Transmission of nerve-impulses

1. Transmission of nerve impulses
Course: B.Sc.(Micro./Biotech) sem II
Subject: Animal & Plant Physiology
Unit 4.1

2. 1

3. Transmission of nerve impulses
 Neurones transmit
impulses as electrical
signals
 These signals pass
along the cell surface
membrane of the
axon as a nerve
impulse
2

4. Transmission of nerve impulses
 It is NOT the same as
an electric current
passing down a wire
(which is much
faster)
 The mechanisms is
the same throughout
the animal kingdom
3

5. Transmission of nerve impulses
 Experiments have been carried out using squid
axons which are big enough to have electrodes
inserted in them.
 One electrode can be placed inside the axon and
one on its surface.

6. Transmission of nerve impulses
 When at rest, the inside of the membrane
has a negative electrical potential
compared to the outside.
 This difference in potential is called the
resting potential and is typically about
between -70 mV to -80 mV
 In this resting state the axon is said to be
polarised.

7. Transmission of nerve impulses
 This is maintained because the neurone has
an internal composition which is different
to the outside.
 Sodium ions and potassium ions are
transported across the membrane against
their concentration gradients by active
transport

8. Transmission of nerve impulses
 Carrier proteins pick up
Na+
ions and transport
them to the outside.
 At the same time K+
ions
are transported into the
axon.
 This is known as the
sodium-potassium
pump and relies on ATP
from respiration
4

9. Transmission of nerve impulses
 Inside the axon there
are large numbers of
negatively charged
organic ions which
can not move out of
the axon.

10. Transmission of nerve impulses
 The Na+
ions are passed out faster than the K+
ions
are bought in.
 Approx. three Na+
ions leave for every two K+
ions that enter.
 K+
ions can also diffuse back out quicker than Na+
ions can diffuse back in.
 Net result is that the outside of the membrane is
positive compared to the inside.

11. The action potential
 A nerve impulse can be initiated by mechanical,
chemical, thermal or electrical stimulation
 When the axon is stimulated the resting potential
changes.
 It changes from –70 mV inside the membrane to
+40 mV
 For a very brief period the inside of the axon
becomes positive and the outside negative

12. The action potential
 This change in potential is called the
action potential and lasts about 3
milliseconds
 When an action potential occurs, the axon
is said to be depolarised.
 When the resting potential is re-established
the axon membrane is said to be
repolarised

14. depolarisation

15. depolarisation
reploarisation

16. depolarisation
reploarisation
‘overshoot’

17. depolarisation
reploarisation
‘overshoot’
direction of impulse

18. Depolarisation
 When the membrane
depolarises changes
occur in the
membrane to the
permeability of both
Na+
ions and K+
ions

19. Depolarisation
 When the axon is stimulated, channels
open on its cell surface which allow Na+
ions to pass through.
 Na+
ions flood in by diffusion
 The Na+
ions create a positive charge of
+40 mV inside the membrane, reversing
the resting potential and causing the action
potential

20. Repolarisation
 Potassium channels open in the membrane
and K+
ions diffuse out along their
concentration gradient.
 This starts of repolarisation
 At the same time, sodium channels in the
membrane close preventing any further
influx of Na+
ions.

21. 5

22. Repolarisation
 The resting potential is re-established as
the outside of the membrane becomes
positive again compared to the inside.
 So many K+
ions leave that the charge
inside becomes more negative that it was
originally.
 This shows up as an ‘overshoot’.

23. resting potential
(no net ion movement)

24. resting potential
(no net ion movement)
Na+
start to move
in

25. resting potential
(no net ion movement)
Na+
start to move
in
Na+
ions diffuse in
rapidly

26. resting potential
(no net ion movement)
Na+
start to move
in
Na+
ions diffuse in
rapidly
K+
ions diffuse out
rapidly

27. resting potential
(no net ion movement)
Na+
start to move
in
Na+
ions diffuse in
rapidly
K+
ions diffuse out
rapidly
Sodium ions pumped out
potassium ions pumped in

28. Repolarisation
 The potassium channels close and the
sodium-potassium pump starts again.
 Normal concentrations of sodium and
potassium ions is re-established.
 The membrane is once again at its resting
potential

29. 6

30. a) In the resting axon, there is a high conc. of Na+
ions outside
and a high conc. of K+ ions inside. But the net effect is that
the outside is positive compared to the inside giving the resting
potential
b) The axon is stimulated producing an action potential, setting
up local circuits on the axon membrane
c) Sodium ions rush into the axon along a diffusion gradient
depolarising the membrane causing an action potential

31. d) As the action potential passes along the axon potassium ions
diffuse out along a concentration gradient, starting off the
process of repolarisation
e) The sodium-potassium pump is re-established, fully repolarising
the membrane

32. References:
Image 1: http://universe-review.ca/I10-75-impulse.jpg
Image 2: http://postimg.org/image/ibu6ul45b/
Image 3: http://postimg.org/image/k4x3iwpbz/
Image 4: http://postimg.org/image/klmwj32gv/
Image 5: http://postimg.org/image/9krre2a7z/
Image 6:
http://www.biologydiscussion.com/neural-control/mechanism-of-tra
Book: Human Physiology 4th
edition by Lauralee Sherwood