1. 10.6: Cell Membrane Potential
A cell membrane is usually electrically charged, or polarized, so that the inside of the membrane is negatively charged with respect to the outside of the membrane (which is then positively charged).
This is as a result of unequal distribution of ions on the inside and the outside of the membrane.

2. Distribution of Ions
Potassium (K+) ions are the major intracellular positive ions (cations).
Sodium (Na+) ions are the major extracellular positive ions (cations).
This distribution is largely created by the Sodium/Potassium Pump (Na+/K+ pump).
This pump actively transports 3 sodium ions out of the cell and 2 potassium ions into the cell.

3. Resting Potential Resting Membrane Potential (RMP):
70 mV difference from inside to outside of cell
It is a polarized membrane
Inside of cell is negative relative to the outside of the cell
RMP = -70 mV
Due to distribution of ions inside vs. outside
Na+/K+ pump restores
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High Na+
Low Na+
Impermeant
anions
High K+
Low K+
Cell body
Axon
Axon terminal + + + + – – –
(a) – + + – + + + – – – – – – – – – + + – + – – – + + + + + +
–70 mV
(b) + +
High Na+ + + – – –
Low Na+ –
Na+ + +
Pump – – + + – – K+ – – – – + – –
Low K+
High K+ – + + + + + + – +
–70 mV
(c)

4. Local Potential Changes
Caused by various stimuli:
Temperature changes
Light
Pressure
Environmental changes affect the membrane potential by opening a gated ion channel
Channels are 1) chemically gated, 2) voltage gated, or 3) mechanically gated
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Gate-like mechanism
Protein
Cell
membrane
Fatty acid
tail
(a) Channel closed
Phosphate
head
(b) Channel open

5. Local Potential Changes
If membrane potential becomes more negative, it has hyperpolarized
If membrane potential becomes less negative, it has depolarized
Graded (or proportional) to intensity of stimulation reaching threshold potential
Reaching threshold potential results in a nerve impulse, starting an action potential

6. Local Potential Changes
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Na+
Na+
–62 mV
Chemically-gated
Na+ channel
Neurotransmitter
Presynaptic
neuron
(a)
Voltage-gated
Na+ channel
Trigger zone (axon hillock)
Na+
Na+
Na+
Na+
Na+
–55 mV
(b)

7. Action Potentials At rest, the membrane is polarized (RMP = -70)
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Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+ K+ K+ K+ K+ K+ K+ K+ K+ –0
Threshold stimulus reached (-55) K+ K+ K+ K+ K+ K+ K+ K+
–70
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
(a)
Sodium channels open and membrane depolarizes (toward 0)
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+ channels open
K+ channels closed K+
Na+
Na+
Na+ K+ K+ K+ K+ K+
Threshold
stimulus K+ K+ –0 K+ K+ K+
Na+
Na+
Na+ K+ K+ K+ K+ K+
–70
Potassium leaves cytoplasm and membrane repolarizes (+30)
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Region of depolarization
(b) K+ K+ K+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
K+ channels open
Na+ channels closed K+
Na+
Na+
Na+ K+ K+ K+ K+ K+ –0
Brief period of hyperpolarization (-90) K+
Na+
Na+
Na+ K+ K+ K+ K+ K+
–70 K+ K+ K+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Region of repolarization
(c)

8. Action Potentials +40 Action potential +20 –20 Resting potential
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+40
Action potential
+20
–20
Resting potential
reestablished
Membrane potential (millivolts)
–40
Resting
potential
–60
–80
Hyperpolarization 1 2 3 4 5 6 7 8
Milliseconds

9. Action Potentials Region of action potential
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Region of
action potential + + + + + + + + + + + – – – – – – – – – + + – – – – – – – – – + + + + + + + + +
(a) + + + + + + + + + – – – + + – – – – – –
Direction of nerve impulse – – – + + – – – – – – + + + + + + + + +
(b) + + + + + + + + + – – – – – – – + + – – – – – – – – – + + – – + + + + + + + + +
(c)

10. All-or-None Response
If a neuron axon responds at all, it responds completely – with an action potential (nerve impulse)
A nerve impulse is conducted whenever a stimulus of threshold intensity or above is applied to an axon
All impulses carried on an axon are the same strength

11. Refractory Period Absolute Refractory Period
Time when threshold stimulus does not start another action potential
Relative Refractory Period
Time when stronger threshold stimulus can start another action potential

12. Impulse Conduction

13. 10.7: Synaptic Transmission
This is where released neurotransmitters cross the synaptic cleft and react with specific molecules called receptors in the postsynaptic neuron membrane.
Effects of neurotransmitters vary.
Some neurotransmitters may open ion channels and others may close ion channels.

14. Synaptic Potentials EPSP Excitatory postsynaptic potential Graded
Depolarizes membrane of postsynaptic neuron
Action potential of postsynaptic neuron becomes more likely
IPSP
Inhibitory postsynaptic potential
Graded
Hyperpolarizes membrane of postsynaptic neuron
Action potential of postsynaptic neuron becomes less likely

15. Summation of EPSPs and IPSPs
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EPSPs and IPSPs are added together in a process called summation
More EPSPs lead to greater probability of an action potential
Neuron
cell body
Nucleus
Presynaptic
knob
Presynaptic
axon

16. Neurotransmitters

17. Neurotransmitters

18. Neuropeptides
Neurons in the brain or spinal cord synthesize neuropeptides.
These neuropeptides act as neurotransmitters.
Examples include:
Enkephalins
Beta endorphin
Substance P

19. 10.8: Impulse Processing
Way the nervous system processes nerve impulses and acts upon them
Neuronal Pools
Interneurons
Work together to perform a common function
May excite or inhibit
Convergence
Various sensory receptors
Can allow for summation of impulses
Divergence
Branching axon
Stimulation of many neurons ultimately

20. Neuronal Pools
Groups of interneurons that make synaptic connections with each other
Interneurons work together to perform a common function
Each pool receives input from other neurons
Each pool generates output to other neurons

21. Convergence Neuron receives input from several neurons
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Neuron receives input from several neurons
Incoming impulses represent information from different types of sensory receptors
Allows nervous system to collect, process, and respond to information
Makes it possible for a neuron to sum impulses from different sources 2 1 3
(a)

22. Divergence One neuron sends impulses to several neurons
Can amplify an impulse
Impulse from a single neuron in CNS may be amplified to activate enough motor units needed for muscle contraction
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(b)