Nerve impulse synapses

HOW THE IMPULSE TRAVELS FROM NERVE TO NERVE

1. The Nerve Impulse
KRVS CHAITANYA
Assistant Professor
Lovely professional university

2. The nerve impulse
This is the electrical signal which is
transmitted by the neurones around the
nervous system.
See Interactive Tutorial for Activity 8.2

3. Tissue fluid outside neurone [Na
+
] high [K
+
] low
Neurone cell membrane ______________________________
Cytoplasm inside neurone [Na
+
] low[K
+
] high
+ proteins
-
+ ve
- ve
Distribution of ions and charge across the surface of a neurone.
The difference in charge results in a resting potential of –70 mV across
the membrane.

4. The resting potential is the result of:
•the action of an active
transport system called the
‘sodium pump’:
•i.e. involves a transporter
protein
+ the use of energy from
respiration
this ‘pumps
3 Na+ ions out and
2 K+ ions in

5. •Also the differential permeability of the neurone
membrane to Na+ and K+ ions
these ions can only cross the membrane via specific ion
channel proteins that allow facilitated diffusion:
(there are also so special ‘gated ions channels which
only allow ions to diffuse through them when they are
‘opened’ but they are not relevant here.)
ion channel protein
sodium pump

6. •The membrane is very impermeable to Na+ ions so
these cannot diffuse back in.
•The membrane is slightly more permeable to K+ ions
so these diffuse out slowly (down their concentration
gradient).
•This establishes electrochemical diffusion gradients
for the ions across the membrane.
•The net result is that relatively more +ve ions end
up outside the neurone than remain inside to balance
the – ve charged proteins
•so the outside is more positive with respect to the
inside;
•this charge difference is responsible for the resting
potential.
•At this point the neurone is said to be polarised.

7. When a nerve impulse is transmitted along a neurone a wave of
electrical activity passes along it
(NB: it is NOT a flow of electrons so it is not an electrical
current)
This can be detected (e.g. by
using a cathode ray oscilloscope
with electrodes placed inside and
outside the neurone) as a
transient change in electrical
charge on the membrane
surface – this is called an
ACTION POTENTIAL and is the
basis of the nerve impulse.
The nerve impulse.

8. An action potential

9. Events during
an action
potential

10. http://www.blackwellpublishing.com/matthews/channel.html
Action potential/ion channels

11. The action potential
[1] Resting potential: Na+ and K+ special ‘voltage gated’
ion channels closed: neurone is polarised.
[2] Na+ gated channel proteins open (due to a change
in shape of the protein itself: makes the membrane
more permeable to Na+ ions now) which allows Na+
ions to diffuse into the neurone down the
electrochemical diffusion gradient.
[3] This causes the inside of the neurone to become
increasingly more + ve: this is depolarisation and
neurone has become depolarised

12. [4] Na+ channels close and K+ gated channels now
open
[5] K+ ions diffuse out of the neurone down the
electrochemical diffusion gradient, so making the
inside of the neurone less positive (= more negative)
again: this is repolarisation and the neurone has
become repolarised
[6] The neurone has its resting potential restored.
During this time the Na+ and K+ ions which have
diffused in/out of the cell are redistributed by
active transport (sodium pump)
[NB: the ion movements during the action potential
occur by facilitated diffusion; active transport is not
involved]

13. http://www.blackwellpublishing.com/matthews/channel.html
Action potential/ion channels

14. Events during
an action
potential

16. Properties of the action
potential.
Action potentials
•have a threshold:
•this is the minimum level of stimulus necessary to
cause depolarisation (i.e. open the ion channels)
•are all or nothing:
•all action potentials are the same size, irrespective of
the intensity of the stimulus
•consequently information about the intensity of a
stimulus is coded in the number of impulses - strong
stimulus  many action potentials etc

17. •have a refractory period (about 1 ms);
•the ion channels remain closed and cannot be
made to reopen so no further depolarisation is
possible; the consequences are
•it separates impulses from one another by a
fraction of a msec, and so imposes a maximum
on the number of impulses which can be
transmitted along a neurone in a given time
•it ensures impulses can only pass in one
direction along an axon, i.e. forward into the
next resting region but not back into a region
still in its refractory period.

18. Role of action potentials in the
transmission of a nerve impulse.
•A STMULUS causes the Na+ gated
channel proteins to open (effectively
causes a change in shape of the protein
itself)
•which allows Na+ ions to diffuse down the
concentration gradient across the
membrane into the cell and so set off an
action potential.
•[see handout]

21. Sodium gates open
Sodium ions flow
into neurone
How action potentials are transmitted along a neurone

22. Sodium gates close
Potassium gates open
Potassium ions flow
out of neurone
Sodium gates in next bit of membrane open and sodium ions flow in
Sodium pump redistributes
sodium and potassium ions
after impulse has passed

23. Speed of conduction of nerve
impulses.
•Transmission speeds can range from 0.5
m per sec to over 100 m per sec.
•Impulse transmission can be speeded up
by:

24. •increasing the diameter of the axon

25. Myelin sheath
Bundle of nerve fibres
Connective tissue coat of nerve
TS Nerve
but this imposes constraints on packing a large number of large axons into a nerve

26. •having a myelin sheath;
the myelin sheath is
composed of tightly
packed layers of the cell
membrane of the
Schwann cells which are
wrapped around the axon

28. ·this is rich in lipid (called myelin) which
makes the axon impermeable to ions so
they are unable to diffuse between the
tissue fluid and the neurone
·so action potentials cannot be generated
by the myelinated regions (it acts as an
insulator);

29. ·action potentials can only be generated
at the nodes of Ranvier
·so the local currents involved in nerve
impulse transmission flow over longer
distances:
·thus action potential seem to ‘jump’
from node to node (this is called
salutatory conduction):

30. Saltatory conduction

32. •since the intervening parts of the axon
membrane do not have to be successively
depolarised it takes
•less time for the action potentials to pass
from node to node
•this results in nerve impulse transmission that
is much faster,
•the consequence of which is that smaller
myelinated nerves can transmit impulses much
faster than larger non-myelinated ones
•which alleviates the ‘packing problem’.

33. ·temperature
·which affects the rate of diffusion and
the rate of energy release by
respiration for active transport (since it
is controlled by enzymes)
·the consequence is that nerve impulse
transmission is faster in endothermic
animals which maintain a high body
temperature.

36. Synapses

37. Nerve endings

38. synaptic knobs (neurone endings)

39. Motor end plate
Synapses between motor nerve and muscle

40. Neurones are
connected together
(normally via axons
and dendrites) at
synapses

42. N.B. this is not a
physical junction,
there is actually a
small gap of approx 20
nm between the cells
so there is no
membrane continuity so
nerve impulses cannot
cross directly.
synaptic vesicles
synaptic bulb
pre-synaptic membrane
post-synaptic membrane

44. •Instead transmission is by chemicals
called neurotransmitters
•These made in the Golgi body (synthesis
requires energy from respiration)
•stored in vesicles in the synaptic bulb

45. •There are several different types of
neurotransmitters, of which two are
•Acetyl choline
•(used in the voluntary [motor neurones 
muscles] and parasympathetic nervous
systems)
•Noradrenaline
•(used in the sympathetic nervous system)

46. How nerve impulses are transmitted across a synapse by neurotransmitters

47. Interactive tutorial: Activity 8.4
Crossing a synapse

51. neurotransmitter molecules
diffuse across the synaptic
cleft neurotransmitter receptors are
membrane protein with binding sites with
a shape complementary to the
neurotransmitter
neurotransmitters broken
down by enzymes so the
ion channels to close and
the receptors are available
again and so the resting
potential can be re-
established
this sequence takes 0.5 msec which results in a synaptic delay in the
transmission of impulses.

53. Types of synapse.
Excitory synapses
•Binding of neurotransmitter to postsynaptic
neurone
•opens Na+ gated channels
 Na+ diffuses IN
depolarisation
 action potentials
•so nerve impulses can continue around the
nerve circuit.

54. Inhibitory synapses
•Binding of neurotransmitter to postsynaptic
neurone opens K+ gated channels
•  K+ diffuses OUT
 inside of neurone becomes even more – ve
and so impossible to depolarise
 no action potentials
•so nerve impulses cannot continue around the
nerve circuit.

55. Functions of synapses.
Enables impulses to be transmitted from
one neurone to another, so enables nerve
circuits to function.
•Neurotransmitter only released from the
• presynaptic neurone,
•so nerve impulses can only be transmitted
• IN ONE DIRECTION around a nerve
circuit

56. •Sometimes neurotransmitter released by a
single neurone is insufficient to depolarise the
postsynaptic neurone;
•however simultaneous release of
neurotransmitter from the synapses several
neurones
•(sometimes from other nerve circuits – known as
integration)
•will be sufficient to cause sufficient
depolarisation to generate nerve impulses
•this is called SUMMATION.

57. impulses
arrive at the
same time

58. impulses very
close together

59. Summative effect of
excitory and inhibitory
synapses also determines
whether nerve impulses will
be generated in the
postsynaptic neurone.

60. •Nerve impulses are only transmitted if
threshold of postsynaptic neurone is
exceeded,
•so ensures only significant information is
passed around the circuit
•(so insignificant, low level stimuli get
“filtered out”).

62. animations of how drugs work
•http://www.pbs.org/wnet/closetohome/science/html/animations.html
•http://learn.genetics.utah.edu/units/addiction/drugs/mouse.cfm