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: 5452

#Unit 7. System Physiology – Animal

The relative refractory period begins at the end of the absolute refractory period and overlaps primarily with the period of the hyperpolarizing afterpotential.

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#Id: 5453

#Unit 7. System Physiology – Animal

During the relative refractory period, stronger than normal stimuli can cause excitation

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#Id: 5454

#Unit 7. System Physiology – Animal

The basis for the relative refractory period is the higher K+ conductance than is present at rest. 

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#Id: 5455

#Unit 7. System Physiology – Animal

The spatial distribution of ion channels along the axon plays a key role in the initiation and regulation of the action potential

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#Id: 5456

#Unit 7. System Physiology – Animal

Voltage-gated Na+ channels are highly concentrated in the nodes of Ranvier and the initial segment in myelinated neurons. 

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#Id: 5457

#Unit 7. System Physiology – Animal

The number of Na + channels per square micrometer of membrane in myelinated mammalian neurons has been estimated: 

50–75 in the cell body, 350–500 in initial segment, less than 25 on the surface of the myelin, 2000–12,000 at the nodes of Ranvier & 20–75 at the axon terminals