BMS11004 - WEEK 3 - TUESDAY Flashcards
neuronal propagations, factors affecting neuronal conductions, saltatory conduction, demyleinating disease
speed of AP propagation is determined by how fast next segment of membrane gets depolarised to thresold. which 2 factors determine this:
- space constant (length constant)
- time constant
what is space constant (also known as length constant)
how far current can spread passively along axons
why is current spread down axon passive
AP very brief and only affects small segment of membrane at at a time
passive due to laws of physics “like charges repel”
explain mathematical cable theory
weakening of signal as spread along axon, axon like water hose and electricity is water electrical charge = water sitting still
electric current = charge in motion, water flow
voltage (forces pushing charges) = water pressure
resistance = what stops water flowing
capacitor = 2 plates with gap inbetween allowing charge to build up on 1 side creating voltage
axon like leaky hose, current can flow down but also leaks out via channel
give the formula for calculating space constant (using membrane resistance, internal resistance)
λ = √Rm/Ri
space constant = square root of membrane resistance divided by internal resistance
explain membrane resistance
inversely proportional to surface area of membrane (more area = more leaks)
depends on axon circumference (c=2πr)
more membrane resistance = more space constant
explain internal resistance
inversely proportional to cross-sectional area of axon (wider hose = water travels more easily)
depend on area (rA=πr²)
wider axon = longer space constant
“how hard is it for current (water) to pass along axon (hose pipe)”
what is time constant
how long it takes membrane to charge up (how long it takes for pressure to build up across wall)
give equation for time constant
T = RmCm
membrane resistance = how unleaky hose wall is
membrane capacitance = how stretchy hose wall is
explain how membrane can be both resistor and capacitor
resistor = current can pass through but not very well
capacitor = charge can build up on one side
myelin insulation increases membrane resistance, what impact does this have for membrane capacitance
longer space constant so current can spread further down axon, so decreases membrane capacitance meaning it counteract effect of increased membrane resistance on time constant, allowing membrane to charge upa s quickly as usual
explain how saltatory conduction work
- charge enter via Na Channel inbetween myelin
- spreads passively through insulated axon part, then charge can enter again at next NoR/Na channel
- charge decays slow enough that by time reaches next NoR still above threshold (myelin insulation)
explain how saltatory conductions saves energy
only Na+ etner at NoR not all along axon, so less work for Na/K pump to restore Na+ gradient, as less Na+ to be pumped in, so less energy wasted for removing excess Na+
explain how myelination can save space in brain
without myelin, can only increase speed of conduction by widening axon following the rule =
to increase speed 10x, need increase radius 100x, so axon volume increased 10,000x
explain how demyelination diseases impairs neuronal conduction
distribution of ion channels are designed for use alongside myelin. if myelin disappear, signal can’t travel correctly - not insulated
cause ectopic spike in impulse conduction, due to maladaptive homeostatic compensation (neurons trying to fix problem without new myelin)
