Summers

Neurobiology

text: Principles of Neural Science - Kandel, Schwartz and Jessell: Read pages 105-149 for this lecture end

VII. Membrane Potential 			back to VI. Nerves, Neurons and Glia

	A. Axonal membranes have uneven distributions of ions
	    (Na+, K+, Cl-) inside and out
	
		
		1. Inside the resting cell
		    =  high [K+] & [protein-], 		low [Na+] & [Cl-]
		
		
		2. outside
		    =  low [K+],				high [Na+] & [Cl-] 	
		
		
	B. Uneven distribution of + and -
	    results in separation of charge = potential = V
	
	
	C. membrane potential results from:
	
	
		1. diffusion potential
			
			
			a. [gradient] is the driving force for diffusion, but
				
				
				i. different ions have different rates of diffusion
				    = diffusion coefficients, because

					   
					(1) membranes have pores - small H2O soluble
						molecules pass faster
						
						
					(2) electrostatic attraction complicates ion diffusion
						
						
			b. 

						Vd = diffusion potential
						D = respective diffusion coefficients


				i. if [out] = [in]    log 1 = 0

					
				ii.  if D+ (pos. ions) = D-        V=0
					
					(1) if [out] doesn't = [in]    D+ doesn't = D-

						
					(2) then V proportional to the difference
					    in diffusion coefficients


		2. often the membrane is essentially permeable to only one ion


			a. i.e. ion channels for that ion are the only ones open


				i.  if e.g. D+= 0  (i.e. membrane only permeable to neg. ions)


					(1)0-D-/0+D-  therefore  Vd=D-/D-61 log[out]/[in]mV

			b. Nernst Equation
				
				Forcetotal = ZFVeq + 2.3 RT log [out]/[in]

						Z = valance
						F = Faraday's constant (96,500 coulombs/mole)
						Veq = equilibrium potential
						R = gas constant (8.2 Joules/degree mole)
						T = temperature (oK)
						

				i. ZFVeq = electrical force

					
				ii.  2.3 RT log [o]/[i] = chemical force


				iii. ZFVeq = -2.3 RT log [o]/[i]
				
				
					(1) \ for one ion (e.g. Na+; Z=1)
					
					
						Veq for Na = -2.3 RT/F log [Nao]/[Nai]


						(a) at 37o V = 61/Z log [o]/[i]
					
							
		3. Na+/K+ ATPase (pump)

								    								
			a. active transport via ATPase (a protein with 4 parts)
			
			
				i. 2 a = catalytic & transmembranal
				
				
					(1) binds ATP, Na+ and K+
				
				
				ii. 2 b are glycoprotein
				
				
					(1) direct and anchor a

				
			b. moves 3 Na+ out, for every 2 K+ in


				i. via conformational change
				    when ATP is converted to ADP
				
				
					(i) Mg++ cofactor


			c. ouabain poisons pump: resting potential increases 4 mV
			
			
				i. pump contributes -4 mV to resting potential
				
				
					(1) if it is already established

			
	E.  cells which are not currently transmitting have an unequal
	     balance of ions such that there is a resting potential of
		 -40 to -90 mV

		
		1. Veq=Na+ = 61 log 440/50 mV = 57.6 mV
		
		
		2. Veq=K+ = 61 log 20/400 mV = -79 mV
		
		
			a. most of resting potential is due to K+
			
			
				i. but some Na+ trickles in and some K+ trickles out
				
				
					(1) when Na+in = K+out = resting potential


   Vm = 61 log pNa+[Na+o] + pK+[K+o] + pCl-[Cl-o]/pNa+[Na+i] + pK+[K+i] + pCl-[Cl-i] mV
		
						pNa+ = permeability of Na+

		3. usual resting potential = -60 to -70 mV
		
		
			a. range of the resting potential may reveal
			    the function of the cell
			
			
				i. higher resting potentials (e.g. -50 mV) result in
				    easier action potential stimulation


					(1) but all cells have resting potential
					
					
							(a) neurons and muscle cells are excitable
			

	F. input to the cell may increase (hyperpolarize) this potential
	   or reduce (depolarize)it


		1. multiple inputs make local membrane potential graded

		
		2. dramatic changes in polarity of the cell occur
		   via ion channel action

VIII. Ion Channels

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