2/7/16. Neurons maintain a negative membrane potential. Membrane potential. Ion conductances determine the membrane potential

Transcription

1 Neurons maintain a negative membrane potential. V Ion channels are key regulators of membrane potential. Low Na + 2mM High K + 125mM Low Ca + (10-7 ) Low Cl - (5mM) Membrane potential. V ENa= RT/nF ln[na+]o/[na+]in Ion conductances determine the membrane potential g Na E Na + g K E K + g Cl E Cl Vm = g Na + g K +g Cl E Na ~ +60mV E K ~ - 70mV ~ - 65mV E cl Low Na + 2mM High K + 125mM Low Ca + (10-7 ) Low Cl - (5mM) (E = Nernst or Equilibrium potential) (Possible concentrations) High Na + (130mM) Low K + (2mM) High Ca + (1mM) High Cl - (140mM) (Ca ++ is a major second messenger) Q. 1: A neuron at rest has 100ps conductance to Na 10,000ps conductance to K 100 ps conductance to Cl- What is its (approximate) resting potential? Q.2 For 10 milliseconds, Na conductance rises to 10 6 ps Draw a curve membrane potential vs time. 1

2 Neuronal excitability and synaptic transmission Action potential and ionic conductances V1 V2 Membrane potential Action potential Synaptic transmission (fast active membrane response) Conduction velocity (squid giant axon) = 21.3m/ sec Patch clamp recording of a single channel Neuronal excitability and synaptic transmission! Closed " Open! Inactivated > Membrane potential Action potential Synaptic transmission (fast active membrane response) Neurotransmitters (ligands) open ligand gated channels: recovery 2

3 Properties of ion channels 1. Gating (voltage; ligands/neurotransmitters and others.) 2. Ion Selectivity Activating voltage gated channels g Na E Na + g K E K + g Cl E Cl Vm = g Na + g K +g Cl E Na ~ +60mV E K ~ - 70mV ~ - 65mV E cl 3. Conductance 4. Inactivation/ desensitization 5. Refractory period (recovery from inactivation) Voltage gated channels underlie action potentials. Initial depolarization (positive current entry). Active response of ion channels Action potential threshold Only a small fraction of ion channels participate in each a/p A- D represent firing in the same neuron with increasing depolarizing stimulus - Frequency coding 3

4 Properties of ion channels help sculpt a/p wave forms and neuronal firing patterns Diverse action potential waveforms 1. Gating (voltage; ligands/neurotransmitters and others.) 2. Ion Selectivity 3. Conductance 4. Inactivation/ desensitization (a state different from the closed state) 5. Refractory period (recovery from inactivation) The genome encodes 100s of different ion channels, each with its intrinsic physiological features. A complex action potential in a cell that shows rhythmic firing. E Na ~ +60mV E K ~ - 70mV E cl ~ - 65mV Structure and function of ion channels Describe how different kinds of ion channels contribute to each phase of the action potential indicated in the figure below. Which ion channel conductances do you think control the frequency of firing? 4

5 Venoms and Neurotoxins Target Neurotransmission Leg shaking (hyperexcitability) mutants identify K channels. Noda et al. Nature

6 Shaker mutations affect a 4AP-sensitive K-channel Temperature-sensitive paralytic mutants identify positive elements of neural excitability (A model for human episodic ataxia - EA1) e.g. Na channels/ regulators. (nap ts ; para ts ) Elements of synaptic transmission Organization of voltage-gated channels Tanouye, 1988 Jan and Jan, Rod McKinnon, 2003 Na + -Channel Ca ++ -Channel K + -Channel 6

7 Voltage-clamp recording of sodium channel currents Mechanism of voltage-dependent gating ShB R362Q R365Q Activation Inactivation Ball and chain model for channel inactivation Mechanism of ion-selectivity Regulation 7

8 Mechanisms to generate channel diversity K channel inactivation (and its regulation) E.g. Humans have about 40 different K channels that fall into classic K + channel families + the Twik class) - And >400 genes for ion channel subunits 1. Gene duplication and divergence 2. Alternative Splicing 3. Non-essential (e,g. beta) Subunits 4. Heteromultimer formation/ subunit shuffling 5. RNAi editing LGi1 is mutated in a dominant epilepsy, associated with auditory hallucinations (I estimate about 50 different channelopathies ) - Henry Stewart Lecture (online from TCD library) Kaczmarek, Neuron 2006; Schulte et al., Neuron Channelopathy phenotypes depend on.. Channel properties Effect of the pathogenic mutation on channel properties Cell type in which the channel is expressed Function of the cells affected. Eg1. Lgi mutations (auditory hallucinations and epilepsy) Eg 2. Tang Goldin et al SCNA1 febrile epilepsy 8