positive/negative DF
positive DF- current leaving the cell
- current - flow of + particles
negative DF= current entering cell
capacity current equation
C = q/V q= CV dq/dt= I = CdV/dt
voltage alonx an axon
current will go some distance and leak out
- length constant determines how far the signal spreads in time
length constant equation
lambda = Sqr (Rm/Ri)
- the easier it is to leave the axon, the lower the length constant
action potential
solves length problem:
- long range communication is possible
- AP continue because of active propogation
- all or none events generated by voltage gated ion channels
voltage gated Na channel role in AP
has an inactivation gate that turns off even even in depolarization which allows you to take AP back down
K channel role in Ap
ensures unidirectional propagation:
- K channels activate with a delay and pull membrane potential back which overshoots resting
- activate and immediately inactivate so you cannot go backwards
saltatory conduction
myelin decreases capacity current, tightens resistance and concentrates ion channels to nodes which allows current to spread faster
AP role in sensory systeems
- some (mechano) use AP directly
- some dont (photoreceptors would lose sensitivity)
quantal hypothesis
postsynaptic muscle recordings made by Fatt and Katz revealed that synapse never fails
- quanta: packets of transmitter being released gives rise to MEPP
synaptic release
decreasing Ca reveals the probabilistic and quantal nature of synaptic release
steps in synaptic transmission
- bring pre neuron to threshold at hillock
- conduct down axon
- open voltage gated Ca at terminal
- diffuse Ca
- exocytosis and diffusion of Ca
- activation of receptors
SNARE hypothesis
proteins in vesicle membrane and presynaptic terminal are SNARE proteins that form complex (Core) that primes vesicle for release
- requires energy
molecules in SNARE
synaptobrevin- synaptic vesicle
syntaxin, snap 25 - presynaptic membrane
synaptotagmin - Ca sensor
- mediate docking and priming, release and retrieval
what happens when synaptotgamin is knocked out
no fast synchronous release but instead slow asyonchrounous
- Ca binding leads to insertion into membrane which pulls v and t together and leads to fusion
