1. Neuronal signaling
Domina Petric, MD

2. Electrical signaling
Fundamental neuronal process that underlies all aspects of brain function.

3. Unique properties of neurons
Intrinsically neurons are poor generators of electricity.
Electrical signals are based upon the flow of the ions across the neuronal plasma membrane.

4. Resting potential vs. action potential
Resting potential of the neuron varies in different neurons between minus 40 mV and minus 90 mV.
All or none nature of neurons: only depolarisation above treshold will generate action potentials or spikes.
Stronger stimulus gives rise of more action potentials, but the amplitude is always the same (depending on the type of the neuron).
Number of action potentials is proportional to the strength of the stimulus.

5. Nernst equation Ex is equilibrium potential for any ion.
R is gas constant.
T is absolute temperature (degrees Kelvin).
z is valence of the permeant ion (electrical charge).
F is Faraday constant (amount of electrical charge in a mole of an univalent ion).
(X)0 is concentration of ion X outside of the cell.
(X)i is concetration of ion X inside the cell.

6. Simplified Nernst equation
Ex = 58/z multiplied with log (X)0 /(X)i
The equilibrium potential for an ion is proportional to the concentration gradient.
Proportionality is influenced by the valence of the permeon ion.

7. Goldman equation
Em is equilibrium potential when multiple ions may permeate the membrane.
V (or Em) is voltage across the plasma membrane.
P is permeability of the plasma membrane to each ion.
Simplified
58 log x

8. Potassium and sodium ions
Changes in potassium (K) and sodium (Na) ions permeability underlie the action potential.

9. Action potential Absolute refractory period is
innactivation of sodium channels.
Action potential

10. Electrical current Early current is carried by sodium.
Late current is carried by potassium.
Current is a function of conductance and driving force.

11. Action potential
The action potential is explained by the voltage-dependent and time-dependent changes in the permeability of the neuronal membrane to sodium and potassium ions.

12. Important molecules
Sodium-potassium pump (ATPase pump) is the most important molecule in the nervous system.
Other ATPase pump is calcium pump.
Ion exchangers are Na+/Ca++ exchanger and Na+/H+ exchanger.
Co-transporters are Na+/K+/Cl- co-transporter, K+/Cl- co-transporter and Na+/neurotransmiter co-transporter (GABA, dopamine).

13. Passive conduction decays over distance

14. Axon diameter
Small diameter axons impose greater resistance to the axial diffusion of ions (axial conduction of charge).
Larger diameter axons are for less resistance and facilitate the rapid conduction of passive current.
Small diameter axons will have a slower propagation rate of action potential conduction compared to larger diameter axons.
Myelin facilitates action potential propagation.

15. Regeneration of the action potential in the node of Ranvier

16. Myelin
Increase of the propagation of action potentials: efficiency and conduction velocity.
Advantage of economy: fewer ion channels need to be produced by the cell body and inserted into a much more reduced surface area of axonal plasma membrane.

17. Literature
al-neuroscience/lecture: Leonard E. White, PhD, Duke University