Neuronal Conduction Flashcards
speed of action potential propagation is determined by…
how fast the next segment of membrane gets depolarised to threshold
determined by 2 factors:
1 - space constant
2 - time constant
space constant
how far a current spreads passively along an axon
passively = electrical chargers moving according to laws of electricity, not opening and closing ion channels
space constant equation
square root of Rm/Ri
membrane resistance is inversely proportional to
surface area of membrane
more area = more leaks
internal resistance is inversely proportional to
cross sectional area of axon
wider = travel easierr
therefore ….
space constant is proportional to
square root of radius of axon
wider axon = longer space constant
time constant
how long for membrane to reach certain % of its final voltage value
T = Rm X Cm
Cm = membrane capacitance - how stretchy
Rm = membrane resistance - how leaky
myelin effect on Rm and Cm
increases Rm
decreases Cm
oligodendrocytes/schwann cells wrap around axons many times = creates myelin = increased Rm
increase in distance between extra cellular matrix and intracellular matrix = decreased Cm
myelin effect on space and time constant
inc space constant
time stays same
increased Rm = longer space constant, current can spread farther down axon
decreased Cm = counteracts effect of inc Rm on time constant, membrane can still charge up as normal
saltatory conduction
nodes of ranvier, packed with Na gated ion channels
current enters through Na channels at node, membrane = depolarised, passively propagates down next myelinated bit until it reaches next node
saves energy, means that Na only enters at nodes rather than entire axon = less work to restore gradient
myelination saves space
invertebrates can only increase conduction speed by widening axon, they dont have myelin
different axons conduct at different speeds
myelin = costly
wide axon = also costly
therefore only myelinate and widen axons that need fast conduction
where are axons myelinated
proprioception - where muscles are
motor axons
where are unmyelinated axons
pain and temp areas
demyelinating diseases impair neuronal conduction
distribution of ion channels is made with myelin in mind - only at nodes
so if myelin disappears, signal cant travel correctly =
- decreased velocity
- frequency related block
- total conduction block
- ectopic impulse generation
- increased mechanosensitivity
last 2 because of maladaptive homeostatic compensation
