CSB332H1 Lecture Notes - Lecture 6: Axon Hillock, Electric Current
14 May 2018
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Lecture 6(a): AP Flow in Axon
Action Potential:
• Action potential is generated due to the initial rapid rise in the Na+
conductance due to the opening of Na+ voltage-gated channels
o K+ voltage-gated channels are delayed in their activation
allowing for the outward conductance of K+
ð Absolute refractory Period:
o All Na+ voltage-gated channels are inactivated
o Cannot generate another AP no matter potent the stimulus
ð Relative Refractory Period:
o Some Na+ voltage-gated channels are activated
o Can generate another AP; larger threshold than initial AP
Current Flow During an AP:
• Current is the movement of ions determined by the driving force
(difference between equilibrium potential & membrane potential)
o Positive current from Na+ influx will spread away from
its entry site to depolarize adjacent segments of the
membrane potential to reach threshold allowing an AP
• Action potentials usually run toward the distal end where
neurotransmitters are found to cause for their release
o Do not have inherent directionality, however: AP normally
do not reverse their direction
Regenerating Action Potentials:
• Action potentials are normally initiated at the axon hillock in a
neuronal cell body
o Electrical current; the movement of the Na+ ions will be
spreading through the axon
§ Current spread is due to the voltage-gated channels
• An axon is surrounded by a cell membrane that is embedded with
different type of channels (i.e. Na+ and K+ voltage-gated channels)
Artificial Experiment:
• Injecting positive current at a specific location in the axon
o Cause membrane potential to change opening VG
channels (1st: Na+ | 2nd: K+)
ð In theory, AP should travel in both directions – however the
spread is unidirectional
o Refractory period is responsible for the unidirectional
movement of the AP
Document Summary
Absolute refractory period: all na+ voltage-gated channels are inactivated, cannot generate another ap no matter potent the stimulus. Relative refractory period: some na, can generate another ap; larger threshold than initial ap voltage-gated channels are activated. Regenerating action potentials: action potentials are normally initiated at the axon hillock in a neuronal cell body, electrical current; the movement of the na+ ions will be spreading through the axon. Current spread is due to the voltage-gated channels: an axon is surrounded by a cell membrane that is embedded with different type of channels (i. e. na+ and k+ voltage-gated channels) Injecting positive current at a specific location in the axon: cause membrane potential to change opening vg channels (1st: na+ | 2nd: k+) In theory, ap should travel in both directions however the spread is unidirectional: refractory period is responsible for the unidirectional movement of the ap.
