Lecture 17.pdf - LECTURE 17 27 September 2019(A L Kasinski BIOL 231 Read pp 410-416 movie 12.2 others(links below THE NERVE IMPULSE Problems 6.8 6.9

Lecture 17.pdf - LECTURE 17 27 September 2019(A L Kasinski...

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1 LECTURE 17 27 September 2019 (A. L. Kasinski) BIOL 231 Read: pp. 410-416; movie 12.2 & others (links below) THE NERVE IMPULSE Problems: 6.8, 6.9; Exam II '17, #8, Exam II'18, #12 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. (Note: since the membrane is only 5nm thick, that “small” potential difference is actually >10 5 V/cm!) (3) We define a value for membrane potential, V m , 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. (4) We also define the EQUILIBRIUM POTENTIAL, V x , for just one ion, X. This is the membrane potential that would be reached if that ion and only that ion were allowed to flow across the membrane until it reached its electrochemical equilibrium. This can be calculated from conditions using the Nernst equation – more on that soon. (5) Remember that open channels in the membrane allow ions to flow toward their electrochemical equilibrium; consider typical values for [Na + ], [K + ], V Na and V K . <Now let’s look at how nerve cells use the V m to rapidly conduct signals over long distances> B. Nerve conduction: LONG cells, FAST signal (1) The axons of some 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 its 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 μm or less (10 −5 x its length!). (2) This long axon makes it possible for information to travel rapidly, without having to jump among many cells along the way, but only if conduction along a single axon is EXTREMELY fast. And it is indeed fast: information travels along axons at 50-100 meters/sec. As you probably know already, it is carried in the form of a self- propagating wave of characteristic changes in the axon’s transmembrane potential.
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