Propagation of Ap

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10/22/2012

Communication Along and Between Neurons (Ch.6)
• Receptors receive stimuli, and convert them
to nerve impulses
• Alternating graded and all-or-none signals
on the membrane of a single neuron
• Graded potentials decay with distance
(electrotonic conduction); longdistance transmission depend on transforming signals to APs
• Typically, alternating electric (within
neuron) and chemical signals
(between neurons)
• Postsynaptic potential affected by number
and frequency of APs

Passive spread of electric signal
Sub-threshold current flowing along the membrane decays with distance: • Resistance of cytoplasm
• Leakage of charge across the membrane

Passive spread of electric signal
Electrotonic conduction depends only on the physical properties of a cell
Signal decay with distance depends on membrane and longitudinal resistance to current flow:
• Resistance of cytoplasm (Rl)
• Leakage of charge across the membrane (Rm)
Vm and the time it takes to stabilize (charge
has to accumulate) depend on the
membrane capacitance

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10/22/2012

Passive spread of electric signal

Passive electrotonic conduction:
• The change in Vm decays exponentially with distance
• The rate of the decay depends on Rl and Rm- the spread of a current along the membrane is enhanced with low Rl and high Rm

Passive spread of electric signal

The signal decay can be expressed by:

Vx = Vo· e-x/λ
Vλ = 0.37Vo
The length constant (λ), depends on of Rl and Rm, is defined as the distance at which the steady-state potential shows 63% drop in amplitude

Passive spread of electric signal

Vx = Vo· e-x/λ

(λ of neurons is 1-2mm)

Assuming λ=1mm, what would be the magnitude of the spinal
depolarization if it was to travel a distance of 1m to the axon terminal by electrotonic conduction and induce a 10mV change in Vm?

0.01 = Vo · e-1000/1 = Vo · e-1000
Vo = 0.01 · e1000 ~ 10430V

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Propagation of AP
Electrotonic conduction decays with distance:
• Non-spiking neurons are widespread throughout the animal kingdom (CNS)
• Typically only few mm’s in length
• High Rm, resulting in high λ
• Attenuated signal still sufficient for NT release

Communication over large distances depends on the propagation of AP without loss of intensity

Propagation of AP

Alan Hodgkin (1914-1998)

in 1937 Alan Hodgkin (as an undergrad!) confirmed that currents are conducted electrotonically ahead of a propagating AP

Propagation of AP

• APs are produced by voltage-gated ion channels
• Large but transient current (Na+) crosses the membrane: positive charges pulled outside, and pushed away inside membrane
• Current must flow out to complete the electric circuit (Leaked K+ from unexcited nearby regions); Excited regions generate
local currents which excite nearby regions
• Depolarization of 20mV is enough to bring a nearby membrane patch to threshold- AP is 100mV!

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Propagation of AP
Propagation of a nerve impulse depends on:
1. Electrotonic spread of current from area of Na+ flux to
neighbouring regions of inactive membrane
2. Excitability of Na+ channels to amplify and maintain
depolarization caused by local currents




Refractory period maintains unidirectional propagation
K+ channels are essential for quickly restoring Vm and thus
neuron excitability
Propagation velocity depends on how fast the membrane ahead
of the active region is brought to Vthr

Speed of propagation
The velocity of AP propagation
is so rapid it will never be
accurately measured

Johannes Muller, late 1830’s

Speed of propagation
The velocity of AP propagation
is so rapid it will never be
accurately measured

HA!

Johannes Muller, late 1830’s

Hermann Ludwig Ferdinand von Helmholtz,
20 years later

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10/22/2012

Speed of propagation

Determined experimentally by calculating ∆d/∆t… found to be 30m/s in frog nerves

Speed of propagation
The spread of a...
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