Nerve Impulse

A2 biology edexcel

1. Nerve impulse
•Starter activity
•Explain the role of the ‘nodes of ranvier’

2. Nerve impulse
•Learning objectives
•Understand the nature of the nerve
impulse
•Describe the propagation of action
potentials of nerve impulses
•Describe the permeability of membranes to
sodium ions during action potential

3. Nerve impulse
•http://www.phschool.com/atschool/science_activity_library/path_nerve_impulse.html
•Each nerve impulse involves the movement of ions through the
axon membrane
•Na+ move into the axon
•K+ move out of the axon
•When a neurone is resting (not conducting an impulse) it has a
negative electrical charge (resting potential), but when Na+ enter
it briefly becomes positive (depolarised) which generates the
action potential. When the K+ leave the inside becomes negative
again therefore restoring the resting potential. The resting
potential is the effect of differences in the ion concentration
across the axon membrane (Na+, K+, Cl-, and organic ions)

4. Nerve impulse (resting state)
Exclusive to
inside
Organic ions
Greater Cl-
GreaterK+
Greater Na+
Outside the
axon
Inside the axon
Ions

region of axon

5. Nerve impulse (resting state)
•The difference in the ion concentrations is all dependent on the
permeability of the axon membrane
•During the resting state the permeability of the membrane to K+ is
high, due to the presence of the protein channels (gates)
•Due to there being no (gates) for the organic ions, they remain on the
inside of the membrane
•Due to the ions, this is the reason why there is a negative charge on
the inside of the axon membrane
•Very few K+ ions will diffuse out of the membrane, due to the fact that
there is an overall negative charge on the inside of the axon
membrane
•Cl- concentration gradient is towards the inside of the axon membrane
but due to overall negative charge, they are repelled back out.
•Na+ would be expected to move into the axon membrane, due to the
overall negative charge. But there is a low permeability on the axon
membrane to sodium ions, so in turn they are moved inside slowly and
are captured by the ion pumps and expelled to the outside

6. Nerve impulse (resting state)
http://www.biology4all.com/resources_library/source/63.swf

7. Nerve impulse (generating)
•When a nerve impulse is generated the following occur
•Na+ permeability slightly increases, so more enter the axon
membrane than are being expelled
•Due to this movement the potential difference reaches a positive
value (action potential)
•So as the permeability to Na+ decrease in turn the permeability
to K+ increases due to more (K) channels being open.
•So as the K+ flow out, the potential difference inside the axon
decrease back to the negative resting value. There is slight
decrease in the membrane potential which is slightly lower than
resting potential, due to more K+ released than needed
•This effect is called the Hyperpolarisation. But this is temporarily
until the Na+ and K+ return to their resting concentrations

8. Action potential

9. Action potential
•A nerve impulse is the propagated (spread) action potential or a
wave of depolarisation which travels along the axon membrane.
•The ion channels in the axon membrane are voltage dependent,
which only open when the membrane is depolarised
•This sets up a current which spreads from the action potential
region of the membrane to the region ahead of it.
•This stimulates the Na+ channels to open. This occurs in a small
segment of the axon membrane
•Due to the propagated action potential, this moves at a constant
velocity along the axon membrane.
•Saltatory conduction is a type of conduction which only occurs
when the action potential jumps from one ‘node of ranvier’ to
another. This is because of the axon being myelinated. The
result is an increase in the conduction velocity
•During the passage of the nerve impulse the axon will gain Na+
and lose K+, but these ions are re-exchanged by the Na+/ K+
pumps which will actively pump out Na+ and pump K+ into the
axon