Action Potential (Chap-1) Last Lec.pdfAction Potential (Chap-1) Last Lec.pdfAction Potential (Chap-1) Last Lec.pdf

Action Potential (Chap-1) Last Lec.pdf

1. Action Potential Propagation
The Nervous System

2. What is an action potential?
An action potential is an electrical impulse or signal passed
from one neuron to another
Electrical properties result from ionic concentration
differences across plasma membrane and permeability of
membrane
Ions are any charged particles found in the cell

3. Important Ions
Cations= Positive Ions
Sodium = Na
+1
charge
Potassium = K
+1
charge
Calcium = Ca
+2
charge
Anions = Negative Ions
Chlorine = Cl
-1
charge
Nerve cells are surrounded by a semi-permeable membrane
that allows some things to pass through and not others

4. Membrane Potential
Caused when opposite charges are separated by a cell
membrane and want to move to balance each other out.
More cationson outside
of cell than inside of cell
so you have a membrane
potential

5. Electrode measures membrane
potential of a neuron
Axon

6. Phases of Membrane Potential
1. Resting potential: –70mV (inside of axon more
negative than outside)
2. Depolarization: 0 to +30 mV (inside of axon more
positivethan outside)
3. Repolarization: +30 back to –70mV (inside of axon
becoming more negative than outside)
4. Hyperpolarization: -70 mV to –100mV (inside
axon way more negative than outside)

7. Phases of Membrane Potential
Hyperpolar-
ization

8. Resting Potential
Whenaneuronisnotsendingasignal,itis"atrest”.Whena
neuronisatrest,theinsideoftheneuronisnegativecompared
totheoutside.Althoughtheconcentrationsofthedifferentions
attempttobalanceoutonbothsidesofthemembrane,they
cannotbecausethecellmembraneallowsonlysomeionsto
passthroughchannels(ionchannels).Thedifferenceinthe
voltagebetweentheinsideandoutsideoftheneuronis
measured,youhavetherestingpotential.Theresting
membranepotentialofaneuronisabout-70mV
(mV=millivolt)-thismeansthattheinsideoftheneuronis70
mVlessthantheoutside.

9. Resting Potential
At rest, there are relatively more sodium ions outside the
neuron and more potassium ions inside the neuron. The
sodium channels are closed, preventing sodium from
entering the cell.

10. Depolarization
Theactionpotentialisanexplosionofelectricalactivity
thatiscreatedbyadepolarizingcurrent.Thismeansthat
someevent(astimulus)causestherestingpotentialto
movetoward0mV.Thisevent(stimulus)happenswhen
aneurotransmitterbindstoreceptorsonthedendrites.
Thisbindingcausessodiumchannelstoopen,and
sodiumbeginstoenterthecell.Whenthe
depolarizationreachesabout-55mVaneuronwillfire
anactionpotential.Thisisthethresholdstimulus.

11. Depolarization

12. All or None Principle
If the neuron does not reach this critical threshold
level, then no action potential will fire. Also, when
the threshold level is reached, an action potential of a
fixed sized will always fire...for any given neuron,
the size of the action potential is always the same.
There are no big or small action potentials in one
nerve cell -all action potentials are the same size.
Therefore, the neuron either does not reach the
threshold or a full action potential is fired this is the
"ALL OR NONE" principle.

13. Repolarization
Duringrepolarization,weseeareturntothenormal
restingpotentialstate.Thesodiumchannelsbeginto
close,andinanefforttobalancethechargesinside
andoutsidethecell,potassiummovesoutofthecell.
Whenpotassiumleavesthecell,themembrane
potentialbecomesmorenegative(returnsto-70
mV).

14. Hyperpolarization
During hyperpolarization, the potassium channels
continue to stay open, allowing more potassium to
leave the cell. This drops the electrical voltage down
to -100mV, hyperpolarizing the cell. Once potassium
channels close, a return to resting potential occurs.

15. Hyperpolarization

17. Action Potential Generated
Themovementofsodiumionsintothecelldepolarizes
adjacentsitesontheaxon,triggeringtheopeningof
additionalchannels.Theresultisachainreactionthatspread
acrossthesurfaceofthemembranelikealineoffalling
dominoes.Theactionpotentialcontinuestomovedownthe
axonuntilitreachestheaxonterminalandcausesthe
calciumchanneltoopen.Calciumenterstheterminal,binds
tothevesicles,andcausesthevesiclestobereleasedintothe
synapticcleft.Theneurotransmittermovesthroughthe
synapticcleftandbindstothenextneuron,continuingthe
propagationoftheactionpotential.

18. Copyright Pearson Prentice Hall
The Nerve Impulse

19. Copyright Pearson Prentice Hall
The Nerve Impulse

20. Copyright Pearson Prentice Hall
The Nerve Impulse

21. Copyright Pearson Prentice Hall
The Synapse
Synaptic cleft

22. Basic Steps to an Excitatory Action
Potential
2. Neurotransmitters bind to
receptors on dendrites
3. Causes Na+ to rush
into cell body
4. Membrane potential
becomes more positive
+30mV
1. Neuron is at resting
potential (-70 mV)
6. K+ rushes out of cell to
balance out differences in
membrane potential
5. Na+ channels close

23. Basic Steps to an Excitatory Action
Potential (continued)
8. Continues along length of
axon
9. Action Potential
reaches axon terminal
and Ca
2
+ channel open
10. Ca2+ enters axon
terminal and binds to
vesicles
11. Vesicles with NT inside
travel to end of axon
terminal
7. Membrane potential
goes to -100mV but then
goes back to -70mV
12. Vesicles release NTs into
synaptic cleft
13. NT’s travel through
synaptic cleft and bind to next
neuron