Excitation Contraction Coupling

Question 1

Neuromuscular junction – things to know:

Answer 1

Motor neurons only release ACh to skeletal muscle
* ACh is always excitatory in skeletal muscle * Nicotinic (ionotropic) cholingeric receptors
are Na+ channels
* Na+ induces depolarization

Question 2

Excitation - STEPS:

Answer 2

1) Depolarization of the muscle membrane (aka sarcolemma) by Na+ entry (action potential in muscle)
2) Cytosolic Calcium levels increase.
3) Ca^2+ binds to troponin facilitating cross bridge formation

Question 3

Depolarization-induced Ca2+ release

Answer 3

• skeletal muscle
• Extracellular Calcium channels are connected to SR Calcium channels

Question 4

Ca2+-induced Ca2+ release

Answer 4

• cardiac muscle

Question 5

Depolarization Induced Ca^2+ Release - Transport/ Channels

Answer 5

• NaCaX (exchanger)
• DHPR (voltage-gated membrane channel/ receptor)
• RyR (Ca^2+ Channel in SR)
• SERCA (Ca^2+ pump; ATPase;$)
• Ca^2+ ATPase ($)

Question 6

Depolarization-Induced Ca^2+ Release (Steps):

Answer 6

1) Excitation (no contraction yet)
2) Calcium Release
3) Relaxation

Question 7

Fast skeletal muscles may have…

Answer 7

• more channels that permit Na+ and Ca2+ entry
• channels that open faster
• channels that stay open longer
• Fast action potential = faster contraction rate

Question 8

Depolarization-Induced Ca^2+ Release (Skeletal Muscle)

Answer 8

• Skeletal muscle concentration occurs long after cell has repolarized

Question 9

Ca^2+ Induced Ca^2+ Release (Cardiac Muscle)

Answer 9

• Cardiac muscle contraction occurs while the cell is repolarizing. Long repolarization is caused by voltage gated calcium channels.

Question 10

Absolute Refractory Period

Answer 10

• A new action potential cannot be generated, no matter the strength of the stimulus; Na+ ion channels are closed by inactivation gate.

Question 11

Relative Refractory Period

Answer 11

• A new action potential may be generated but only if the second stimulus is sufficiently strong; Na+ ion channels have had their inactivation gate reset to “open”.

Question 12

Effective Refractory Period

Answer 12

• muscle cannot contract again (under normal physiological conditions)

Question 13

Relative Refractory Period

Answer 13

• muscle has partially repolarized; a second action potential is possible

Question 14

Contractile Summation

Answer 14

• contractions add on to the previous contraction because of high frequency stimulation

Question 15

Tetanus

Answer 15

• Contraction is maximal and sustained by a strong stimulus.

Question 16

Fatigue (Muscle)

Answer 16

• Decrease in the strength of muscle contractions due to repeated stimulation without periods of rest.
• Occurs when ATP supply is depleted and oxygen is not replenished fast enough (lactic acid builds up in the muscle fibres.

Question 17

The strength of a muscle fibre depends on…

Answer 17

• How often the individual muscle fibres are stimulated to contract.
• How many muscle fibres (and motor units) contract within a given muscle