Alteration of resting membrane potential

Transcription

1 Observation electric current easuring electrodes Alteration of resting ebrane potential ebrán extracellular spece intracellular space 1. passive electric properties of the ebrane Inward current Depolarization of the ebrane What is it like? Charge and discharge of C-circuit Interpretation with equvivalent circuit odel: I ion + Ic = I = 0 U C =I 0 (1-e -t/τ ) 100% U C =U 0 e -t/τ I I I I Cl Na K g Na g K g Cl U g Na (U -E Na )= I Na g ion (U -E)= I ion 63% 37% E Na E K E Cl C Δt E + I stiulus = 0 Tie fro the beginning of stiulus τ=c Mebrane potentiel after t U ( t) U t 1 e t = C Saturation value of ebrane potential

2 inward current Capacitance of the ebrane τ = C esistance of the ebrane U ( t) U t 1 e t = C outward current depolarization τ : tie constant of ebrane -the tie required for the ebrane potential to reach 63% of its saturation value -during which the ebrane potential decreases to the e-th of its original value U t is proportional to the stiulating current The rate of the change depends on U t hyperpolarization Local changes of ebrane potential The local changes are not isolated fro the neighbourhood Observation obligate graded agnitude varies directly with the strength of the stiulus direction varies with the direction of the stiulus.. localised Mebrane potential change at the site of stiulation 100% 37% λ x x = 0 e -x/λ Decrease in aplitude with distance due to leaky ebranes

3 λ: space constant of the ebrane: distance in which the axial value of induced ebrane potential change decreses to its e-th value Mebrane potential change at the site of stiulation λ ~ 100% 37% i λ x x = 0 e -x/λ esistance of intracellular space Local changes of resting ebrane potential can be induced - by electric current pulses - by adequate stiulus at receptor cells - by neurotransitters at postsynaptic ebrane -excitatory inhibitory postsynaptic potential - depolarization - inhibitory postsynaptic potential - hyperpolarization Significance of the local changes of resting ebrane potential Sensory function Ipulse conduction Signal transduction Alteration of resting ebrane potential 2. active electric properties of the ebrane in excited state

4 Observation Phases and landark of the action potential inward current +20V Peak potential outward current hyperpolarization depolarization Action potential Threshold potential critical ebrane potential level at which an action potential can occur 0V -70V epolarizing phase depolarization afterpotential stiuli facultative All-or-none aplitude conducted with constant aplitude Hodgkin-Katz hypothesis of action potential generation Hodgkin-Katz hypothesis of action potential sequence Voltage-Gated, potential sensitive ion channels ϕ ϕ = e i T ln F Σp c Σp depolarization + + k ke + + k cki + Σp c + Σp k ki k cke inward Na + current p Na (g Na ) increases

5 Measureent of separated ionic currents U (V) Inhibited K + - Na + -current Andrew Fielding Huxley (1917- ) Alan Loyd Hodgkin ( ) The Nobel Prize in Physiology or Medicine 1963 for their discoveries concerning the ionic echaniss involved in excitation and inhibition in the peripheral and central portions of the nerve cell ebrane" I (μa) channels t (s) Inhibited Na + - channels t (s) t (s) Voltage-Gated Na + and K + Channels K + -current States of voltage-gated sodiu channels Condictivities during action potential at depolarization threshold

6 Propagation of action potential (1) Factors Influencing Conduction Direction and Velocity helyi áraok The evolutionary need for the fast and efficient transduction of electrical signals based on local current flow and depolarization of adjacent ebrane area Propagation of action potential (2) Generation of the next peak potential Where? λ x The greater the space constant, the ore rapidly distant regions will be brought to threshold and the ore rapid will be the propagation velocity When? Speed and distance of propagation? How are the tie constant and the space constant related to propagation velocity of action potentials The saller the tie constant, the ore rapidly a depolarization will affect the adjacent region. Velocity is the function of passive properties τ and λ of ebranes

7 Effect of axon diaeter: i (~1/r 2 ) r (~1/r) τ λ τ = C λ ~ i Myelination! very high C very sall big space constant sall tie constant huan nerve cell r= 10 μ Squid giant axon r=250μ v~ 100 /s v=25/s huan nerve cell r= 10 μ v 0.5/s? Saltatory conduction quick, energy saving Effect of axon diaeter and Myelination Na + Node of anvier Na + Myelin sheath Myelin prevents ions fro entering or leaving the axon along yelinated segents. Na + Daage of Myelin sheath Leak of current Na +

8 Signal transission in synapses Effect of passive electric properties on signal transduction in synapses presynaptic terinal postynaptic terinal Action potential neurotransitter How can neurons transit inforation fro presynaptic to Teporal Suation : cobined effects of neurotransitter release fro the sae sites over tie postsynaptic cells if ost synaptic effects are subthreshold? Postynaptic signal Postsynaptic signal Spatial Suation : cobined influences at the sae cell at a particular oent in tie -40 V U -40 V τ=10 s τ=1 s U -60 V Presynaptic signal -60 V Presynaptic signal Teporal Suation : cobined effects of neurotransitter release fro the sae sites over tie idő 2 s idő 2 s

9 Teporal and spatial suation