neuronal conduction Flashcards
what determines the speed of action potential propagation
-how fast the next segment of membrane gets depolarized to threshold which is determined by:
1)Space constant (aka length constant)
2)Time constant
whats space constant
-length constant
-When positive current enters the axon, it spreads along the axon passively, like current in a wire
-“Passively” means: electrical charges moving according to laws of electricity, not opening or closing of ion channels
-These ideas come from cable theory, first worked out for undersea telegraph cables
why is the axon like a leaky water hose
-membrane isn’t a perfect insulator so some stuff spills to outside
-But it also leaks out through channels in the membrane (“holes in the hose pipe”)
whats membrane resistance
-current (flowing water) spreads further if the membrane (hose wall) is less leaky
-rm
whats internal resistance
-current (flowing water) spreads further if there is little resistance to it moving down the axon (hose)
-ri
space constant equation
space constant= square root of rm/ri
what does the size of the axon determine
-the space constant
-Membrane resistance is inversely proportional to the surface area of the membrane (more area, more leaks)
-Internal resistance is inversely proportional to the cross-sectional area of the axon (wider hose -> water travels more easily)
-Membrane resistance is inversely proportional to the axon’s circumference (circumference = 2piradius)
-Internal resistance is inversely proportional to the area
(area = pi*radius^2)
how long does it take the membrane to “charge up”?
-A cell membrane is both a resistor (current can pass through but not very well) and a capacitor (charge can build up on one side)
-So an axon is like a leaky water hose with stretchy walls. If water pressure is applied, how long does it take for pressure to build up across the wall?
-This time is described by the time constant:
-T= rmcm
-rm= membrane resistance – how un-leaky is the hose wall
-cm= membrane capacitance – how stretchy is the hose wall
what does myelin increase and decrease
-membrane resistance and decreases membrane capacitance
what creates myelin
-Oligodendrocytes / Schwann cells wrap around axons many times, surround them with many layers of membrane – like a Swiss roll
-This is like wrapping a wire with insulating tape – it increases membrane resistance
-It also increases the distance between the extracellular and intracellular solution
-Myelin moves the two capacitor plates farther apart therefore decreasing capacitance
how does Myelin increases the space constant while keeping the time constant the same
-Myelin increases membrane resistance
=longer space constant
=current can spread farther down the axons
-Myelin decreases membrane capacitance
=counteracts the effect of increased membrane resistance on the time constant
=membrane can still charge up as quickly as normal
what does myelin speed up
-conduction of action potentials
-Many myelinated mammalian axons conduct at over 100 m/s!
-(Even the squid giant axon can only manage ~25 m/s)
-need a really good myelinated axon in order for the signal to travel at an appropriate speed
whats saltatory conduction
-Current enters through Na+ channels at a node of Ranvier
-Then depolarization spreads passively down the axon (this is sped up by longer space constant)
-At the next node of Ranvier, depolarization triggers voltage-gated Na+ channels to regenerate the action potential
-Saltatory conduction means Na+ only enters at nodes of Ranvier (instead of all along axon) less work for Na+/K+ pump to restore Na+ gradient*
how does myelination save space
-Without myelin, you can only increase conduction speed by widening the axon following
-To increase speed 10x, have to increase radius by 100x -> axon volume increases by 10,000x
-Example: the human optic nerve is 3 mm wide. Without myelin, to conduct at the same speed, it would have to be 300 mm wide (1 foot)
why do Different axons conduct at different speeds
-Myelin is costly
-Wide axons are also costly
-Only myelinate and widen axons that need to carry information very quickly
-Thus: squid giant axons (escape reflex)
-Myelinated axons: proprioception, motor axons
-Unmyelinated axons: pain, temperature
what does demyelinating disease
-diseases impair neuronal conduction
-The distribution of ion channels is “designed” with myelin in mind (e.g. voltage-gated Na+ channels are only at the nodes of Ranvier), so if myelin disappears, signals will not travel correctly
-Ectopic spikes because of maladaptive homeostatic compensation
what does demyelinating diseases cause
-symptoms explained by failure of neuronal conduction
multiple sclerosis
-auto-immune disorder, immune system attacks myelin
-episodic: symptoms get worse, then better, then worse, etc.
-diverse neurological symptoms, e.g., vision problems, numbness/tingling, muscle spasms/weakness, many others
-symptoms might be worse when under stress or at high temperatures – neuronal conduction is “safer” at low temperatures, because Na+ channels inactivate more slowly
Guillain-barre syndrome
-auto-immune disorder affecting PNS myelin
-symptoms: numbness, tingling, weakness
-patients usually recover because PNS myelin can regenerate (unlike CNS myelin)
