1. Bioelectrical phenomena in nervous cells

3. Measurement of the membrane potential of the nerve fiber using a microelectrode membrane potential membrane potential

4. Basic Concepts Forces that determine ionic movement  Electrostatic forces  Opposite charges attract  Identical charges repel  Concentration forces  Diffusion – movement of ions through semipermeable membrane  Osmosis – movement of water from region of high concentration to low

5. Basic mechanisms of transport mechanisms

6. Gating of protein channels provides a means of controlling ion permeability of the channels. Gating of protein channels provides a means of controlling ion permeability of the channels.

7. I. Membrane Resting Potential  A constant potential difference across the resting cell membrane  Cell’s ability to fire an action potential is due to the cell’s ability to maintain the cellular resting potential at approximately –70 mV (-.07 volt)  The basic signaling properties of neurons are determined by changes in the resting potential

8. Concept of Resting Potential (RP)  A potential difference across the cell membrane at the rest stage or when the cell is not stimulated.  Property:  It is constant or stable  It is negative inside relative to the outside  Resting potentials are different in different cells.

9. Resting Membrane Potential  Na + and Cl - are more concentrated outside the cell  K + and organic anions (organic acids and proteins) are more concentrated inside.

10. Active Transport Movement of molecules and ions against their concentration gradients. Movement of molecules and ions against their concentration gradients. –From lower to higher concentrations. Requires ATP. Requires ATP. 2 Types of Active Transport: 2 Types of Active Transport: –Primary –Secondary

11. Primary Active Transport ATP directly required for the function of the carriers. ATP directly required for the function of the carriers. Molecule or ion binds to carrier site. Molecule or ion binds to carrier site. Binding stimulates phosphorylation (breakdown of ATP). Binding stimulates phosphorylation (breakdown of ATP). Conformational change moves molecule to other side of membrane. Conformational change moves molecule to other side of membrane.

12. P Extracellular fluid 1. Three sodium ions (Na + ) and adenosine triphosphate (ATP) bind to the carrier protein. 2. The ATP breaks down to adenosine diphosphate (ADP) and a phosphate (P) and releases energy. 3. The carrier protein changes shape, and the Na + are transported across the membrane. 4. The Na + diffuse away from the carrier protein. 5. Two potassium ions (K + ) bind to the carrier protein. 6. The phosphate is released. 7. The carrier protein changes shape, transporting K + across the membrane, and the K + diffuse away from the carrier protein. The carrier protein can again bind to Na + and ATP. Cytoplasm Na + ATP K+K+ Breakdown of ATP (releases energy) Carrier protein changes shape (requires energy) ADP 1 3 2 Carrier protein resumes original shape Na + K+K+ P K+K+ 4 5 6 7 ATP binding site Carrier protein

13. Intracellular vs extracellular ion concentrations Ion Intracellular Extracellular Na + 5-15 mM 145 mM K + 140 mM 5 mM Mg 2+ 0.5 mM 1-2 mM Ca 2+ 10 -7 mM 1-2 mM H + 10 -7.2 M (pH 7.2) 10 -7.4 M (pH 7.4) Cl - 5-15 mM 110 mM

14. Resting Membrane Potential  Potassium ions, concentrated inside the cell tend to move outward down their concentration gradient through nongated potassium channels  But the relative excess of negative charge inside the membrane tend to push potassium ions out of the cell

15. Resting Membrane Potential But what about sodium? Electrostatic and Chemical forces act together on Na + ions to drive them into the cell The Na + channel close during the resting state Na + is more concentrated outside than inside and therefore tends to flow into the cell down its concentration gradient Na + is driven into the cell by the electrical potential difference across the membrane.

16. Resting Potential

18. Postulated mechanism of the sodium-potassium pump mechanism

19. The formation of resting potential depends on:  Concentration difference of K + across the membrane  Permeability of Na + and K + during the resting state  Na + -K + pump

20. Factors that affect resting potential  Difference of K + ion concentration across the membrane  Permeability of the membrane to Na + and K +.  Action of Na + pump

21. Basic Electrophysiological Terms I:  Polarization: a state in which membrane is polarized at rest, negative inside and positive outside.  Depolarization: the membrane potential becomes less negative than the resting potential (close to zero).  Hyperpolarization: the membrane potential is more negative than the resting level.

22. Basic Electrophysiological Terms I:  Reverspolarization: a reversal of membrane potential polarity.  The inside of a cell becomes positive relative to the outside.  Repolarization: restoration of normal polarization state of membrane.  a process in which the membrane potential returns toward from depolarized level to the normal resting membrane value.

23. Effect of stimuli of increasing voltages to elicit an action potential action potentialaction potential

24. Typical action potential

26. II Action Potential Successive Stages: (1)Resting Stage (2)Depolarization stage (3)Repolarization stage (4)After-potential stage (1) (2)(3) (4)

28. The Action Potential Components/characteristics Components/characteristics –RMP –Depolarizing stimulus –Threshold –Rapid Na + entry (depolarization) –Isopotential –Overshoot –Repolarization (K + moves out) –Undershoot (after- hyperpolarization) –Absolute refractory period –Relative refractory period

29. Propagation of action potentials in both directions along a conductive fiber Propagation of action potentials in both directions along a conductive fiber Propagation

31. Saltatory conduction Saltatory conduction along a myelinated axon. Flow of electrical current from node to node is illustrated by the arrows. Saltatory conduction