When polymerase binding causes transition to the open complex and initiates transcription. This gives a low level of constitutive expression called the basal level.  Binding of RNA polymerase is the rate-limiting step in this case.

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#Unit 7. System Physiology – Animal

In cardiac muscle, the action potential is caused by opening of three types of channels: 

Voltage activated fast sodium channels 

L-type calcium channels (slow calcium channels or calcium-sodium channels)

Voltage activated potassium ion channels

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#Unit 7. System Physiology – Animal

Phase 0 (depolarization), fast sodium channels open. 

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#Unit 7. System Physiology – Animal

Phase 1 (initial repolarization), fast sodium channels close.

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#Unit 7. System Physiology – Animal

Phase 2 (plateau), calcium channels open and fast potassium channels close

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#Unit 7. System Physiology – Animal

Phase 4 (resting membrane potential) averages about −90 millivolts.

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#Unit 7. System Physiology – Animal

The SA node exhibits automaticity; it can spontaneously generate action potentials without neural input.