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 because the
signal for contraction is myogenic, originating within the heart muscle itself.
- The heart contracts, or beats, rhythmically as a result of
action potentials that it generates by itself, a property which is called auto
rhythmicity (auto means “self”).
- The signal for myocardial contraction comes NOT from the
nervous system but it is from specialized myocardial cells also called auto
rhythmic cells.
- These specialised cells are also called pacemaker cells of
heart because they set the rate of the heart beat.
The myocardium
Two specialized types of cardiac muscle cells exist
Each of these 2 types of cells has a distinctive action
potential.
Electrical Activity of the Heart
Myocardial Auto rhythmic cells (1%)
These cells are smaller and they contain few contractile fibers or
organelles. Because these cell do not have organized sarcomeres, they do not
contribute to the contractile force of the heart.
Myocardial Contractile cells (99%) -Contractile cells include
most of the heart muscle
- Atrial muscle
- Ventricular muscle
These cells contract and are also called as the working
myocardium
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.
- Instead 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, it 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 contain channels different from other
excitable cells.
- When cell membrane potential is at -60mv, channels are
permeable to both Na and K.
- This will 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.
Ionic basis of action potential of autorrythmic cells
Phase 1: Pacemaker Potential:
Opening of voltage-gated Sodium channels called Funny
channels (If or f channels ).
Closure of voltage-gated Potassium channels.
Opening of Voltage-gated Transient-type Calcium (T-type Ca2+
channels) channels .
Phase 2: The Rising Phase or Depolarization:
Opening of Long-lasting voltage-gated Calcium channels
(L-type Ca2+ channels).
Large influx of Calcium.
Phase 3: The Falling Phase or Repolarization:
Opening of voltage-gated Potassium channels
Closing of L-type Ca channels.
Potassium Efflux.
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
excited by electrical activity propagated by the pacemaker cells
Action potential of a contractile myocardial cell:a typical
ventricular cell
Depolarization
- 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.
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.
Summary of Action Potential of a Myocardial Contractile Cell
- Depolarization= Sodium Influx
- Rapid Repolarization= Potassium Efflux
- Plateau= Calcium Influx
- Repolarization= Potassium Efflux
