1. Electrochemical Impulses

2. How do nerve cells send and receive messages?
Dendrites
Cell body
Nucleus
Axon hillock
Axon
Signal direction
Synapse
Myelin sheath
Synaptic terminals
Presynaptic cell
Postsynaptic cell

3. 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

4. What causes membrane potential?
Unequal distribution of positively charged ions inside and outside the cell
Why only positively charged ions?
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

5. At rest, more permeable to K+ than it is to Na+
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

6. Maintaining the membrane potential at rest
ADP
Sodium Potassium Pump (Na-K Pump)

7. Resting Potential

8. Action Potential
Fill in worksheet as you watch the following animation

9. 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

10. Action Potential – Three States
resting potential: -70mV
required before a signal can be detected
Threshold: values vary
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
change that occurs to transmit signal

11. Propagation of Action Potential

12. REPOLARIZATION (Refractory Period)– +
Na+
Action potential K+
Axon
An action potential is generated as Na+ flows inward across the membrane at one location. 1 2
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. 3
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.
RESTING
DEPOLARIZATION
REPOLARIZATION (Refractory Period)
Refractory Period
Na+ channels closed
Refractory Period

13. Propagation of AP

14. Action Potential Factors
refractory period
threshold
axon diameter
saltatory conduction

15. 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

16. 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

17. 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

18. 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

19. Signal Transmission Dendrites Cell body Nucleus Synapse
Figure 48.5
Dendrites
Cell body
Nucleus
Axon hillock
Axon
Signal direction
Synapse
Myelin sheath
Synaptic terminals
Presynaptic cell
Postsynaptic cell

20. Signal Transmission
postsynaptic neurons
presynaptic neurons

21. Voltage-gated Ca2+ channel
Synapse
Presynaptic cell
Postsynaptic cell
Synaptic vesicles containing neurotransmitter
Presynaptic membrane
Postsynaptic membrane
Voltage-gated Ca2+ channel
Synaptic cleft
Sodium Channels
Na+ K+
Ligand- gated ion channel
Neuro- transmitter 1
Ca2+ 2 3 4 5 6
synapse – structure formed by two adjacent neurons
action potentials cause axon terminals to release neurotransmitters into the synaptic cleft

22. Neurotransmitters acetylcholine – common neurotransmitter
released upon neuron depolarization
from presynaptic neuron
causes Na+ channels to be opened
in the postsynaptic neuron
Animation

23. Neurotransmitters
How does the postsynaptic neuron know when the signal has stopped?
enzymes released will degrade the chemical
cholinesterase – released by postsynaptic neuron

24. Signals positive (Excitatory) Negative (Inhibitory)
causes action potential to proceed in postsynaptic cell
Negative (Inhibitory)
prevents action potential to proceed
hyperpolarization – membrane is more negative; therefore stronger signal needed

25. 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

26. Membrane potential (mV)
Resting potential
Threshold of axon of
postsynaptic neuron
(a) Subthreshold, no summation
Terminal branch of presynaptic neuron
Postsynaptic neuron
–70
Membrane potential (mV)
E1 + E2
Action potential
(c) Spatial summation E1 E2 E1
E1 + I I
(d) Spatial summation of EPSP and IPSP