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
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
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