The conducting system of the heart and the
normal ECG
The sino-atrial node (SAN, also called the sinus node) is a group of specialised cells positioned on the wall
of the right atrium, near the entrance of the superior vena cava.
- This is the impulse generating (pacemaker) tissue.
- These nodal cells are modified cardiac muscle cells, not nerve cells. They are connected to
adjacent atrial cells by gap junctions.
Cardiac electrical activity starts in the SA node & spreads across the atria to the Atrio-Ventricular (AV)
node where it stops (it is not directly transmitted to the ventricles).
The AV node is located on the inter-atrial septum close to the tricuspid valve.
SAN receives blood from the RIGHT CORONARY ARTERY
- If there is insufficient blood supply from the left
coronary artery and ANASTOMOSIS it can result
in a myocardial infarction.
This is because affected cells around the SAN will not
allow a heartbeat to be initiated
- Atria will beat but slower!!
Goes into the branches called the Purkinje fibres around
the bottom of the heart to papillary muscles which
attach to valves
NERVE ACTION POTENTIAL
In a normal nerve action potential, there is no sodium influx into the cell at rest. Sodium enters briefly for
about 0.5 milliseconds at the start of an action potential. Potassium leaves for a short period at the end
of the action potential.
The whole action potential only lasts about 2 milliseconds.
CARDIAC PACEMAKER CELL
In a cardiac pacemaker cell, there is a constant inward (leak)
sodium influx into the cell at rest.
This is would normally depolarise the cell, but a simultaneous
outward potassium current triggered by the action potential
prevents the depolarisation. The membrane potential depends
on the balance between the inward and outward currents.
No action potential occurs!
The outward potassium current however decays with time. This means the inward sodium current
gradually becomes dominant and the membrane potential slowly depolarises.
- When the outward potassium current reaches a critically low level the cell is sufficiently
depolarised to trigger an action potential. During the action potential the outward potassium
current is ‘reset’ to a high level and then starts to decay again.
From -70mV it gets depolarised to about -40mV
The heart rate depends on the rate of decay of the potassium current.
, DEPOLARISATION which causes the potassium current to decay
normal ECG
The sino-atrial node (SAN, also called the sinus node) is a group of specialised cells positioned on the wall
of the right atrium, near the entrance of the superior vena cava.
- This is the impulse generating (pacemaker) tissue.
- These nodal cells are modified cardiac muscle cells, not nerve cells. They are connected to
adjacent atrial cells by gap junctions.
Cardiac electrical activity starts in the SA node & spreads across the atria to the Atrio-Ventricular (AV)
node where it stops (it is not directly transmitted to the ventricles).
The AV node is located on the inter-atrial septum close to the tricuspid valve.
SAN receives blood from the RIGHT CORONARY ARTERY
- If there is insufficient blood supply from the left
coronary artery and ANASTOMOSIS it can result
in a myocardial infarction.
This is because affected cells around the SAN will not
allow a heartbeat to be initiated
- Atria will beat but slower!!
Goes into the branches called the Purkinje fibres around
the bottom of the heart to papillary muscles which
attach to valves
NERVE ACTION POTENTIAL
In a normal nerve action potential, there is no sodium influx into the cell at rest. Sodium enters briefly for
about 0.5 milliseconds at the start of an action potential. Potassium leaves for a short period at the end
of the action potential.
The whole action potential only lasts about 2 milliseconds.
CARDIAC PACEMAKER CELL
In a cardiac pacemaker cell, there is a constant inward (leak)
sodium influx into the cell at rest.
This is would normally depolarise the cell, but a simultaneous
outward potassium current triggered by the action potential
prevents the depolarisation. The membrane potential depends
on the balance between the inward and outward currents.
No action potential occurs!
The outward potassium current however decays with time. This means the inward sodium current
gradually becomes dominant and the membrane potential slowly depolarises.
- When the outward potassium current reaches a critically low level the cell is sufficiently
depolarised to trigger an action potential. During the action potential the outward potassium
current is ‘reset’ to a high level and then starts to decay again.
From -70mV it gets depolarised to about -40mV
The heart rate depends on the rate of decay of the potassium current.
, DEPOLARISATION which causes the potassium current to decay
