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# lec16 note - LECTURE 16 29 September 2010(P J Hollenbeck...

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-1- LECTURE 16 29 September 2010 (P. J. Hollenbeck) BIOL 231 Read: pp. 409-416; DVD 12.8 THE NERVE IMPULSE Problems: 44-45; Exam II '05, 4a and 4c I. Electrical charge and information flow A. Charge disequilibrium across membranes (1) Virtually all charged molecules are out of equilibrium across the membrane, including ions and “fixed” charges, such as proteins. Charge separated by a membrane => VOLTAGE across membrane, or “transmembrane potential.” (2) In most cells, most of the time, the net charge on the INSIDE is NEGATIVE. The convention is to describe the transmembrane potential as inside relative to outside, so it usually falls in the range from ! 15 to ! 100mV, typical for a neuron might be ! 70mV. (By the way, since the membrane is only 5nm thick, that “small” potential difference is actually >10 V/cm!) 5 (3) We define a value for membrane potential, m V , that is the sum of all contributions to voltage across the membrane. It is what you measure if you put electrodes inside and outside the cell and empirically determine the voltage difference. x (4) We also define the EQUILIBRIUM POTENTIAL for just one ion, V . This is the membrane potential that would be reached if that ion and that ion only were allowed to flow across the membrane until its electrochemical equilibrium was reached. This can be calculated using the Nernst equation - more on that soon. (5) Remember that open channels in the membrane allow ions to flow toward their Na K electrochemical equilibrium; consider typical values for [Na ], [K ], V and V . ++ m <Now let’s look at how nerve cells use the V to rapidly conduct signals over long distances> B. Nerve conduction: LONG cells, FAST signal (1) The axons of many nerve cells in your peripheral nervous system span up to 1 meter – for example, a motor neuron that has its cell body in your lumbar spinal cord and sends an axon to a muscle in your toe. By cellular standards this is a VERY long distance. Note that this axon has a diameter of 10 ì m or less.

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lec16 note - LECTURE 16 29 September 2010(P J Hollenbeck...

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