Muscle Action

1
Muscle Action

• Muscle cells are polarized, have charge
• Excess of Na ions in ECF
• Excess of K ions in ICF mixed with PO4-3 cause
  muscle fiber to have negative charge
• The negative RMP is maintained by Na/K pumps

2
Muscle Action

• Depolarization when ion gates of fiber are
  opened and ICF briefly becomes positive
• Repolarization the return to RMP caused by K
  leaving cell
• Action potential the quick voltage shift from
  RMP(-90mV) to 75mV and back to RMP

3
Excitation

1. Voltage reaches knob causes Ca2 to be released
2. Ca2 causes ACh to be released into cleft
3. ACh diffuses across cleft and binds to ACh
   receptors

4
Excitation

1. Bound receptors cause ion gates to open allowing
   Na to flow in and K to flow out, charge spikes
   to 75mV and then quickly falls to RMP as K ions
   flow out
2. Action potential is achieved muscle cell is
   excited

5
Contraction

1. Charged wave of action potential travels from the
   synapse to the tissue around the sarcomeres like
   ripples in a pond
2. The charge causes tissue to release Ca that
   diffuses into the sarcomere
3. Ca binds to the thin filaments causing the G
   actin active sites to be exposed for binding
   myosin heads

6
Contraction

1. The myosin head breaks down ATP into ADP and Pi
   and uses the energy to move and bind to the
   active site forming the cross-bridge
2. The head releases the ADP and Pi causing a power
   stroke where the thin filament is pulled over the
   thick
3. The head will stay bound until another ATP binds
   to it breaking the cross-bridge

7
Relaxation

1. Nerve impulses stop, ACh is not released
2. AChE breaks down the ACh left in the cleft and
   the components are reabsorbed by the knob
3. Active transport Ca pumps return the gradient to
   pre action potential levels

8
Relaxation

1. Low Ca concentrations cause Ca to unbind from G
   actin covering the active sites
2. Muscle returns to resting length