Skeletal Muscle

1
Skeletal Muscle

• Regulation of Contraction
• Action Potentials

2
Action Potential

• Message from nervous system that stimulates
  muscle to contract
• Electrical current propagated along the sarcolemma

3
Events of an Action Potential

• 1. Depolarization sodium channels open, sodium
  goes into the cell, cell becomes less negative
• 2. Propagation of action potential along
  sarcomlemma
• 3. Repolarization sodium channels close (no
  more sodium coming in), and potassium channels
  open (potassium goes out of cell), resting
  membrane potential is re-established

4
Excitation- Contraction Coupling

• Series of events linking the electrical signal
  (message from brain) to muscle contraction

5
Neuromuscular Junction

• Motor neurons stimulate muscle cells
• Cell body is in brain or spinal cord, but long
  cellular extensions (axons) travel to the muscle
• Axon divides as it enters the muscle, and each
  axonal ending forms a neuromuscular junction with
  a single muscle fiber

6
Neuromuscular Junction Anatomy

• See Diagram!!!
• Axonal ending
• Synaptic vesicles, contain ACh
• Synaptic cleft space between the nerve ending
  and the muscle
• Motor end plate trough-like part of muscle
  fibers sarcolemma
• ACh receptors

7
Generation of an Action Potential

• Polarized sarcolemma inside the cell is more
  negative in resting state
• Binding of ACh causes change in membrane
  potential, so that the inside of the cell becomes
  less negative (depolarized)
• Rippling effect, AP travels along the sarcolemma

8
Excitation- Contraction Coupling

• 1. AP propagates along the sarcolemma and down
  the T-tubules
• 2. AP passes across triad region, causing SR to
  release Ca2 into the sarcoplasm

9
E-C Coupling Continued

• 3. Cytoplasmic Ca2 binds to tropinin, causing
  the protein to change shape, removing the
  blocking action of tropomyosin
• 4. Myosin cross bridges attach and pull thin
  filaments toward center of sarcomere

10
E-C Coupling Continued

• 5. After action potential ends, Ca2 is taken
  back up and stored in SR
• 6. Tropomyosin blockade of actin is
  reestablished, myosin ATPases are inhibited.
  Crossbridge activity ends and relaxation occurs

11
Summary of Ca2 Role in Muscle Contration

• Ca2 concentrations in sarcoplasm are kept low in
  resting state
• Promotes neurotransmitter release
• Released from SR when AP is present
• Triggers sliding of myofilaments