ATP and Events of Cross bridge cycling

Question 1

What does it mean that myosin is in high energy form?

Answer 1

The energy released during ATP hydrolysis changes the angle of the myosin head, and resets myosin shape to “resting” and the myosin is now considered to be in high energy form because it has stored potential energy in the form of ADP+Pi and ready to attach to actin again when calcium ions are available.

Question 2

What are the three functions of ATP in muscle contraction?

Answer 2

1. ATP hydrolysis by an ATPase activates the myosin head so it can bind to actin and rotate;
2. Its binding of myosin causes detachment from act in after the power stroke;
3. It powers the pumps that transport calcium ions out of the cell or into the SR.

Question 3

Powerstroke refers to the pulling action of myosin on actin. This energy uses stored energy.

During powerstroke, _________ is released from myosin head
After power stroke, ______ is released from myosin head

Answer 3

Inorganic phosphate

Adenosine phosphate

Question 4

In a crossbridge event, when is myosin is in its lowest energy form ?

Answer 4

At the end of a power stroke, when myosin is bound to actin, and before new ATP attaches to myosin.

Question 5

What is asynchronous cycling referring to in a crossbridge cycle?

Answer 5

During muscle contraction, myosin molecules may be undergoing cross-bridge cycle while others are at rest. But once power stroke occurs, those myosin and actin that are bounded will stay bounded until ca2+ is removed… but the next phase of myosin and actin molecules can bind to generate force.

Question 6

Sarcomeres are the structural units of a myofibril. Each myofibril contains 100, 000 sarcomeres. These sarcomeres are bounded by Z lines on either end.

Answer 6

Smooth muscles do not have myofibrils, therefore no sarcomeres, or Z lines, or the regulatory protein troponin.

Calcium ions binds to calmodulin which activates myosin kinase causing phosphorylation of myosin. This chemical change in myosin is what caUses the contraction in smooth muscle

Question 7

Where can muscle fiber acquire ATP for energy/contraction?

Answer 7

1. Availability of AtP in the sarcoplasm
2. ATP can be generated from the hydrolysis of creatine phosphate. 2 ATP
3. Glycolysis - 2 ATP
4. Oxidative phosphorylation - 34 ATP

Question 8

What are the factors that determine the tension produced by individual muscle fibers

Answer 8

Frequency of stimulation - more action potential generated –> ACh released into neuromuscular junction.
can cause Twitch summation: stimulate the muscle again before it has time to relax.

The availability of cross-bridge binding sites; the more binding sites uncovered, the higher the potential for more tension to be developed.

Muscle is at its optimal length. so not too lengthened or shortened. Myosin can easily crossbridge with actin.

Question 9

When is a muscle fiber considered to be contracting isometrically or isotonically ?

Answer 9

Whether a muscle fiber is contracted isometrically or isotonically depends on whether the MF changes length during the contraction. Isometric = equal length
Isotonic: muscle fibers