Cerebrospinal Fluid (CSF)

Transcription

1 1. CSF Cerebrospinal Fluid (CSF) Neurons are surrounded by an aqueous saline solution called cerebrospinal fluid (CSF). The CSF contains sodium (Na), potassium (K), chloride (Cl-), and other ions in solution. The neuronal membrane itself (lipid bilayer) is impermable to the movement of ions. However, ions can cross the membrane by two means...

2 2. Transporters and Channels Move Ions Across the Neuronal Membrane Ion Transporters Ion Channels

3 3. The Na /K pump

4 4. Restiing Potential The Resting Potential The resting potential is negative because: 1. The K concentration is greater inside than outside the neuron, due to the action of the sodium / potassium transporter pump. 2. At rest, the neuron is primarily permeable to K, because the membrane contains K leak ion channels.

5 5. Diffusion and electrostatic (Coulomb) force Two Physical Phenomena Establish the Resting Potential Diffusion: K diffuses out down its concentration gradient. Electrostatic force: As K diffuses out, the inside becomes negative compared to the outside. Eventually, K is attracted strongly enough to the negative inside that the diffusion stops. (Opposite charges attract).

6 6. Coulomb s Law Electrostatic force (Coulomb s Law) (Opposite charges attract) F " (q 1)(q 2 ) r 2

7 7. Nernst Equation The Nernst Equation E x = 58 log [x] out z [x] ( ) in Ex [x] out Equilibrium potential for ion x Outside concentration [x] z in Inside concentration Electric charge (valence of the ion)

8 8. Logarithm Review Logarithm Review log(n) 0 Log(1) = 0, because 10 = 1 1 Log(10) = 1, because 10 = 10-1 Log(0.1) = -1, because 10 = 0.1 When n = 1, log(n) is 0 n > 1, log(n) is positive ( ) n n < 1, log(n) is negative ( )

9 9. Electrochemical Equilibrium for K Ions - an example K Clground (Vout = 0) (reference location)

10 10. Electrochemical Equilibrium for K Ions - another example K Clground (Vout = 0) (reference location)

11 11. Electrochemical Equilibrium K Na Cl-

12 12. Ionic Concentrations and Equilibrium Potentials E x = 58 log [x] out z [x] ( ) in Inside (mm) Outside (mm) Ex (mv) Potassium (K) Sodium (Na) At rest, the neuron is primarily permeable to K, so the resting potential is close to the K equilibrium potential: negative!

13 13. Goldman Equation The Goldman Equation Vm P x = 58 log Pk [k] out [k] ( out ) P k Permeability to ion x in P Na P Na [Na] [Na] in The resting potential is -65 mv, not -84 mv, because there is some permeability to Na, even at rest. So some Na is continually flowing into the neuron at rest, and some K flowing out.

14 14. Receptor Potential Causing Action Potenial Receptor (generator) potential Vm (mv) threshold Action potential (spike)

15 15. Synaptic Potentials Causing Action Potential Membrane Potential (mv) Time (ms)

16 16. Action Potential The Action Potential Membrane Potential (mv) ( polarized ) threshold resting potential rising phase strong receptor or synaptic potential ( depolarization ) falling phase undershoot ( afterhyperpolarization ) Time (ms)

17 17. Voltage-Gated Sodium Channel The Voltage-Gated Sodium Channel gating charge 3) channel opens Na 1) sensory receptor or synaptic activity Na Axon 2) depolarization

18 18. Action Potential Rising Phase The action potential rising phase occurs because the neuronal membrane contains voltage-gated Na channels that open in a regenerative manner if depolarization passes the threshold level: Na rushes in POSITIVE FEEDBACK LOOP voltage-gated Na channels open strong receptor potential or synaptic potential

19 19. Voltage-Gated Sodium Channel The Real Voltage-Gated Sodium Channel Has Two Gates Na Axon activation gate inactivation gate Na

20 20. Voltage-Gated Sodium Channel The Voltage-Gated Sodium Channel Has Three States closed open inactivated Axon

21 21. End of Rising Phase The action potential rising phase ends because the voltage-gated Na channels inactivate Na no longer enters POSITIVE LOOP ENDS voltage-gated Na channels inactivate

22 22. Voltage-Gated Sodium Channel The Voltage-Gated Potassium Channel 3) K channel opens, K leaves gating charge Creaky door (slow) K 1) Na entry (rising phase) Na Axon 2) depolarization

23 23. Action Potential Falling Phase Na no longer enters POSITIVE LOOP ENDS voltage-gated Na channels inactivate The action potential falling phase occurs because the voltage-gated K channels open (with a delay) in response to depolarization. K flows out NEGATIVE FEEDBACK LOOP voltage-gated K channels open

24 24. Action Potential Membrane Potential (mv) rising phase Na channels open; Na enters threshold resting potential end of rising phase Na channels inactivate; Na stops entering falling phase K channels open; K leaves undershoot Time (ms)

25 25. Action Potential Action Potential Summary 1. The membrane is depolarized past threshold, by a receptor potential or synaptic potential. 2. Voltage-gated sodium channels open in a positive-feedback loop, causing the rising phase. 3. The sodium channels inactivate, ending the rising phase and initiating the refractory period. 4. Voltage-gated potassium channels open, causing the falling phase and the undershoot (after-hyperpolarization). 5. The sodium channels close, ending the refractory period. 6. The potassium channels close, ending the undershoot and restoring the membrane to its resting potential.

26 26. Action Potential voltage-gated Na channels refractory closed open inactivated closed velocity time voltage closed closed open closed voltage-gated K channels

27 27. Importance of The Refractory Period The Refractory Period 1. Ensures unidirectional ( polarized ) action potential conduction, away from the cell body. 2. Places an upper limit on the firing rate of a neuron.

28 28. Importance of The Refractory Period Unidirectional Conduction Under normal conditions, action potentials only move in one direction: away from the cell body. This is not due to an intrinsic property of the axon. It is due to the refractory period... Normally, the action potential only moves away from the cell body, because it starts there, so the refractory wake prevents it from going backwards. Note: Cell body is to left; axon terminal is to right