Lecture 18

Question 1

Describe cross-bridge formation in skeletal muscle

Answer 1

tropomyosin bound to actin, myosin can’t strongly bind. Ca binds to troponin, pulls tropomyosin away from actin, myosin can now strongly bind.

Myosin picks up ATP, this unbinds it from actin. ATP hydrolysis swivels head, binds strongly to actin. Pi release = power stroke, end of power stroke = release of ADP

Question 2

Describe excitation-contraction coupling in skeletal muscle

Answer 2

action potential depolarizes motor neuron, release of Ach. Ach diffuses across synaptic cleft, binds to Ach receptor, net influx of Na+ depolarizes, and creates end plate potential (graded). Leads to AP in nearby voltage gated Na channels. AP spreads down membrane into T-tubule, causes conformation change of DHP (L-type Ca channel) (dihydropyridine) which causes conformation change in RyR causing release of Ca2+ from SR. This leads to it binding to actin.
relaxation occurs through SERCA

Question 3

Describe how muscle tension is determined

Answer 3

frequency of action potentials
1:1 ratio of AP to muscle twitch
If APs arrive before muscle twitch completed, leads to summation and unfused or complete tetanus
sarcomere length, has optimal length
motor unit recruitment = small units recruited first, then larger ones (requires larger potential to activate)