5. Finally, the large diameter Purkinje Fibres rapidly conduct the AP from the apex of the heart UPWARD to the remainder of the ventricular myocardium causing the ventricles to contract and pushing blood upwards to the semilunar valves.
Nerve impulse from the ANS and blood hormones (epinephrine) can modify the timing and strength of each heartbeat but DO NOT establish the fundamental rhythm.
Cardiac Action Potential and Contraction of Contractile Fibres
APs are initiated by the SA node. The AP spreads along the conduction system and spreads out to excite the “working” fibres known as Contractile Fibres . Unlike auto rhythmic fibres, contractile fibres do have a stable resting membrane potential close to about -90mV .
Depolarization
1. Contractile fibre is brought to threshold. 2. Voltage gated Na+ channels open allowing Na+ inflow 3. Rapid depolarization occurs 4. Na+ channels inactivate and the Na+ inflow decreases
Plateau
This is a period of “maintained” depolarization
1. Voltage gated Ca++ channels open 2. Increased Ca++ content triggers contraction 3. Voltage-gated K+ channels open and the K+ flows out
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