Understanding How a Skeletal Muscle Contracts

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Understanding How a Skeletal Muscle Contracts
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A skeletal muscles contraction begins at the
neuromuscular junction.

• What do you think the definition of
  neuromuscular junction would be?

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Neuromuscular Junction
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Neuromuscular junction animation

• 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
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Sliding Filament theory

• Boat Myosin (thick filament)
• Oar Myosin side arm
• Water Actin (thin filament)
• Life ring Calcium

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Resting

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

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Step 1 Action potential

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

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Step 2 Myosin-actin binding

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

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Step 3 Power Stroke

1. Side arm pivots so myosin and actin slide by each
   other shortening the sarcomere.
2. ADP and P released (low energy)

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Step 4 ATP BindingActin-myosin release

1. A different ATP molecule binds to active site.
2. Actin released

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Step 5 ATP cleavage

1. Return to high energy state
2. Cycle will repeat if Ca still available.

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Think it over

• The boat (myosin) does not move far in one cycle,
  can a muscle contraction occur with one cycle?
• No
• 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.

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Sarcomere summary
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Sliding Filament Theory

• Focus questions
• What happens to the length of the sarcomere
  during a contraction?
• The sarcomere shortens.

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Sliding Filament Animation

• animation 2
• 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?
• ATP

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Muscle contraction at the macroscopic level

• Place your fingers along the angle of your jaw
  just in front of your ear. Grit your teeth and
  fell what happens to the hardness of the masseter
  muscle.
• During muscle contraction the muscle becomes
  ________________________.

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• With your thumb and little finger of one hand,
  span the opposite arms biceps 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.

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• 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 _____________________.