Membrane and Action Potentials

Membrane Potential

• Voltage measured across the plasma membrane (-50mV - 100mV)

• Difference in electrical charge between inside and outside of a cell

Membrane Structure

• Phospholipid bilayer, proteins

• Lipids – phospholipids, glycosphingolipids, sterols

• Proteins - pumps, channels, receptors, enzymes, markers, structural components

Types of transport

Active transport: needs energy

Diffusion/passive transport

Sodium-Potassium Pump

• Creates membrane potential by unequal movement of charge

• Uses ATP to move 3 Na+ out for every 2 K+ ions in

Resting Membrane Potential

• Typical membrane potential -70mV

• Balance of intracellular and extracellular ions dictate membrane potentials

Ions important in neural signals

– Na+ , Cl- , and Ca2+ have inward gradient » greater concentration outside cell

– K+ has outward gradient » greater concentration inside cell

Nernst Equation

• Calculates equilibrium potential for each ion based on its concentration gradient

• Difference between equilibrium potential and membrane potential is net gradient for that ion

Goldman Equation

• Goldman constant field equation gives predicted membrane potential based on concentration gradients for principal permanent ions

• p = relative permeability for each ion (other ions e.g. Calcium are essential impermeable in an unstimulated neuron)

• Position of positive and negative ions due to opposite effects on membrane potential

• At rest, K most permeable (pK=1.00), with pCl=0.45 and pNa=0.04.

Hence: E (mV)=-67.4 mV

Resting Membrane Potential Summary

• Changes in [K+ ] cause most changes in EM because it is most permeable

• Since K+ is actively transported and is most permeable, its concentration is most important in determining EM at rest

• If an ion becomes more permeable, EM will change towards that ion’s equilibrium potential

– high permeability of one ion reduces equation to Nernst equation for that ion

Action Potential

• Needs a stimulus

• Short-lasting event, all-or-none, travels in one direction

• Occurs in excitable cells

• Neurons summate synaptic inputs to a threshold

– Threshold may be 10-25 mV depolarization above the resting potential

Voltage-gated Channels

• Voltage-sensitive Na+ and K+ channels alter the membrane potential

– Depolarization (inside of cell becomes less negative)

– Na+ channels open but do not stay open very long

• Na+ influx depolarizes the membrane even more

– K+ channels open slowly and remain open until EM returns to normal, then slowly close

• K+ efflux causes hyperpolarization

– Number of open channels can be measured as membrane conductance for each ion

• Larger stimulus will open more channels and result in larger change in membrane potential

Phases of an Action Potential

• Rise and fall of action potential due to Na+ and K+

– Na+ channels rapidly open, increasing pNa and changing EM towards ENa

– Membrane potential depolarizes

– Membrane potential reverses (overshoot)

– Na+ channels rapidly close (inactivate) at same time as K+ channels open

– EM changes towards EK , hyperpolarized relative to membrane potential at rest

• At rest, membrane is most permeable to K+ , least to Na+

• At peak, Na+ permeability rises

• Na+ channels open and close rapidly

• K+ channels take longer to open and close

Refractory Period

occurs due to inactivated Na+ channels and hyperpolarization