1. Understanding How a Skeletal Muscle Contracts

2. A skeletal muscles contraction begins at the neuromuscular junction.
What do you think the definition of “neuromuscular junction” would be?

3. Neuromuscular Junction

4. Neuromuscular junction animation
Focus Questions:
What is the name of the stimulus that travels down the axon to the muscle fiber?
An action potential
Does the terminal (end) of the axon enter the muscle fiber?
No. There is a gap between the two.
Does acetylcholine enter the muscle fiber?
No.
What chemical does enter the muscle fiber, resulting in an action potential through the muscle fiber?
Sodium

5. Sliding Filament theory
Boat = Myosin (thick filament)
Oar = Myosin side arm
Water = Actin (thin filament)
Life ring = Calcium

6. Resting ATP is bound to myosin side arm.
ATP cleaves into ADP + P (high energy)

7. Step 1 Action potential
A nerve action potential releases acetylcholine into the synaptic cleft opening the Na+ channels.
Action potential spreads across sarcolemma releasing Ca into sarcoplasma

8. Step 2 Myosin-actin binding
Ca binds to troponin
A shape change in troponin moves tropomyocin out of the way of actin binding site
Actin and myosin bind using energy from cleaved ATP.

9. Step 3 Power Stroke
Side arm pivots so myosin and actin slide by each other shortening the sarcomere.
ADP and P released (low energy)

10. Step 4 ATP Binding Actin-myosin release
A different ATP molecule binds to active site.
Actin released

11. Step 5 ATP cleavage Return to high energy state
Cycle will repeat if Ca still available.

12. Think it over
The boat (myosin) does not move far in one cycle, a muscle contraction requires many cycles
If a muscle is contracted what happens if a new molecule of ATP is not available?
Muscle stays contracted- cramps
Why does rigor mortis occur? (Hint: What chemical is no longer available to the body?)
ATP is not available to control Ca release so contractions are continuous 6-8 hours after death. Body relaxes hours as enzymes break down contractile structures.

13. Sarcomere summary

14. Sliding Filament Animation
Focus Questions:
What chemical exposes the binding site for actin and myosin? Ca
What is the source of energy for a contraction?
ATP
What is the name of the step in which the actin filament is actively contracted?
Powerstroke
What chemical must be present in order for the actin and myosin filaments to separate?

15. Observing muscles in action
Place your fingers along the angle of your jaw just in front of your ear. Grit your teeth and feel what happens to the hardness of the masseter muscle.
During muscle contraction the muscle becomes ________________________.

16. With your thumb and little finger of one hand, span the opposite arm’s bicep’s from the elbow to as close to the shoulder as possible. Bend the arm and observe the change in the length of the muscle.
During muscle contraction the muscle ___________________ in length.

17. Wrap a string around your extended upper arm and determine the circumference.
Clench your fist tightly and flex your arm to contract the muscle.
During muscle contraction the diameter of the muscle _____________________.