MEMBRANE POTENTIAL PLASMA MEMBRANE Plasma membrane of neurons

MEMBRANE POTENTIAL

PLASMA MEMBRANE §Plasma membrane of neurons is filled with channels that allow specific ions to cross.

1. Passive or leakage channels – usually open and allow specific ions to pass (i. e. , K+) 2. Gated channels ü only open when appropriate signal received. Chemically (ligand) gated channels – only open when the appropriate chemical or neurotransmitter present. Voltage gated channels only open when the membrane voltage is at an appropriate level.

TYPES OF ION CHANNELS

4 4 140

§ BASIC PHYSICS OF MEMBRANE POTENTIAL

Membrane Potential Caused by Diffusion of K+

Membrane Potential Caused by Diffusion of Na+

§ The concentration difference of ions across a selectively permeable membrane can , under appropriate conditions cause a creation of a membrane potential

NERNST EQUATION § ALLOWS THEORETICAL MEMBRANE POTENTIAL TO BE CALCULATED FOR A PARTICULAR ION. – Membrane potential that would exactly balance the diffusion gradient and prevent the net movement of a particular ion. – Value depends on the ratio of [ion] on the 2 sides of the membrane. §

§ The potential level across the membrane that prevents the net diffusion of an ion in either direction through the membrane is called “NERNST POTENTIAL FOR THAT ION” EMF(millivolts) = ± 61 log Conc inside/Conc outside §Equilibrium potential for K+ = - 94 m. V. §Equilibrium potential for Na+ = + 61 m. V.

Goldman-Hodgkin-Katz equation

0 m. V

-90 m. V

§ Potential difference exists across every cell’s plasma membrane. – cytoplasm side is negative pole, and extracellular fluid side is positive pole § Inside of cell negatively charged because: – large, negatively charged molecules are more abundant inside the cell – sodium potassium pump – voltage-gated ion channels

THE RESTING MEMBRANE POTENTIAL • If we measure voltage between the inside of a neuron and the outside we find that the neuron is more negative inside than outside with a potential of about – 90 m. V. 9

Resting membrane potential: AT REST The differences in electrical charges between the inside of the cell and the outside cell

How Do Neurons Operate? • Neuron at Resting Potential q. Occurs when the neuron is at rest. q. A condition where the outside of the membrane is positively(+) charged compared to the inside which is negatively(-) charged. q. Neuron is said to be polarized. q. Neuron has a voltage difference of -90 m. V

Active transport of sodium and potassium ions through the membrane Na+( outside)- 142 m. Eq/L Na+( inside)- 14 m. Eq/L K+ ( outside)- 4 m. Eq/L K+( inside)- 140 m. Eq/L Ratio of these two respective ions from inside to outside Na+ = 0. 1 and K+ =35. 0

• Na+ ions are actively transported (this uses energy) to maintain the resting potential. NOTE: Na+/K+ pump = Pump. Kin (Pump K + in) Na + Na+ outside K+ K+ inside

Leakage of potassium and sodium through the nerve membrane

Origin of normal RMP § Contribution of pottasium diffusion potential § Contribution of sodium diffusion through the nerve membrane § Contribution of sodium potassium pump

Local potential

§ Local response § Sub minimal stimulus is applied. § Axon is stimulated with slowly increasing strength of stimuli. MP is decreased by 7 mv. § Cathelectrotonic potential developed at the cathode. § Anelectrotonic potential is developed at the anode.

Graded potential Action potential Sub minimal stimulus Threshold stimulus Does not obey all or none law Obey all or none law Can be summated Cannot be summated Cannot be propagated Can be propagated

SUMMARY

1. A Nerve Cell has ELECTRICAL POTENTIAL across its cell membrane because of a difference in the number of Positively and Negatively Charged IONS on each side of the Cell Membrane.

§ 2. The Electrical Potential is due to PROTEINS in the Neuron known as Sodium -Potassium Pumps move Sodium ions (Na+) OUT of the Cell and Actively Pump Potassium ions (K+) INTO the Cell.

§ 3. The result of this Active Transport of ions is the Cytoplasm of the neuron contains MORE K+ IONS and FEWER Na+ IONS than the surrounding medium.

§ 4. The Cytoplasm also contains Many NEGATIVE CHARGES PROTEINS Molecules and Ions. § 5. K+ ions can leak out across the membrane more easily than Na+ ions can leak in.

§ 6. The Negatively charged protein molecules and ions do not leak in or out. § 7. The Net Result of the leakage of positively charged ions out of the cell is a Negative Charge on the INSIDE of the neuron's Cell Membrane

§ 8. The Charge Difference is known as the RESTING POTENTIAL of the Neuron's Cell Membrane. § 9. As a result of its Resting Potential, the Neuron is said to be POLARIZED.

§ 10. POLARIZED = Negatively Charged on the inside of the Cell Membrane, and Positively Charged on the Outside. § 11. A Neuron maintains this polarization until it is stimulated. § 12. A STIMULUS is a change in the environment that may be of sufficient strength to initiate an impulse.