Write a note on the physiology of the cardiac muscle. Also explain the phases of cardiac muscle action potential?

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 HEART:

Heart contains two pumps: 

  1. a right heart that pumps blood through the lungs, 
  2.  a left heart that pumps blood through the systemic circulation provides blood flow to the organs and tissues of the body. 
  • Heart contains two chambers; Atrial and Ventricles.
  • Cardiac rhythmicity: Special mechanisms in the heart cause a continuing succession of heart contractions.

https://pixabay.com/vectors/human-heart-pumping-care-medicine-41546/

Physiology Of Cardiac Muscle:

The heart is composed of three major types of cardiac muscle: Atrial muscle, Ventricular muscle and specialized excitatory and conductive muscle fibers.

Cardiac muscles are actually Skeletal muscles. They have many similarities and differences.

Simmilarities:

  • The cardiac muscle is striated just like skeletal muscle.
  • Cardiac muscle has typical myofibrils that contain actin and myosin filaments almost identical to skeletal muscle.
  • These filaments lie side by side and slide during contraction just like skeletal muscle.
Differences:
  • The atrial or ventricular types of muscles contract in much the same way as skeletal muscle, but there duration of contraction is much longer than skeletal muscles.
  • There are major differences between the membrane properties of cardiac and skeletal muscles account for prolonged action potential in cardiac muscle.
Action Potential In Cardiac Muscles:
The action potential in ventricular muscle fiber averages about 105 millivolts which means that the potential rises from a very negative value, about -85 millivolts between beats to a slightly positive value, about + 20 millivolts, during each beat.
  • After the initial spark, the membrane remains depolarized for about 0.2 second exhibiting a plateau, followed by abrupt repolarization.
  • The presence of plateau in the action potential causes ventricular contraction to last as much as 15 times as long in cardiac muscle as in skeletal muscle.
In Cardiac Muscle, the action potential is caused by opening of two types of channels of channels: 
  1. The same voltage activated fast sodium channels as those in skeletal muscle.
  2. Caused by another different population of L-type calcium channels, which are also called calcium-sodium channels.
https://www.shutterstock.com/image-vector/nerve-impulse-action-potential-neuron-scheme-190034219


Phases Of Cardiac Muscle Action Potential:
There are following phases of cardiac muscle action potential:
Phase 0: ( Depolarization )
  • Fast sodium channels open. When the cardiac cell is stimulated and depolarizes, the membrane potential becomes positive.
  • Voltage gated sodium channels open and permit sodium to rapidly flow into the cell and depolarizes it.
  • The membrane potential reaches above +20 millivolts before the sodium channel close.
Phase 1: ( Initial Repolarization )
  • Fast sodium channels close.
  • The cell begins to repolarize.
  • Potassium ions leave the cell through open potassium channels.
Phase 2: ( Plateau )
  • Calcium channels open and fast potassium channels close.
  • Initial repolarization occurs, then plateau as a result of:
  • 1. Increased calcium ion permeability 2. Decreased potassium ion permeability.
  • The combination of decreased potassium ion efflux and increased calcium ion influx causes the action potential to plateau.
Phase 3: ( Rapid Repolarization )
  • Calcium channels close and slow potassium channels open.
  • The closure of calcium ion channels and increased potassium ion permeability, permitting potassium ions to rapidly exit the cell, ends the plateau and returns the cell membrane potential to its resting level.
Phase 4: ( Resting membrane potential )
  • Averages about -90 millivolts.

https://www.shutterstock.com/image-vector/connection-between-ecg-electrical-activity-heart-76383643
Velocity of signal conduction in cardiac muscle:
The velocity of conduction of excitatory action potential signal along both atrial or ventricular muscle fibers is about 0.3 to 0.5 m/sec or about 1/250 is the velocity in a very large nerve fibers and about 1/10 the velocity in skeletal muscles.
References:
Guyton and Hall ( Textbook of Medical Physiology )
John E. Hall 
Author C. Guyton Professor and Chair Department of Physiology.
https://www.shutterstock.com/image-vector/connection-between-ecg-electrical-activity-heart-76383643
https://www.shutterstock.com/image-vector/nerve-impulse-action-potential-neuron-scheme-190034219
About Author:
Tayyba Saleem ( PT )
Doctor Of Physical Therapy.

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