An action potential is the fast, sudden and propagating modification of the resting membrane potential. Action potential arises either due to threshold or the suprathreshold stimuli on a neuron. It broadly comprises the following phases – depolarisation, overshoot, repolarisation.
When an action potential is initiated in a cell, there is a flow of current through the gap junctions, depolarising the adjacent cells. If depolarisation leads the membrane potential to be more positive compared to the threshold, the self-propagating action potentials take place in the adjacent cells too. As a result, the generation of an action potential is as vital in initiating a contraction in the cardiac muscles as it is in the skeletal muscles.
Skeletal Muscle Action Potential
- Generation of action potential in cells of skeletal muscles is quite similar to that seen in neurons
- The duration of action potential in skeletal muscle cells is about 10 milliseconds which is somewhat longer compared to neurons; however, the refractory period is shorter
- The contraction in most of the muscles takes place only at the end of an action potential
Cardiac Muscle Action Potential
- It is a fleeting change in the voltage across the cell membrane of the cells of the heart
- These are caused due to the movement of the charged atoms between the outside and inside of the cell, via proteins referred to as the ion channels
- There is a difference between cardiac action potential and the action potentials seen in other types of electrically excitable cells (nerves, for instance)
- The action potential also differs in the heart as a result of the presence of various ion channels in various cells
- This potential is not initiated by nervous activity, rather seen in cells in the SAN (sinoatrial node) in healthy hearts, generating about 60-100 action potential per minute
Key Differences between Skeletal Muscle and Cardiac Muscle Action Potential
The table below depicts the differences between Skeletal Muscle and Cardiac Muscle Action Potential.
|
Skeletal Muscle Action Potential |
Cardiac Muscle Action Potential |
|
Range of action potential |
|
|
About 2-5 ms |
About 200-400 ms |
|
Elaborate sarcoplasmic reticulum |
|
|
Yes |
Lesser compared to skeletal muscle (1–8% of cell volume); limited terminal cisterns |
|
Presence of gap junctions |
|
|
Not present |
Yes, present; at the intercalated discs |
|
Source of Ca2+ for the calcium pulse |
|
|
Sarcoplasmic reticulum |
Sarcoplasmic reticulum and from the extracellular fluid |
|
Cells show individual neuromuscular junctions |
|
|
Yes, they exhibit |
No, they do not |
|
Pacemaker |
|
|
Absent |
Present |
|
Impact of nervous system stimulation |
|
|
Excitation |
Inhibition or excitation |
|
Number of Ca2+ binding sites in each troponin C molecule |
|
|
Two high-affinity Ca2+ binding sites |
A single active low-affinity Ca2+ -binding site |
You read some differences between Skeletal Muscle and Cardiac Muscle Action Potential.
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