The Cardiac Electrical system and the ECG Flashcards
Specialized excitatory and conductive system of the Heart
Sinus Node/Sinoatrial Node/SA Node
Internodal Pathways
Atrioventricular Node/AV Node
AV Bundle
Left and Right Bundle of branches of Purkinje Fibres
Sinus(Sinoatrial) Node
Small,flattened,ellipsoid strip oof specialized cardiac tissue.
Located: Superior posterolateral wall of the right atrium immediatly below and slightly lateral to the opening of the SVC
Fibrres of this node have no contractile muscle filaments
SA nodal fibres connect directly to the atrial muscle fibres so that any AP that begins in the sinus node spreads immediatly into the atrial muscle wall
Automatic Electrical Rhythmicity of the Sinus Fibres
Some cardiac fibres have the capability of self excitation, a process that can cause autonomic rhythmical discharge and contraction
Mechanism of Sinus Nodal Rhythmicity
The RMP of the sinus nodal fibre between discharges has a negativity of about -55 - -60mV in comparison with the -85 - -90 mV for the ventricular muscle fibre.
What is the cause for the negative RMP for the sinus fibre
The reason is that the cell membrane of the sinus fibre are naturally leaky to sodium and calcium ions and positive charges of the entering calcium and sodium ions neutralize some of the intracellurlar negativity.
Types of membrane ion channels which play important roles in causing the voltage changes of the AP in cardiac muscle.
Fast Sodium channels
L-type calcium channels/Slow sodium-calcium channels
Potassium Channels
Discuss the action potential which is observed in ventricular muscle
- Opening of the fast sodium channels is responsible for the upstroke spike in AP because of rapid influx of positive ions to the interior of the fibre
- The plateau of the Ventricular AP is caused by the slower opening of the the sodium-calcium channels/L-type calcium channels
- Opening of the potassium channels allows diffusion of large amounts of positive k+ ions in the outward direction through through fiber membrane and returns the Membrane potrntial to its resting level.
Self-excitation of the Sinus Nodal Fibre
Because of the high sodium ion concentration in the ECF outside the nodal fibre as well as a moderate number of the already open sodium channels,positive sodium ions from outside the fibre normally tend to leak to the inside.
Therefore between heartbeats,influx of +ve charged sodium ions cause a slow rise in the RMP in the +ve direction.
The RMP gradually rises and becomes less +ve between each two heartbeats.
When the potential reaches a threshold voltage of about -40 mV, the L-type calcium channels become activated causing an AP.
Therefore,basically the inherent leakiness of the sinus nodal fibres to the sodium and calcium ions cause their self-excitation
Why does the leakiness to the sodium and calcium ions not cause the sinus nodal fibre to remain depolarised all the time.
First, the l-type calcium channels become inactivated after opening
Second, At about the same time,greatly increased numbers of the potassium channels open.
Therefore the influx of positive calcium and sodium ions through the l-type calcium channels ceases while at the same time large quantities of positive ions diffues out the fibre.
Both of the effects reduce the intracellular potential back to its negative resting level and therefore terminating the AP.
The potassium channels remain open for a few seconds temporarily continuing the movement of positve charges out the cell with resultant excess negativity inside the fibre-Hyperpolarization
Hyperpolarization
It initially carriers the RMP down to about -55 - -60 mV at the termination of the action potential
Why is the new state of hyperpolarization not maintained forever?
During the next feew seconds after the AP is over,progressively more and more potassium channels close.
The inward-leaking sodium and calcium ions once again over balance the outward flux of potassium ions which cause the resting potential to drfit upward reaching the threshold for discharge
fibres
fibre
The AV node delays impulse conduction from the atria to the ventricles
The atrial conductive system is organized so that the cardiac impulses does not travel from the atria to the ventricles to rapidly
The av node and its adjacent conductive fibres that delay this transmission.
What is the location ot the AV-Node
It is located in the posterior wall of the right atrium immediatly behind the tricuspid valve.
The duration in the AV Nodal delay
The Total delay in the AV bundle and the AV bundle system is about 0.13 second-0,09 0,04
This delay as well as the intial conduction delay of 0.03 seconds from the sinus node to the av node
What the causes of the slow conductance
The slow conduction in nthe transitional,nodal and penetrating AV bundle fibre is caused mainly by diminished number of gap junctions between succesive cells inthe conducting pathways. son theree is grat resistance to conduction of excitatiry ions from one conducting fibre.