Transcript 12-Electrochemical Impulse-website - kyoussef-mci

How do nerve cells send and
receive messages?
Dendrites
Cell body
Nucleus
Synapse
Signal
Axon direction
Axon hillock
Presynaptic cell
Postsynaptic cell
Myelin sheath
Synaptic
terminals
Membrane Potential
 Every cell has a
membrane potential or
“voltage” across its
plasma membrane
 Membrane Potential
 Separation of charge
 Ability to do work
 Resting Potential of a
neuron is -70 mV
What causes membrane potential?
1. Unequal distribution of positively charged ions
inside and outside the cell
Why only positively charged ions?
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Cells have large negative ions inside the cell that cannot
cross the plasma membrane
High concentration of K+ inside the cell
High concentration of Na+ outside the cell
2.
Unequal permeability (diffusion)
of positively charged ions
across the membrane

At rest, more permeable to K+ than
it is to Na+




More potassium ions diffuse out of
the cell than sodium ions diffuse
into the cell
Cell looses more positive ions than
it gains
The inside of a cell is negative
relative to the outside
Negative and positive ions will
accumulate along the inside of the
membrane due to charge attraction
Maintaining the membrane potential at rest
ADP
Sodium Potassium Pump
(Na-K Pump)
Resting
Potential
Action
Potential
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Action Potential
Three
Phases
Resting potential – positive exterior, negative interior
 maintained by sodium-potassium pump
Depolarization – negative exterior, positive interior
 Sodium channels open to allow sodium ions to rush into the
cell
Repolarization – positive exterior, negative interior
 Sodium channels close
 Potassium channels open
 Resting state restored by sodium-potassium pump
Action Potential – Three States
resting potential: -70mV
1.

required before a signal can be detected
Threshold: values vary
2.

If membrane potential reaches this value, will get an
action potential because this triggers MANY Sodium
channels to open = ALL OR NONE Principle
action potential: +40mV
3.

change that occurs to transmit signal
Propagation of Action Potential
RESTING
Axon
DEPOLARIZATION
REPOLARIZATION
(Refractory Period)
Action
potential
–
–
+
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+
Na+
+
+
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–
Refractory
Period
Action
potential
K+
Na+
channels
closed
+
+
–
–
–
+
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+
–
–
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–
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Na+
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The depolarization of the action
potential spreads to the neighboring
region of the membrane, re-initiating
the action potential there. To the left
of this region, the membrane is
repolarizing as K+ flows outward.
K+
Refractory
Period
Action
potential
K+
+
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+
+
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–
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+
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K+
An action potential is generated
as Na+ flows inward across the
membrane at one location.
2
–
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1
3
–
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+
+
+
+
+
+
–
–
Na+
–
The depolarization-repolarization process is
repeated in the next region of the
membrane. In this way, the action potential
Is propagated along the length of the axon.
Propagation of AP
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Action Potential Factors
1. refractory period
2. threshold
3. axon diameter
4. saltatory conduction
Refractory Period
 Time is required to re-establish the resting potential after
an action potential has been initiated.
 no other action potential may be initiated, no matter how
strong the signal
Threshold Level

Different neurons have
differing threshold levels
before an action potential
will proceed.

Sensitive neurons will
have low threshold
values.

all-or-none response –
neurons will or will not fire
Axon Diameter
 greater axon diameter = greater axon surface area
 Larger surface area results in more ion channels and
therefore less time to depolarize and repolarize.
 faster signals
Saltatory Conduction
 Na+ and K+ exchange can only occur where the
axons are exposed to the extracellular fluid.
 allows for faster signal conduction along the axon
 Insulation
Signal Transmission
Dendrites
Cell body
Nucleus
Synapse
Signal
Axon direction
Axon hillock
Presynaptic cell
Postsynaptic cell
Myelin sheath
Figure 48.5
Synaptic
terminals
Signal Transmission
presynaptic
neurons
postsynaptic
neurons
Synapse
Postsynaptic cell
Presynaptic
cell
synapse – structure
formed by two
adjacent neurons
 action potentials
cause axon terminals
to release
neurotransmitters into
the synaptic cleft
Synaptic vesicles
containing
Presynaptic
neurotransmitter
membrane
Voltage-gated
Ca2+ channel
1 Ca2+
4
2
Synaptic cleft
3
Sodium Channels
Postsynaptic
membrane
Neurotransmitters
 acetylcholine – common neurotransmitter
 released upon neuron depolarization
 from presynaptic neuron
 causes Na+ channels to be opened
 in the postsynaptic neuron
Animation
Neurotransmitters
How does the postsynaptic neuron know when the
signal has stopped?
 enzymes released will degrade the chemical
 cholinesterase – released by postsynaptic neuron
Signals
1. positive (Excitatory)
 causes action potential
to proceed in
postsynaptic cell
2. Negative (Inhibitory)
 prevents action potential
to proceed
 hyperpolarization –
membrane is more
negative; therefore
stronger signal needed
Putting it all Together
 positive and negative signals collect (summation) on
postsynaptic neuron
Why are negative signals important?
 experience has told you what should be concentrated
on and what can be ignored
Terminal branch of
presynaptic neuron
E1
Postsynaptic
neuron
E1
E2
E1
I
Threshold of axon of
postsynaptic neuron
Membrane potential (mV)
0
Action
potential
Resting
potential
–70
E1
E1
(a) Subthreshold, no
summation
E1 + E 2
(c) Spatial summation
E1
I
E1 + I
(d) Spatial summation
of EPSP and IPSP