CELLULAR MEMBRANE POTENTIALS

1. Dr. Kenneth OrimmaB.Sc., M.Sc., M.B.B.S, D.I.R, D.M(Doctor of Medicine) Psychiatry GENERATION CELLULAR ELECTRICITY & EXCITABLE TISSUES

2. MEMBRANE POTENTIAL • Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell • This is the difference in the energy required for electric charges to move from the internal to exterior cellular environments and vice versa • The concentration gradients of the charges directly determine this energy requirement.

3. MEMBRANE POTENTIAL • Cells are surrounded by a membrane composed of a lipid bilayer with proteins embedded in it. • The membrane serves as both an insulator and a diffusion barrier to the movement of ions. • Transmembrane proteins, also known as ion transporter or ion pump proteins, actively push ions across the membrane and establish concentration gradients across the membrane, and ion channels allow ions to move across the membrane down those concentration gradients.

4. MEMBRANE POTENTIAL • Ion pumps and ion channels are electrically equivalent to a set of batteries and resistors inserted in the membrane, and therefore create a voltage between the two sides of the membrane. • Plasma membrane have transmembrane capacitance, like batteries it has ability to store electrical charges across it’s membrane creating electrical energy that drives ion charges

5. MEMBRANE POTENTIAL • Membrane potential therefore is Voltage across membrane • This voltage is energy due unequal distribution of ions (K+ & Na+) • The voltage is a result of selective/differential permeability of specific plasma membrane proteins (mainly ion channels) • Membrane potential is evident in all living cells (ranges between -20 & -200mV)

6. MEMBRANE POTENTIAL

7. MEMBRANE POTENTIAL Membrane Potential Depends on: o Relative permeability of plasma membrane to ions o Each ion have its own concentration gradient o E.g Na electrochemical gradient is different from that of K+ Resting Membrane Potential: o Is a stable membrane potential of cells when unstimulated o For Nerve Cells – approx -70mV and For cardiac muscle cells – approx. -90mV

8. Ion Distribution Across Plasma Membrane K+: [greater inside cell] o At rest, membrane is much more permeable to K+ than to Na+ o Ie: K+ diffuses out of the cell through leakage channels down its concentration gradient § Therefore, loss of positive charge from cell makes: Inside the cell negative and Outside the cell positive § Eventually the negativity of the inner membrane face attracts K+ back into the cell Therefore, concentration gradient drives K+ out and is equally opposed by electrical gradient (equilibrium potential has been reached)

9. MEMBRANE POTENTIAL Na+: [greater outside cell] o Membrane has much lower permeability to Na+ o Negative inner membrane-face attracts Na+ into cell, but is opposed by low permeability o Therefore low diffusion of Na+ into cell

10. MEMBRANE POTENTIAL Resting Membrane Potential • is established due to greater diffusion of K+ out than Na+ diffusion into the cell Maintaining the Resting Membrane Potential • Na passively diffuses into cell and K passively diffuses out • The Concentration Gradients for both Na & K are maintained by the Na/K ATPase

12. MEMBRANE POTENTIAL Excitable Tissues (Nerves/Muscle) • In excitable cells, stimuli can alter the permeability of the membrane to K+ and/or Na+ via opening/closing gated ion channels (ligand/chemically gated, voltage gated, mechanically gated, vibration gated, temperature gated) • This changes the membrane potential • If the membrane potential is sufficiently altered, an action potential is initiated • Action potential: an electrical impulse generated and conducted along a nerve’s axon or muscle fiber in response to stimuli

14. ACTION POTENTIAL CONDUCTION AP are conducted along the length of the axon • A wave of action potentials cause opening and closing of voltage gated ion channels • This in turn causes depolarisation, repolarisation and hyperpolarisation of membrane

16. Excitable tissues In excitable tissues – • stimulation causes change in Membrane Potential which results in activation of the cell • Nervous & Muscle Tissues are self electrical potential generating tissues so there are generally referred to as excitable Tissues

17. Neuronal tissue Action Potentials & features Phase 1 – Resting Phase: o Membrane is much more permeable to K+ than to Na+ o Greater diffusion of K out than Na in o Therefore inside is negative/Outside is positive o Both Na & K voltage gated channels are CLOSED

18. Neuronal tissue Action Potentials & features Phase 2 – Depolarisation Phase: o Mechanical/chemical/vibratory/other stimulus opens some Na+ channels → Na+ flows into the cell o Therefore membrane potential becomes less negative (Ie: It depolarises) o If the MP reaches approximately -55mV (threshold), the voltage gated Na+ channels open → Na+ influx increases dramatically – until MP reaches approximately +30mV where the voltage gated Na+ channels close

19. Neuronal tissue Action Potentials & features Phase 3 – Repolarisation Phase: o At approximately +30mV K+ voltage gated channels open (permeability of K increases & Na decreases) o Large outflow of K+ → membrane potential becomes more negative (repolarises) and returns to - 70mV

20. Neuronal tissue Action Potentials & features Phase 4 – Hyperpolarisation (undershoot) Phase: o K+ channels remain open past -70mV and MP becomes more negative than at rest o K+ channels close and Na/K ATPase returns the MP to normal (-70mV)

22. Neuronal tissue Action Potentials & features Refractory Periods During the Action Potential: • Is the total time between a stimulus creating an action potential and the MP returning to rest • Usually 3-4ms • Determines how soon a neuron can respond to another stimulus

23. Neuronal tissue Action Potentials & features Divided into 2 sub-periods: o Absolute Refractory Period – no additional stimulus (no matter how large) can initiate a further action potential • Stimulus →Depolarisation • Repolarisation o Relative Refractory Period – If an additional stimulus is to initiate another action potential during this time, it must be larger in order to reach threshold • Hyperpolarisation → Rest

25. Neuronal tissue Action Potentials & features • THE END • THANK YOU