Lec 08 Flashcards
Spatial summation
Currents from nearly simultaneous graded potential combine
-more than one epsp/ipsp
Temporal summation
Two graded potential from one presynaptic neuron occur close together in time
Membrane potential during AP
1- graded potential
2- depolarizing stimulus
3- membrane depolarizes to threshold
4- voltage-gated sodium channel open
5-PEAK: sodium channel deactivated
6- UNDERSHOOT: potassium channels are ooen
7- Repolarization
8- -84mV =E(k)
9- OVERSHOOT: Na leak channels AND Na/k pump AND potassium channels close up
Sodium channel bs potassium channel speed
Na channels open and close faster than k channels
Rising phase is driven by
Positive feedback loop of Na channel
UNTIL E(Na) is reached
Na voltage-gated channels
Inactiveate in the peak
Then during repolarization gates will reset to original position
Refractory period
Limits how soon after an AP another one can be triggered
Absolute refractory period
-Majority of Na channels are inactivated
-No stimulus
- Excitability=0
Relative refractory period
Some Na channels are recovered
K channels are still open
-stronger stimulus is required for having fewer available Na channels and hyperpolarization k efflux
-Excitability is recovering
Current flow faster if
1- R(i) low — large cell
2- Few leak channels
3- R(m) high — myelinated
Biological current is
Movement of positive charge
Why AP does not move backward?
Refractory period
1- Na channels inactivation
2- increase permeability of k
Ap is ——directional
Unidirectional
Loss of myelin
1- decrease R(m)
2- increase C(m)
3- increase loss of current and voltage signal
When myelinated:
1- R(m) increases — no ion movement— act as insulator
2- AP only at nodes of Ranvier
3- decrease membrane capacitance C(m)
