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Announcements

Announcements. Mid term room assignments posted to webpage. Lecture 01. A. Excitor. B. Inhibitor. Record voltage. Simple case:. Threshold. A. B. Vm. hyperpolarizing  inhibitory IPSP. Depolarizing  excitatory EPSP. Threshold. A+B=smaller. Vm.

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Announcements

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  1. Announcements • Mid term room assignments posted to webpage Lecture 01

  2. A. Excitor B. Inhibitor Record voltage

  3. Simple case: Threshold A B Vm hyperpolarizing  inhibitory IPSP Depolarizing  excitatory EPSP Threshold A+B=smaller Vm

  4. How to get hyperpolarizing potential? • Neurotransmitter receptor is permeable to an ion whose Eion is more negative than resting membrane potential • usually Cl- or K+

  5. Hyperpolarizing Synaptic Potential +60 mV 0 mV + -80 mV K+ +

  6. More complex case: Threshold A B Vm Why??? Depolarizing  excitatory Depolarizing  inhibitory Threshold A+B=smaller Vm

  7. Reversal Potential • Membrane potential at which there is no net synaptic current

  8. Current source Control resting membrane potential Record membrane potential Measuring Reversal Potential eg. Frog NMJ +25 stimulus 0 Reversal potential -50 -100 Stimulate nerve

  9. Many neurotransmitter receptors are permeable to more than one ion • Non-selective • The reversal potential depends on the equilibrium potential and permeability of each ion • It will usually be between the equilibrium potential of the permeable ions

  10. eg. Acetylcholine channel • Permeable to both K+ and Na+ • For Frog muscle: • EK = -90 mV • ENa = +60 mV

  11. K+ VmENa ENa = +60 mV +25 0 Reversal potential Vm=Erev -50 Erev>Vm>EK -90 VmEK Na+ Neurotransmitter receptor EK = -90 mV

  12. How can depolarizing potential be inhibitory? • Excitatory synapses have a reversal potential more positive than threshold • Inhibitory synapses have a reversal potential more negative than threshold

  13. How can depolarizing potential be inhibitory? Erev Threshold A B Erev Vm Example: Cl- permeable receptor in a cell whose Vthresh >ECl- > Vm

  14. Inhibition • Channels of inhibitory synapses ‘short-circuit’ excitatory synapses • Because neurotransmitter channels will drive the membrane potential toward their reversal potential

  15. Neurotransmitters and receptors • Synaptic Integration

  16. Types of Receptors • Ligand-gated ion channels • Neurotransmitter binding to receptor opens an ion channel • Directly changes the membrane potential of the postsynaptic cell • Also known as ‘fast’ synaptic transmission • G-Protein Coupled Receptors • Transmitter binds to receptor which activates intracellular molecules • Can directly or indirectly change the membrane potential • Also known as ‘slow’ synaptic transmission

  17. Neurotransmitter Receptors Ligand-gated ion channels

  18. Neurotransmitter Receptors G-Protein coupled receptors Same neurotransmitter, different receptors

  19. G-protein coupled receptor receptor  direct effect Open or close ion channel   G-proteins indirect effect Activate intracellular molecules GTP GDP Regulate other cellular functions eg gene expression

  20. What happens to neurotransmitter after it is secreted? • Acetylcholine • Broken down by Acetylcholinesterase into Choline and Acetate • Choline transported back into nerve terminal and resynthesized into Acetylcholine • Glutamate • Transported into glia or the nerve terminal and converted to glutamine

  21. Serotonin • A neurotransmitter used in the emotional centres of the brain • Prozac is a drug that inhibits the reuptake of serotonin • Therefore, Prozac makes serotonin remain in synaptic cleft longer

  22. Synaptic Integration The sum of all excitatory and inhibitory inputs to a cell. • Spatial Summation • Temporal Summation

  23. Spatial Summation • The addition of several inputs onto one cell A B A B A+B A B A+B

  24. Temporal Summation Stim once A Stim twice Stim twice

  25. Synaptic Integration Summation Soma and dendrites Axon Hillock Synaptic inputs Passive current flow Above threshold? No Yes Passive Current Decays to zero Action Potential Conducts down axon

  26. Summary • Excitation and inhibition in relation to the reversal potential • Fate of neurotransmitters after release • Types of transmitters and their receptors • Synaptic integration leading to action potentials

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