Understanding How a Skeletal Muscle Contracts

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

Neuromuscular Junction

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

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

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

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

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.

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

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

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

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 16-24 hours as enzymes break down contractile structures.

Sarcomere summary

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?

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 ________________________.

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.

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 _____________________.