Neurobiology

text:
Principles of Neural Science
- Kandel, Schwartz and Jessell:
Read pages 150-169 for this lecture
end

IX. Action Potential 			back to VIII. Ion Channels


	A. transient  charge reversal at the membrane = depolarization


	B. Generated by flow of ions through voltage-gated
	     Na⁺ and K⁺ channels


		1. Na⁺ channels open first when the membrane
		    reaches threshold potential


			a. Na⁺ rushes in along [Na⁺] and electrical gradients


			b. rising membrane potential (V_m) is caused by
			    influx of Na⁺ and opens even more Na⁺ channels


				i. postive feedback:
				  Na⁺ in  ®  mV  ®  Na⁺ channel opening


				ii. membrane depolarizes and moves toward
				    the Na⁺ equilibrium potential (V_{eq_Na⁺}) 



		2. higher V_m  closes Na⁺  channels and  opens K⁺ channels


			a. K⁺  eflux begins to repolarize the cell


				i. V_m moves from near V_{eq_Na⁺} toward V_{eq_K⁺}


					(1) Na⁺  channels are inactivated
					    for a  short while


						(a) refractory


				ii. K⁺ voltage-gated channels open
				    longer than Na⁺ channels


			b. V_m passes (becomes more negative than)
			    the resting potential


				i. the neuron is hyperpolarized


					(1) V_m = V_{eq_K⁺}


	C. Glial cells take up excess extracellular K⁺


	D. Na⁺/K⁺ ATPase pump +
	   glial cell action restores resting potential


	E. Charge is redistributed along the axonal membrane by
	       electrical and concentration gradients


		1. depolarizing adjacent membrane
		    opens Na⁺ channels there


			a. the Action Potential moves down the axon


			b. greatest [Na⁺ channel] at the axon hillock


				i. trigger zone or integrative component of neuron


					(1) multiple inputs on dendrites and soma


						(a) graded membrane potential


					(2) sums input from regional depolarizations


					(3) axon conductile


						(a) action potential all-or-nothing


						(b) amplitude cannot be affected by stronger
						    stimulus or  stimulus duration post-axon hillock


						(c) action potential same at
						    soma and axon terminal


		2. Once the action potential has moved down the axon it
		    cannot move back because the membrane is refractory


			a. closed Na⁺ and  open K⁺ channels


	F. Saltatory  (=jumpy) Propagation


		1. axons may be wrapped in myelin


			a. myelin insulates the neuron
			    = inhibits movement of ions


			b. charge may move along axon but without
			    reinforcement of newly opened Na⁺  channels


		2. between myelin filled glial cells are open
		    spaces = Nodes of Ranvier


		3. Action Potential is reinforced at the Nodes


			a. Na⁺  channels open, Na⁺  rushes in, etc.


		4. Since ion movement across membranes is slow
		    (and limited to the nodes),
			saltatory propagation is much faster


		5. most vertebrate axons are myelinated
X. Synthesis

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University of South
Dakota......Department of Biology