Transcript The Nervous System - BiologyMad A

The Nervous System
J. Gilbert
March 2004
BiologyMad.com
Overview
Nerve Impulses
(completed12/03/04)
Resting Membrane Potential
(completed12/03/04)
How do nerve impulses start?
(completed 19/03/04)
Action Potential
(completed 19/03/04)
How Fast are Nerve Impulses?
Synapses
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Nerve Impulses
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Nerve Impulses
Neurones send messages
electrochemically – this means that
chemicals cause an electrical impulse.
 Chemicals in the body are ‘electrically
charged’ when they have an electrical
charge, they are called ions.

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Resting Membrane
Potential
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Resting Membrane Potential

When a neurone is not sending a signal, it is
at ‘rest’.
– The inside of the neurone is negative relative to
the outside.
– K+ can cross through the membrane easily
– Cl- and Na+ have a more difficult time crossing
– Negatively charged protein molecules inside the
neurone cannot cross the membrane.
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Resting Membrane Potential

The membranes
contain sodiumpotassium pumps
(Na+K+ATPase).
– Uses ATP to
simultaneously pump
3 sodium ions out of
the cell and 2
potassium ions in.
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Resting Membrane Potential

There are also sodium and potassium ion
channels in the membrane.
– These channels are normally closed, but even
when closed, they ‘leak’, allowing sodium ions to
leak in and potassium ions leak out – down their
concentration gradients.
+
3
N
a
o
u
t
s
id
e
c
e
ll
m
e
m
b
r
a
n
e
N
a
++
N
a
K
A
T
P
a
s
e
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Resting Membrane Potential
Ion
K+
Na+
ClThe
Concentration inside
cell/mmol dm-3
150.0
15.0
9.0
Concentration outside
cell/mmol dm-3
2.5
145.0
101.0
imbalance of ions causes a potential
difference (or voltage) between the inside of
the neurone and its surroundings
The resting membrane potential is –70mV
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Resting Membrane Potential

Overall:
– K+ pass easily into the cell
– Cl- and Na+ have a more difficult
time crossing
– Negatively charged protein
molecules (A-) inside the neurone
cannot pass the membrane.
– The Na+K+ATPase pump uses
energy to move 3 Na+ out for
every 2K+ in to neurone
This imbalance in voltage causes a potential difference
across the cell membrane – called the resting
membrane potential.
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Resting Membrane Potential
Membrane potential is always negative
inside the cell.
 The Na+K+ATPase is thought to have
evolved as an osmoregulator to keep
the internal water potential high and so
stop water entering animal cells and
bursting them.

– Plant cells don’t need this as they have
strong cells walls to prevent bursting.
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How do Nerve
Impulses Start?
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How do Nerve Impulses Start?

Neurones are stimulated by receptor cells
– These contain special sodium channels that are
not voltage-gated, but are gated by the
appropriate stimulus.

stimulus causes the sodium channel to open
– Causes sodium ions to flow into the cell
– Causes a depolarisation of the membrane
potential  affects the voltage-gated sodium
channels nearby and starts an action potential.
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How do Nerve Impulses Start?

Some examples:
– chemical-gated sodium channels in
tongue taste receptor cells open when a
certain chemical in food binds to them
– mechanically-gated ion channels in the hair
cells of the inner ear open when they are
distorted by sound vibrations; and so on.
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How do Nerve Impulses Start?
In each case the correct stimulus causes the
sodium channel to open (reaches the
threshold value)
↓
causes sodium ions to flow into the cell
↓
causes a depolarisation of the membrane
potential
↓
affects the voltage-gated sodium channels
nearby and starts an action potential.
Action Potential
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Action Potential (AP)
The resting potential tells about what
happens when a neurone is at rest.
 An action potential occurs when a
neurone sends information down an
axon.

– Is an explosion of electrical activity
– The resting membrane potential changes
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AP - Depolarisation

Resting potential is –70mv (inside the axon).
When stimulated, the membrane potential is
briefly depolarised
– Stimulus causes the membrane at one part of the
neurone to increase in permeability to Na+ ions
– Na+ channels open. This causes resting potential
to move towards 0mV
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AP - Depolarisation

When depolarisation reaches –30mV
more Na+ channels open for 0.5ms
– Causes Na+ to rush in  cell becomes
more positive
-
open
out
Na
K
in
+
Na
closed
(leak)
+
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AP - Repolarisation
At a certain point, the depolarisation of
the membrane causes the Na+
channels to close
 This causes K+ channels open

K+
out
Na
in
closed
(leak)
+
K
open
-
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AP - Repolarisation

K+ rush out  making inside the cell more
negative.
– Since this restores the original polarity, it is called
repolarisation
– There is a slight ‘overshoot’ in the movement of
K+ (called hyperpolarisation).
– Resting membrane potential is restored by the
Na+K+ATPase pump
K+
out
Na
in
closed
(leak)
+
K
open
-
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AP - Overview
(Click here for animation)
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AP – All or nothing

AP only happens if the stimulus reaches a
threshold value
– Stimulus is strong enough to cause an AP
– It is an ‘all or nothing event’ because once it
starts, it travels to the synapse.

AP is always the same size
 Frequency of the impulse carries information
 strong stimulus = high frequency
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Action Potential

At rest, the inside of the neuron is slightly negative
due to a higher concentration of positively charged
sodium ions outside the neuron.
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Action Potential

When stimulated past the threshold, sodium
channels open and sodium rushes into the axon,
causing a region of positive charge within the axon.
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Action Potential

The region of positive charge causes nearby
sodium channels to open. Just after the sodium
channels close, the potassium channels open wide,
and potassium exits the axon.
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Action Potential

This process continues as a chain-reaction
along the axon. The influx of sodium
depolarises the axon, and the outflow of
potassium repolarises the axon.
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Action Potential

The sodium/potassium pump restores the resting
concentrations of sodium and potassium ions
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Action Potential
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AP – Refractory Period

There is a time after depolarisation where no
new AP can start – called the refractory
period.
– Time is needed to restore the proteins of voltage
sensitive ion channels to their original resting
conditions
– NA+ channels cannot be opened, as it can’t be
depolarised again
– Therefore impulses travel in one direction
– Can last up to 10 milliseconds – this limits the
frequency of impulses
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AP - Refractory Period

Absolute refractory period = During the
action potential, a second stimulus will not
cause a new AP
 Exception: There is an interval in which a
second AP can be produced but only if the
stimulus is considerably greater than the
threshold = relative refractory period
 Refractory period can limit the number of AP
in a given time.
 Average = about 100 action potentials/s
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How Fast are Nerve
Impulses?
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How fast are impulses?
AP can travel 0.1-100m/s along axons
 Allows for fast responses to stimuli
 Speed is affected by:

– Temperature
– Axon diameter
– Myelin sheath
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Myelinated Neurones

The axons of many neurones are encased in a
fatty myelin sheath (schwann cells).
 Where the sheath of one Schwann cell meets
the next, the axon is unprotected.
 The voltage-gated sodium channels of
myelinated neurons are confined to these
spots (called nodes of Ranvier).
Na+
Sodium channel
Na+
Nodes of Ranvier
Na+
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Myelinated Neurones

The in rush of sodium ions at one node
creates just enough depolarisation to reach
the threshold of the next.
 In this way, the action potential jumps from
one node to the next (1mm) – called
saltatory propagation (click here for animation)
– Results in much faster propagation of the nerve
impulse than is possible in nonmyelinated
neurons.
Na+
Sodium channel
Na+
Nodes of Ranvier
Na+
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Facts about Propagation
Nerve impulse conduction is really the
bumping of positive charge down the
axon
 AP initiated at one end of the axon is
only propagate in one direction.

– The AP doesn’t turn back because the
membrane just behind is in its refractory
period i.e. voltage gated Na+ channels are
inactivated
Facts about propagation

To increase conduction velocity:
– Increase the axonal diameter
– Myelin of the axon facilitates current flow down the
inside of the axon.
• Breaks in the myelin wrapping occur at the Nodes of
Ranvier, which have increased concentrations of voltage
gated Na+ channels. Regeneration of the AP occurs at
the nodes
Saltatory conduction – propagation and
regeneration of an AP down myelinated axon
 E.g. Local anaesthesia temporarily blocks AP
generation by binding the interior of voltage
gated Na+ channels

Synapses
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Synapses
Junction between two neurones is
called a synapse
 An AP cannot cross the synaptic cleft
 Impulse is carried by chemicals called
neurotransmitters

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Synapses - Neurotransmitters
Neurotransmitters are made by the cell
sending the impulse (the pre-synaptic
neurone) and stored in synaptic
vesicles at the end of the axon
 The cell receiving the impulse (postsynaptic neurone) has chemical gated
ion channels called neuroreceptors

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Synapses

Click here for animation
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Synapses

At the end of the presynaptic neurone there
are voltage gated
calcium channels.
 When AP reaches the
synapse, the channels
open
 Calcium ions flow into
the cell
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Synapses

Calcium ions cause
synaptic vesicles to
fuse with the cell
membrane
 Neurotransmitters
diffuse across the
synaptic cleft
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Synapses

Neurotransmitter binds
to neuroreceptors in
the post-synaptic
membrane
 Channels open, Na+
flow in
 Causes depolarisation
 AP initiated in postsynaptic neurone
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Synapses

Function:
– Prevents impulses travelling in the wrong
direction.
• An impulse can pass along an axon in either direction,
but can only cross a synapse in one direction because
the synaptic vesicles are only found in the synaptic
knobs and end plates
– A vast number of synaptic connections allow for
great flexibility. They are equivalent to the
switchboard in an elaborate telephone exchange
enabling messages to be diverted from one line to
another and so on
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Integrating Signals

If the diffusion
of ions reaches
a threshold
value, it will
cause the AP
in the
postsynaptic
membrane.
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Neurotransmitters

Neurotransmitters are broken down by a
specific enzyme in the synaptic cleft.
 Breakdown products are absorbed by the
pre-synaptic neurone
 Used to re-synthesise more neurotransmitter
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Neurotransmitters

Acetylcholine (ACh)
– Released by motor neurones onto skeletal muscle
cells
– Released by neurones in the parasympathetic
nervous system
– Cholinergic synapses
– Ach is removed from the synapse by
acetylcholinesterase
• Nerve gasses used in warfare (e.g. sarin) and the
organophosphate insecticides (e.g. parathion) achieve
their effects by inhibiting acetylcholinesterase this
allowing Ach to remain active.
• Atropine is used as an antidote because it blocks ACh
receptors
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Neurotransmitters

Noradrenaline
– Released by neurones in the sympathetic
nervous system
– Adrenergic synapses