Electrical Signals in Neurons

1. Electrical Signals in Neurons Ch. 12

2. Objectives • Describe the factors that maintain a resting membrane potential. • List the sequence of events that generates an action potential.

3. Path of a Nerve Impulse

4. Path of a Nerve Impulse

5. Membrane Potentials • Resting membrane potential – potential for current to pass • Created by a difference in charge between the inside and outside of the cell • Most cells are -70mV • The greater the difference, the greater the potential • Graded potential – a change in resting potential • Action potential – the passing of electrical current as a result of the graded potential • This is how neurons send signals

6. Resting Membrane Potential

7. Resting Membrane Potential – based on diffusion Inside – more negative Outside – more positive • High in K+ • High in proteins (-) • - ions can’t leave • This keeps inside more - also • K+ tends to diffuse out of the cell • This results in an increase – charge here • High in Na+ • High in Cl- • K+ diffuses out • This results in an increase + charge here • However, K+ will also go back inside because of attraction to - charge

8. Resting Membrane Potential – based on pump action • Na+ action • Will diffuse inward • This could destroy RMP • sodium-potassium pump • Pumps 3 Na+ out and 2 K+ in • Maintains RMP

9. Action Potentials • Also called impulse • Occurs in 2 phases • Depolarizing phase • Negative membrane potential becomes less negative • Reaches zero • Then becomes positive • Repolarizing phase • Membrane potential is restored to -70mV (RMP)

10. Action Potential • Channels • Na+ channels open • Na+ rushes into the cell → depolarizing phase • K+ channels open • K+ flows out → repolarizing phase • depol. and repol. last about 1msec (.001 sec.)

11. All-or-None • When depolarization reaches the threshold (-55mV) the action potential occurs • No matter how great the difference is, it will occur • Like dominoes • It doesn’t matter how hard or soft your push the first domino, once you do all the rest will fall

12. Depolarizing Phase • Inward movement of Na+ is favored • Na+ channels have 2 gates • Activation and inactivation • In resting state • Inactivation gate is open, activation gate is closed • Na+ cannot move into the cell through these gates • At threshold (activated state) • Both channels are open • Na+ rushes into the cell and depolarization occurs • The more Na+ rushing in, the more channels open

13. Depolarizing Phase • After activation gates open, inactivation gates close • Inactivated channel • Na+ – K+ pumps return to Na+ to the ECF

14. Repolarizing Phase • At threshold – depolarization opens K channels • This allows K+ to flow out of the cell • the neuron is returned to resting state

15. Action Potential

16. Review • Describe a neuron at rest. Where are the ions? What are the charges? • List the steps in an action potential.

17. Steps • RMP (-70mV) • Action potential is triggered by neurotransmitter • Depolarization - Na+ gates open and Na+ enters the cell (+30mV) • Repolarization – K+ gates open and K+ leaves the cell • Pumps return everything to RMP (-70mV)