging
QuestionChoice
Latch bridges are only formed in smooth muscle. These are used to fix the muscle at a set length without continuous ATP consumption. This occurs when myosin is dephosphorylated when it is bound to actin, whereas dephosphorylation when myosin is not bound to actin causes relaxation. Myosin is constantly activated in skeletal muscle, and the limiting factor is troponin moving tropomyosin to reveal actin binding sites, rather than myosin phosphorylation.
10001174
d
After contraction initiation, if myosin phosphatase removes the phosphate group from myosin, latch bridges are formed
Concept
Chapter
2693
3822
Membrane Potential
Topic
152
Neuroscience
12781
Which of the following statements about skeletal muscle excitation-contraction coupling is true?
a