cable properties of axons 2010
TRANSCRIPT
Figure 4.20b,c
Cable Properties of Axons
Voltage Decreases with Distance
• Change in membrane potential (voltage) during AP decreases over distance due to resistance– Conduction with decrement– Higher resistance of intracellular and extracellular
fluids causes greater decrease in voltage along axon
– Lower resistance of membrane causes greater decrease in voltage along axon
– K+ leak channels (always open)» Some + charge leaks out
– Number of K+ leak channels will affect current loss and voltage decrease along axon
)/( orrr im im rr /
Length Constant () of Axons• Distance over which membrane potential will
decrease to 37% (1/e) of its original value
• Variables affecting length constant:• Resistance of cell membrane (rm)
• Resistance of intracellular fluid (ri)
• Resistance of extracellular fluid (ro)
– ro is usually low and constant; and is often ignored
– is largest when rm is high and ri is low
Length Constant () of Axons)/( orrr im im rr /
Figure 4.21
and the Speed of Conduction• Axonal conduction is a combination of
electrotonic current flow and ions flowing through voltage-gated channels during AP– Electrotonic current flow much faster than
opening of voltage-gated channels– Electronic current flow decreases over distance
• Higher allows more electrotonic current flow and faster speed of conduction
Axon Membrane Capacitance• Capacitance
– Quantity of charge needed to create a potential difference between two surfaces of a capacitor
• Depends on three features of the capacitor:– Material properties
• Generally the same in cells (lipid bilayer)
– Area of two conducting surfaces• Larger area increases capacitance
– Thickness of insulating layer• Greater thickness decreases capacitance
Figure 4.20b and Figure 4.22
Axon Membrane Capacitance
Time Constant (t) • Time over which membrane potential will
decay to 37% of its maximal value• How well does the membrane “hold” its charge?
• Variables affecting time constant:• Resistance of cell membrane (rm)
• Capacitance of the cell membrane (cm)
= rmcm
– Low rm or cm result in low • Capacitor becomes full faster• Faster depolarization• Faster conduction
Figure
Time Constant (t)
Figure
Giant Axons– Easily visible to naked eye (up to 1 mm
diameter)– Not present in mammals
Giant Axons Have High Conduction Speed
– rm inversely proportional to surface area• Large diameter axons have greater surface area and more
leak channels; therefore low resistance
– ri inversely proportional to volume• Large diameter axons have greater volume; therefore low
resistance
• As axon diameter increases, rm and ri both decrease
im rr /
Giant Axons Have High Conduction Speed
– Low rm reduces the length constant and decreases conduction speed
– Low ri increases the length constant and increases conduction speed
– Do not cancel each other out: rm is proportional to radius, ri is proportional to radius2
• Net effect of increasing axon radius is to increase speed of conduction
Figure
Axon Diameter and the Length Constant
Myelinated Neurons in Vertebrates
• Disadvantage of large axons– Take up a lot of space which
• Limits number of neurons that can be packed into nervous system
– Large volume of cytoplasm makes them expensive to produce and maintain
• Myelin enables rapid signal conduction in compact space
Myelin Increases Conduction Speed
• Increased membrane resistance– Insulators decrease current loss through leak
channels, increasing the length constant
• Decreased membrane capacitance– Increased thickness of insulating layer reduces
capacitance, decreasing the time constant
• High length constant and low time constant increase conduction speed
• Nodes of Ranvier are needed to boost depolarization