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.
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
- 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.
- 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.
- 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.
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.
Depolarization
- Opening of fast voltage-gated Na+ channels.
- Rapid Influx of Sodium ions leading to rapid depolarization.
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.
K+ Efflux is very small as K+ permeability decreases &
only few K channels are open.
Repolarization
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.
- Rapid Repolarization= Potassium Efflux
- Plateau= Calcium Influx
- Repolarization= Potassium Efflux
