10.3 The nerve impulse
Learning objectives:
The role of sodium potassium pumps in
establishing the resting potential of a
neurone.
Sequence the events that occur to create
an action potential.
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Mcgrawhill animation
Sodium-potassium pump
Sodium ions bind with internal
surface transport protein
ATP combines with protein
resultant phosphorylation/
activation of protein causes
shape change
Three sodium ions released
externally
Two potassium ions combine
with transport protein which
reverts to its original shape
Potassium ions released
internally
3Na+out/2K+in so inside more negative RP = -65mV
Maintaining RP -65mV
Sodium potassium pumps results in 3Na+out/2K+in
so inside axon negative
Sodium gradient but Na+ ions cannot diffuse back in
as gated sodium channels closed
Potassium gradient K+ ions diffuses out as potassium
channels open
-further increasing pd, making inside more negative
Electrical gradient created as more K+ ions diffuse
out, - outside becomes more positive so fewer K+
ions leave axon as repelled by positive state of tissue
fluid
Equilibrium established where no net movement of
ions and the membrane is polarised at -65mV
A nerve impulse is a
self-propagating wave of depolarisation,
a temporary reversal of the electrical
potential difference across the axon
membrane
a change between – 65mV resting potential
to +40mV is called an action potential
The change in potential is caused by a
change in the permeability of the axon
membrane to sodium and potassium ions.
The action potential
a 2ms temporary depolarisation
Stimulus energy GP causes sodium voltage gated
channels to open
Rapid influx Na+ ions down electrochemical gradient:
inside becomes positive so membrane depolarised.
At +40mV sodium gated channels close, and
potassium voltage gated channels open
K+ ions move out as no electrochemical gradient
preventing them, so membrane repolarised.
Outward diffusion of K+ ions causes overshoot,
hyperpolarisation.
Potassium gated channels close. Sodium-potassium
pump restores resting potential
Annotate
40
35
30
25
20
Potential difference
/mV
15
10
5
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
1
2
3
Time /ms
Action potential
Fixed time …….ms
Fixed change in potential difference - ……
to + ……mV
An all or nothing event
Summary Q
Describe how the movement of ions establishes
the resting potential of an axon.
2. For A-F to state if channels open or closed
1.
Difference
Membrane
potential /mV
Na+ voltage
gated
channels
K+ voltage
gated
channels
Resting
Beginning to
depolarise
Repolarising
-65
-50
-20
A
B
C
D
E
F
1. Sodium-potassium pumps in axon membrane: 3 sodium
ions actively pumped out for 2 potassium ions pumped in.
Inside axon becomes negative in charge. Potassium ions
diffuse out through open potassium channels until
electrochemical gradient is at equilibrium -65mV.
Difference
Resting
Beginning to
depolarise
Repolarising
Membrane
potential /mV
-65
-50
-20
Na+ voltage
gated channels
A closed
B open
C closed
K+ voltage
gated channels
D closed
E closed
F open
HSW
Measuring action potentials
P167 Q1 2 3
Summary RP through AP to RP
Stage
Polarisation
Depolarisation
Repolarisation
Hyperpolarisation
Polarisation
Membrane Na+ in/out
potential
or
+/K+ in/out
Na+ voltage
gated
channels
open/closed
K+ voltage
gated
channels
open/closed
Stage
Membrane Na+ in/out
potential
or
+/K+ in/out
Na+ voltage
gated
channels
open/closed
K+ voltage
gated
channels
open/closed
Polarisation
-65 mV
equilibrium
closed
closed
Depolarisation
+40mv
Na+ in
open
closed
Repolarisation
back to
negative
K+ out
closed
open
K+ out
closed
open
equilibrium
closed
closed
Hyperpolarisation More
negative
than
-65mV
Polarisation
-65 mV