Cardiac Muscle, Dr. D. Penney




Table 1. Follower cell ion channel events: sodium and calcium gates opening / closing during a cycle of depolarization / repolarization (fast conduction: 0.3-3 m/sec.)

State.................. TMP Na+a (m) Na+i (h) Ca++a (d) Ca++i (f)
Resting state: -90 mv closed open closed open
Depolarization:
Activation 1 (Na+ influx --> fast current (high dV/dt))		 -70 mv OPENS open closed open
Depolarization:
Activation 2 (Ca++ influx --> slow current (low dV/dt))		 -40 mv open open OPENS opens
Depolarization:
Inactivation 1 (loss of fast current)		 +20 mv open CLOSES open open
Repolarization:
Inactivation 2 (loss of current)		 0 to -50 mv open closed open CLOSES
Repolarization:
Recovery (of excitability)		 -55 mv open closed CLOSES OPENS
Prevent the fast current (see hyperkalemia) -70 mv CLOSES OPENS closed open

Note: TMP = transmembrane potential
"a" = activation; "i" = inactivation
"m" and "h" are upper and lower gates in the Na+ channel.
"d" and "f" are upper and lower gates in the Ca++ channel.



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