In some cases, a nonsense mutation in one gene of a transcription unit was found to prevent the expression of subsequent genes in the unit even though both genes had their own ribosome binding sites, an effect called polarity.

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#Id: 4006

#Unit 7. System Physiology – Animal

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

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#Id: 4008

#Unit 7. System Physiology – Animal

SA node has an unstable resting membrane potential. The “resting membrane potential” of the sinus nodal fiber between discharges has a negativity of about −55 to −60 millivolts.

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#Id: 4010

#Unit 7. System Physiology – Animal

The cause of less negative RMP of SA node is that the cell membranes of the sinus fibers are naturally leaky to sodium and calcium ions.

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#Id: 4012

#Unit 7. System Physiology – Animal

The inherent leakiness of the sinus nodal fibers to sodium and calcium ions causes their self-excitation.

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#Id: 4013

#Unit 7. System Physiology – Animal

SA node has Prepotential or pacemaker potential which is the gradual potential rise from the resting membrane potential to threshold  ( -40 mV) & this occur spontaneously

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#Id: 4014

#Unit 7. System Physiology – Animal

Membrane ion channels that play important roles in generation of the action potential:

T (transient) channels 

L (long-lasting) channels Voltage gated potassium ion channel