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Electrical activity- the action potential of the heart explained

Cardiac cells can contract without Nervous Stimulation.
  • Cardiac muscle, like skeletal muscle & neurons, is an excitable tissue with the ability to generate action potential.
  • Most cardiac muscle is contractile (99%), but about 1% of the myocardial cells are specialized to generate action potentials spontaneously. These cells are responsible for a unique property of the heart: its ability to contract without any outside signal.
  • The heart can contract without an outside signal since the signal for contraction is myogenic, that is these signals originating within the heart itself.
  • The heart contracts, or beats, rhythmically as a result of action potentials that it generates by itself, this  property of heart is called auto rhythmicity -auto means “self”.
  • Most important thing to understand is that the signal for myocardial contraction NOT comes from the nervous system but occur from the specialized myocardial cells also called auto rhythmic cells.
  • These cells are also called pacemaker cells of the heart as they set the rate of the heart beat. 
The cells of myocardium
Two specialized types of cardiac muscle cells:
Each of these 2 types of cells has a distinctive action potential.
They are
Contractile cells /working cell that include 99% of the cell
Myocardial Auto rhythmic cells (1%)  include 1% of cardiac cell
Myocardial Contractile cells
  • These cell constitute  99% of the cardiac muscle cells,
  • They do the mechanical work of pumping.
  • These cells normally do not initiate their own action potentials.
  • These cells contract and are also known as the Working Myocardium
  • Contractile cells which include most of the heart muscle Atrial muscle and Ventricular muscle
Myocardial Auto rhythmic cells
  • The small but extremely important remainder of the cardiac cells,include (1%) of cardiac cell
  • They  are specialized for initiating and conducting the action potentials responsible for contraction of the contractile cells.
  • They  do not contract because the cells are smaller and contain few contractile fibers or organelles. as they do not have organized sarcomeres, they do not contribute to the contractile force of the heart.
Action Potential of the Autorrythmic cardiac cells
  • The auto rhythmic cells do not have a stable resting membrane potential like the nerve and the skeletal muscles.
  • But  they have an unstable membrane potential that starts at – 60mv and slowly drifts upwards towards threshold.
  • Because the membrane potential never rests at a constant value, this  is called a Pacemaker Potential rather than a resting membrane potential. 
 What causes the membrane potentials of these cells to be unstable?
  • Auto rhythmic cells have  channels different from other excitable cells.
  • When cell membrane potential is at -60mv, channels are permeable to both Na and K ions .
  • This leads to Na influx and K efflux.
  • The net influx of positive charges slowly depolarizes the auto rhythmic cells.It will leads to opening of Calcium channels.
  • This moves the cell more towards threshold. When threshold is reached, many Calcium channels open leading to the Depolarization phase. 
Action potential of a contractile myocardial cell:a typical ventricular cell
Unlike the membranes of the autorrythmic cells, the membrane of the contractile cells remain essentially at rest at about -90mv until it is excited by electrical activity propagated by the pacemaker cells.
  • Opening of fast voltage-gated Na+ channels.
  • Rapid Influx of Sodium ions leading to rapid depolarization.
Small Repolarization
Opening of a subclass of Potassium channels which are fast channels.
Rapid Potassium Efflux.
Plateau phase
  • 250 msec duration (while it is only 1msec in neuron)
  • Opening of the L-type voltage-gated slow Calcium channels & Closure of the Fast K+  channels.
Large Calcium influx
K+ Efflux is very small as K+ permeability decreases & only few K channels are open.
Opening of the typical, slow, voltage-gated Potassium channels.
Closure of the L-type, voltage-gated Calcium channels.
Calcium Influx STOPS
Potassium Efflux takes place.
 Summary of Action Potential of a Myocardial Contractile Cell
  •  Depolarization= Sodium Influx
  • Rapid Repolarization= Potassium Efflux
  • Plateau= Calcium Influx
  • Repolarization= Potassium Efflux