Download Neurobiology: Action Potentials and Ion Dependence and more Study notes Neurobiology in PDF only on Docsity! updated 11/30/2020 Biol 6102: Neurobiology Action Potentials Chapter 4 Important features of the action potential Threshold o all or none o Action potentials are regenerative Action potential signaling Digital Code o Timing – when action potentials occur o Intensity - Frequency of action potential firing Action potential timing can be complex and non-linear (bursting) Action Potential: Ion dependence At rest: o The resting membrane potential is negative (and therefore very attractive to the sodium). o The resting potential is determined by the relative concentrations and resting permeabilities to sodium, potassium, (and chloride). o There is a high concentration of sodium outside the cell and a low concentration inside. o There is a low resting permeability to sodium (so it cannot flow into the neuron along its electrochemical gradient) So, resting potential is not very sodium dependent When the membrane potential is depolarized, the probability of opening voltage-gated sodium channels increases o This causes Na+ to enter the cell. o The addition of positive charge further depolarizes the membrane potential, further increasing the probability of sodium channel openings. o If enough sodium channels open in a short time, then there is a positive feedback of sodium channels opening and causing further depolarization. o The opening of sodium channels drives the membrane potential towards ENa (+55mV) and the action potential reaches its peak. o Therefore the peak depolarization is sodium-dependent After a short period of time: o The sodium channels inactivate and close. o At about the same time, voltage-gated potassium channels begin to open. o The membrane potential heads back towards EK (-75mV) o Eventually the voltage-gated potassium channels close and the membrane potential returns to rest. Positive Feedback System o Cole and Curtis determined that it was due to an increased conductance p. 1 Voltage Clamp used to break the positive feedback with negative feedback. Holds voltage constant and records current Hodgkin and Huxley used the voltage clamp technique to study the ionic basis of action potentials o Early inward current (Na+), late outward current (K+) o Can use toxins to separate early and late currents Tetrodotoxin (TTX) blocks Na+ current Tetra-ethylammonium (TEA) blocks K+ current o Convert current into conductance Ohm’s Law: V = I x R G = 1/R Iion = gion (Vm – Eion) o Voltage and time dependent conductances Differences in the time course of the Na+ and K+ conductances Na+ conductance inactivates with time. K+ conductance has a slower onset. Both conductances are voltage-dependent o Created a set of equations to describe the time and voltage-dependence of the conductances Reconstructed the features of the action potential from these equations This is an example of confirmatory computer model. Voltage-gated Sodium and Potassium Channels As with potassium channel, there are four repeating subunits o Unlike K+-channels, all 4 subunits are on one protein 6 transmembrane segments o one pore loop Ion Selectivity o Selective to Na+ over K+ because Na+ is a smaller ion K+ maintains selectivity due radius of hydration In potassium channel, selectivity is caused by binding site that allows K+ to lose its water molecule and become smaller than the hydrated Na+ ion. Voltage - sensitivity o segment s4 is the voltage sensor o charges on channel move in response to local voltage difference Lidocaine acts by binding to S6 o prevents the Na+ channel from opening Patch clamp method of recording single channel current Large pipette seals against membrane to record current passing through ion channels Interpretation of single channel currents o square shape (open or closed) o amplitude related to conductance and driving force o constant characteristic conductance Four configurations 1. Cell-attached 2. Inside-out 3. Whole-cell 4. Outside-out p. 2
