7/ neuronal conduction Flashcards
propagation of action potential down axon
- like a wave
- Na channels open
- behind wave of depolarisation, k channels open
- wave doesn’t move backwards since sodium channels are inactivated for a few ms after opening
- also k channels opening destabalises memb
what is speed of action potential propagation determined by
- how fast the next segment of membrane gets depolarised to threshold
- determined by: space constant, time constant
what is space constant
- how far current can spread passively along the axon before it decays to a certain fraction of its initial value
- passively = charges moving according to laws of electricity, not opening/closing ion channels
- ideas come from cabal theory (undersea telegraph cables)
electricity is like water analogy
- electric change = water standing still
- electric current = flowing water
- voltage = water pressure
- resistance = what prevents water flowing - narrow pipe
- axon like a leaky water hose
what factors affect space constant? equations
- lambda = sqr(membrane resistance/internal resistance)
- memb resistance: current spready further if memb is less leaky
- internal resistance: current spreads further is there’s less resistance down the axon
- lambda is inversely proportional to sqr(radius)
relationship between memb resistance and SA of memb and circumference
- SA: inversely proportional
- more area=more leaks
- C: inversely proportional
relationship between internal resistance and cross sectional area
- inversely proportional
- wider hose, water travels more easily
note on giant squid axon
- mediates squids escape reflex
- so want signal to be transmitted fast
- no myelin, so v wide axon for fast transmission
capacitor - and water analogy
- 2 plates with a gap between them
- charge can build up on 1 side to create a voltage
- like a stretchy rubber memb in a hose pipe
what 2 things does the cell memb act as
- resistor - current can pass through but not well (lipids insulating)
- capacitor - charge can build up on 1 side
what is time constant
if charge is applied, how long does it take to build up across the cell wall
time constant equation
- T = membrane resistance x membrane capacitance
- memb capacitance = how stretchy the hose wall is
effects of myelin on resistance and capacitance, hence time and space constants
- increases memb resistance - like wrapping insulating tape around a wire
- decreases capacitance - increases distance between extracellular and intracellular solution
- space increases
- time similar (membrane can still charge up as fast as normal)
speed of conduction in myelinated mammal axon
100 m/s
saltatory conduction
- current enters through sodium channels at node of ranvier
- depolarisation spreads passively down axon (decays but stays above AP threshold)
- at next node of ranvier, depolarisation triggers voltage gated sodium channels to regenerate AP
one reason invertebrates have tiny brains
- they don’t have myelin
- without myelin, conduction speed could only be increased by widening the axon
- space constant is inversely proportional to sqt(radius), so to increase speed 10x, increase radius by 100x and volume by 10,000x
- not sustainable to have enough energy for all this biomass
what axons are and aren’t myelinated, why?
- yes myelinated: proprioception (neurons sensing position of muscles), motor neurons - need to move away from danger fast
- not myelinated: pain, temp
overview of how demyelinating diseases impair neuronal conduction
- distribution of ion channels designed with myelin in mind - if myelin disappears signals can’t travel properly
- decreased conduction: decreased velocity, frequency or total block
- maladaptive homeostatic compensation - neurons try to fix problem by inserting extra sodium channels where myelin should be, spontaneous APs
multiple sclerosis
- auto immune disorder where immune system attacks myelin
- episodic - gets better, then worse, then better…
- diverse symptoms: vision problems, numbness/tingling, muscle spasms/weakness (tingling and spasms = signals where they shouldn’t be)
- symptoms potentially worse under stress or at high temps - sodium channels inactivate faster
guillain barre syndrome
- auto immune disorder affecting PNS myelin
- symptoms: numbness, tingling, weakness
- patients usually recover because PNS myelin can regenerate, unlike CNS myelin
- (note glia can regenerate)
how does temp affect sodium channels
colder temps sodium channels close more slowly to enough depolarisation to generate AP later on past the stretch on unmyelinated axon
