Lecture Objectives for: Excitation-Contraction Coupling

Question 1

Mechanistically define what is meant by the “size principle” of motor unit recruitment and the properties of type I and II motor units

Answer 1

The amount of input is reliant on the size of the motor unit. The larger the motor unit, the larger the amount of electrical input needed- type I, slow twitch, is small motor units (a smaller diameter reduces the conduction and the speed of the signal, but he smaller size of the cell body allows for a smaller amount of energy to be required for the cell)

Question 2

List the sequential events of the cross-bridge cycle and describe the mechanisms involved in each step

Answer 2

• Detachment: binding of ATP to myosin relaxes the myosin head as it is able to dissociate from the actin
• Energized cross-bridge: partial hydrolysis of ATP, which is still connected to myosin, cocks the myosin head again
• Cross-bridge attachment: Ca2+ allows for a cross-bridge to form between the myosin-ATP complex and the actin (new position on the actin though)
• Cross-bridge Motion: complete hydrolysis of ATP can occur because ATP is attached to actin and this energy from the hydrolysis allows the myosin to pull on the actin filament
• Rigor Complex: myosin is attached to actin without the assistance of Ca or ATP, causing a stiff, cocked confirmation of the head

Question 3

Describe the role of DHP and RYR receptors in the mechanism of E/C coupling

Answer 3

Action potential open the DHP-gated channels, allowing Ca2+ flow through the channels as a pore itnto the SR and exits this compartment via the RyR, which acts as another pore to allow for contraction

Question 4

Explain how a mutation in the skeletal muscle RYR receptor results in malignant hyperthermia when exposed to volatile anesthetics

Answer 4

Excessive release of Ca2+ via the RYR receptor leads to unregulated interaction of Ca2+ in the cell