NACS641-10L3 - Propagation of electrical signals in neurons...

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Unformatted text preview: Propagation of electrical signals in neurons and the action potential Ricardo C. Araneda raraneda@umd.edu Fall 2010 Some drawings are from “The neuron; cell and molecular Biology” Levitan & Kaczmarek Friday, September 10, 2010 OVERVIEW: * Axon and dendrites as wires (previous lecture) * Action potentials * Phases of the action potential * Voltage-clamp * Ion currents underlying the AP * Na channel inactivation * Structure of Na channels * Gating current Friday, September 10, 2010 Membranes are permeable to more than one ion _ + + K K Na + + Na Cl Cl _ The Goldman-Hodgkin-Katz equation Permeability; how easy is for ions to cross the membrane Friday, September 10, 2010 The cell membrane as an electrical circuit Ionic membrane current Ii Capacitive membrane current Ic Membrane current = Im Friday, September 10, 2010 Im = I i + I c Why do we care? “Small cell” Current -2 pA Rin ∼ 1 GΩ “Large cell” Current -20 pA Rin Friday, September 10, 2010 ∼ 200 MΩ Real Neurons How are signals affected by the passive properties of the membrane ? Friday, September 10, 2010 Experiment 3 Local potentials are graded resting potential Almost no change in potential is observed, why? Friday, September 10, 2010 Current in axons and dendrites Current pulse rm Section of axon or dendrite of determined length (x). In this case 1 cm. ra ra axial resistance (Ω/cm) rm membrane resistance (Ω∗cm) Friday, September 10, 2010 What is the value of V at increasing distances from the site of current injection? voltage-recording electrode current-passing electrode V = VO∗e -x/λ λ is the length constant 37% (1/e) λ λ =√ (rm/ra) (cm) rm Decreasing ra Increasing i.e. Friday, September 10, 2010 increases λ increases λ V is closer to VO Effect of diameter Given a dendrite or an axon with increasing diameters (a, b, c) The length constant increases and the potential decreases less w ith distance. Friday, September 10, 2010 The passive properties of membranes and axon diameter affect the speed of conduction of action potentials speed of conduction of action potentials is inversely related to ra∗cm speed of conduction is increased by increasing the diameter of the axon which decreases ra The giant axon of the squid 1 mm ! Friday, September 10, 2010 Myelination, the alternative to increasing the diameter of the axon. Glial cell wraps around axons many times (20-160 times) this like adding 320 membranes (in series). This increases Rm and decreases Cm Friday, September 10, 2010 The action potential A real one same height faster (latency to threshold) Friday, September 10, 2010 Notice time course and changes in voltage What is threshold? Friday, September 10, 2010 Absolute Refractory period Relative Refractory period Encoding stimulus strength in terms of frequency Friday, September 10, 2010 80 Beating cell mV 40 0 -40 -80 80 mV 40 0 -40 -80 Friday, September 10, 2010 Bursting cell Adapting cell mV pA Friday, September 10, 2010 Action potentials in plants Notice the time course of the action potential Friday, September 10, 2010 Action potential terminology, what do these terms mean? Rising phase Peak overshoot undershoot Threshold Friday, September 10, 2010 Falling phase Changes in permeability associated with the action potential Friday, September 10, 2010 Bernstein (1906) Cole & Curtis, increase in membrane conductance (1939) Hodgkin & Huxley, intracellular recording of squid AP (1939) Development and improvement of Voltage clamp Friday, September 10, 2010 Voltage-clamp & the squid axon Friday, September 10, 2010 Remember But Im = Ii + Ic I c = C ∗ ΔV Δt / Friday, September 10, 2010 Capacitive current only when V is changing Depolarization reveals two different currents Friday, September 10, 2010 Experiment 1 Step to different potentials and measure the currents Na current K current Notice the shape of the of the IV relationship for Na and K Friday, September 10, 2010 How do we study the currents underlying the action potential? Friday, September 10, 2010 How do we study the currents underlying the action potential? 40 20 mV 0 -20 -40 -60 -80 0 1 2 Time (ms) Ion substitution (changes in driving force) Friday, September 10, 2010 How do we study the currents underlying the action potential? 40 20 mV + + 0 -20 -40 -60 -80 0 1 2 Time (ms) Ion substitution (changes in driving force) Friday, September 10, 2010 + + Toxins and pharmacological blockers of ion channels TEA blocks K current TTX blocks Na current Friday, September 10, 2010 Na currents (channels) inactivate Inactivation is both voltage and time dependent Friday, September 10, 2010 Recovery from inactivation is also time and voltage dependent H&H modeled the action potential based on the observed values of conductance and their dependence on voltage and time Friday, September 10, 2010 The structure of voltage-gated channels Friday, September 10, 2010 Gating Current Voltage sensor The movement of the S4 segment produces a measurable gating current Friday, September 10, 2010 “Ball & chain” mechanism of inactivation Friday, September 10, 2010 voltage change Na channel opens (higher probability at the beginning) 3 Na channels Many Na channels Total INa How would K channels look like? Friday, September 10, 2010 Inactivation is responsible for the absolute refractory period Recovery from inactivation is responsible for the relative refractory period Friday, September 10, 2010 Now you can explain each of these terms Rising phase Peak overshoot undershoot Threshold Friday, September 10, 2010 Falling phase ...
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