1. NOTES - UNIT 5 part 2: Action Potential: Conducting an Impulse

2. ACTION POTENTIALS and NERVE IMPULSES
 all cells have an electrical charge difference across their plasma membranes; that is, they are POLARIZED.
 this voltage difference is called the MEMBRANE POTENTIAL (usually –50 to –100 mV)
 arises from differences in ionic concentrations inside and outside cell

3. 150 mM Ion Concentration of Intracellular Fluid [ICF]
Concentration of Extracellular Fluid
[ECF]
Na+
sodium
15 mM
150 mM K+
potassium
5 mM
Cl-
chloride
10 mM
120 mM A-
All the negatively charged molecules (ex: DNA is negatively charged)
100 mM
0 mM

4. the “resting potential” of a nerve cell is approximately –70 mV
Inside of the cell is more negative than the outside
Plasma membrane is impermeable to ions because of their charge
neurons have special
ion channels (GATED ION CHANNELS) that allow the cell to change its membrane potential (a.k.a. “excitable” cells)
RMP Animation

5. Types of gated ion channels:
Gated channels are proteins embedded in the plasma membrane that can be “closed” or “open”
Voltage gated: need specific charge to open
Ligand gated: need specific molecule to bind to open (like a key to a lock)
Mechanically gated/ tension gated (FYI)
Light gated (FYI)

6. Ligand gated: when a stimulus reaches the dendrites of a neuron, it causes the opening of gated ion channels & ions will be able to move through the channel
Ex: ACh at neuromuscular junction

7.  If positive ions are moving into the cell, this is DEPOLARIZATION
membrane potential becomes less negative/more positive
Ex: Na+ channel opens; Na+ flows in
**If the level of depolarization reaches the THRESHOLD POTENTIAL, an ACTION POTENTIAL (electrical impulse) is triggered.
 threshold is -55mV

8. ACTION POTENTIALS (APs):  the nerve impulse
all-or-none event; magnitude is independent of the strength of the stimulus
 strong stimuli result in greater frequency of action potentials than weaker stimuli

9. Resting state (Na + & K + gates closed)
4 Phases of an A.P.:
Resting state (Na + & K + gates closed)
2) Depolarizing Phase (Na + gates open)
3) Repolarizing Phase (Na+ gates closed & K + open)
4) Undershoot (K+ still open a bit)
*Refractory Period- Na+/K+ pump: pump ions back across to resting levels and Na+/K+ gates reset)

11. How do action potentials “travel” along an axon?
Because of myelin, the strong depolarization of one action potential (Na+ entering neuron) causes the neighboring region of the neuron to be depolarized above threshold
This opens this next voltage gated Na + channel, triggering a new action potential

12. Saltatory Conduction:
If the next voltage gated channel is at a Node of Ranvier, the action potential appears as if it has “jumped” down the axon
Called saltatory conduction

14. Action Potential Conduction down unmyelinated axons: Continuous conduction
More action potentials are needed without myelin
Message takes longer to propagate down the axon
Short axons of CNS are unmyelinated

16. Action Potential Reaches Synaptic Knob
Causes the release of neurotransmitters from the presynaptic membrane
These neurotransmitters will diffuse across the synaptic cleft and bind to receptors in the postsynaptic membrane
Located in dendrites if postsynaptic cell is a neuron
Neurotransmitters can be excitatory or inhibitory
Excitatory: bring the membrane potential in the cell body closer to threshold (-55mV)
Inhibitory: move the membrane potential farther from threshold

17. Example of EXCITATORY Post-Synaptic Potential (EPSP ): neurotransmitter causes an ion channel to open that allows positive ions, such as Na+, into the cell body (membrane potential becomes more positive)

18. Example of INHIBITORY Post-Synaptic Potential (IPSP ): neurotransmitter causes an ion channel to open that allows positive ions, such as K+, out of the cell body OR negative ions, such as Cl-, into the cell body (membrane potential becomes more negative)

19. What happens at a postsynaptic neuron when it is receiving multiple neurotransmitter messages?
The change in membrane potential is summed/added together
SUMMATION can be:
Temporal
Spatial
Temporal:
most single EPSPs are not strong enough to generate an action potential
when several EPSPs occur close together or simultaneously, they have an additive effect on the postsynaptic potential

20. TEMPORAL SUMMATION

21. What happens at a postsynaptic neuron when it is receiving multiple neurotransmitter messages?
Spatial:
Sum/add up the change in membrane potential from all the different synaptic knobs sending neurotransmitter message at the same time
Green: +10mV; Black: +15mV;
Red: -10mV change in membrane potential… AP??