Action Potential & Propagation

DENT/OBHS 131Neuroscience 2009

Ionic basis of APs

action potential:faithfully transmit information along the membrane (axon) of excitable cells

allow rapid communication between distant parts of a neuron

Learning Objectives

1. Describe the roles of both sodium and potassium ions / voltage-gated channels before, during and after the action potential

2. Understand how the resistive & capacitive properties of neurons influence electrical signaling

3. Compare and contrast local circuit and saltatory propagation of action potentials

How many distinct ion channels are necessary for the AP?

1. 0

2. 1

3. 2

4. 3

5. 4

3 phases of the action potentialResting

i.e. RMP

Depolarizationreversal of membrane potential

Repolarizationreturn of membrane potential to RMP

relationship between: membrane potential ion equilibrium potentials

if the membrane becomes more permeable to one ion over other ions then the membrane potential will move towards the equilibrium potential for that ion (basis of AP)

membranepotential (mV)

EK

ENa

RMP

+67

-90-98

ECl

General rule

Depolarization

rapid opening of Na-selective channels entry of Na “down” its electrochemical gradient

1. membrane more permeable to Na than K 2. membrane potential moves (rapidly) towards ENa

3. because ENa is positive, the AP overshoots zero

4. At the peak of the AP Na is the primary ion determining the membrane potential

Repolarization

closure (inactivation) of Na-selective channels slower opening of K-selective channel

1. membrane more permeable to K than Na2. K moves out of cell3. membrane potential moves towards EK

selective agents block the 2 components

2 independent channels

the opening and closing of AP Na and K channels are controlled by changes in the membrane potential

Voltage-gated ion channels

all-or-none AP are not graded potentials

threshold in order for an AP to occur the membrane must be depolarized beyond a threshold level

inward Na overcomes resting outward K movement electrical stimulation synaptic activation

What triggers an AP?

APs are regenerative

activation of Na channels is cyclicalinitial depolarizationopening of Na channelsNa entryetc..

Learning Objective #1

Describe the roles of both sodium and potassium ions / voltage-gated channels before, during and after the action potential

AP review

Learning Objective #2

Understand how the resistive & capacitive properties of neurons influence electrical signaling

How does an AP move?

Propagation Aps are conducted along excitable cell membranes away from their point of origine.g. down the axon from cell soma to terminal

Resistance ≈ how far it can get

axon / dendritemembrane resistance (rm)

axial, or internal, resistance (ri)

diameter (d)

rm

riength constant =

“leaky pipe”

Fat axons are fastest!

Capacitance ≈speed

“bulk” solutions IN and OUT are neutralthe transmembrane potential difference exists within a narrow band just across the membrane

a capacitor separates / stores charge

to change membrane potential must add or remove charge this takes time

Summary

Capacitance - speed (time constant)Resistance - distance (length constant)

How does neuron deal with these properties in order to have efficient AP propagation?

local circuit propagationslow of the membrane during the AP is not restricted to a single spot

the inward current carried by Na ions during the AP depolarizes adjacent portions of the membrane beyond threshold and the regenerative AP travels along the membrane

Unmyelinated axons

following a single AP a second AP cannot be generated at the same site for some time (absolute versus relative)Na channels need to recover from inactivationopen K channels oppose inward Na movement

Refractory period

local circuit propagation is slow (< 2 m/s)

In motor neurons propagation is fast 100 m/s

Schwann cell / oligodendrocyte envelop axons / layer of insulation increase membrane resistance

less leaky eliminate capacitance

less discharge

Nodes of Ranvier discontinuity in myelin sheath (every few 200+

m)

Myelination

Saltatory conduction

APs are only generated at Nodes of Ranvier high density of Na / K channels

current flows rapidly between nodes little current leakage between nodes

AP “jumps” down fiber as successive nodal membrane capacitances are discharged

Learning Objective #3

Compare and contrast local circuit and saltatory propagation of action potentials

propagation review

Press button

How can AP rise so fast (< 1 ms)?

m= rmcm

Membrane time constant

changing the membrane potential takes timecharging a capacitor is not instantaneousinject currentrecord voltage

axon/dendrite

I

V

m = rmcm ≈ 50 ms