Tweaking the system blocking K+ channels Block K+ efflux Na+ influx continues, and potential becomes positive the electrical gradient for Na+ efflux becomes equal to the chemical gradient for Na+ influx Na+ influx (chemical) = Na+ efflux (electrical) i.e. equilibrium potential at which this occurs is the K+ equilibrium potential (~-90 mV) Tweaking the system blocking Na+ channels Block Na+ influx K+ efflux continues, and RMP becomes more negative the electrical gradient for K+ influx becomes equal to the chemical gradient for K+ efflux K+ influx (electrical) = K+ efflux (chemical), i.e. Tweaking the system altering out normal out lower out K+ gradient for efflux is steeper (more powerful) membrane potential becomes more negative i.e.

Pump leak model: steady state condition Key points: only 1/10,000 ions move  RMP Na+/K+ pump is electrogenic inhibition of pump (digoxin)  RMP  ~ -65 mV Resting Membrane Potential is predominantly a K+ diffusion potential, modulated by Na+ diffusion Origin of resting membrane potential Set up: in & out permeability to K+ 50x permeability to Na+ Initially: K+ efflux > Na+ influx, therefore cell negative Negative cell slows K+ efflux, speeds Na+ influx Membrane potential stabilizes when K+ efflux balances Na+ influx The squid axon Squid axon experiments show: Na+ & K+ gradients required anions not involved Na+/K+ pump not required if ion gradients exist energy source not required Resting membrane potential fig 6-8a Resting membrane potential of nerve cell = -70 mV Ion distributions a reminder Electrochemical gradients (cell at -70 mV) Na+: strong inward (chemical: in, electrical: in) K+: weak outward (chemical: out, electrical: in) Cl-: at electrochemical equilibrium (chemical in = electrical out) Glial cells Peripheral nervous system Schwann cells: myelin sheath Central nervous system Oligodendrocytes: myelin sheath Astrocytes: remove K+, neurotransmitters, blood-brain barrier Ependymal cells: line cavities Microglia: phagocytic Notes: 90% of CNS cells are glial cells brain tumors almost always tumors of glial cellsĬlasses of neurons fig 6-4 Ratios: 1 afferent : 10 efferent : 200,000 interneurons Myelin sheath Peripheral nervous system: Schwann cells Central nervous system: oligodendrocytes fig 6-2a

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