2 Chemical Transmission Transmitting Step: presynaptic cell release NT 2. Receptive Step: NT binds postsynapticallyto chemically gated (ion) channelpresynapticpostsynaptic
3 Resting membrane potential: AT REST The differences in electrical charges betweenthe inside of the cell and the outside cell“Polarized”
4 Why Negative You ASK? IONS Cl- K+ Charged Molecules Na+ A- Na+: Cation (+)K+: Cation (+)Cl-: Anion (-)A-: Anion (-)Ratio of negative to positiveis greater inside than outside
5 Why Unequal Distribution? 4 Factors EqualizersUnequalizers1. Concentration Gradient (passive)Random movement of ionsIons travel “down” their[ ] gradienthigher concentration movesto region of lowerconcentrationStops when ions are =
6 2. Electrostatic Pressure (Passive) Charges Opposites attractLike charges repelHey baby..What’s your sign?Hey baby..What’s your sign?Talk to theMembrane..Na+Cl-K+Na+
7 3. Selectivity of Membrane (Passive) Cell membrane ispermeable to certainIons – controls in & out
12 IONIC BASIS OF THE RESTING MEMBRANE POTENTIAL General overviewResting membrane potential is a characteristic feature of ALL cells in the bodyMembranes of neurons contain channels that allow the movement of ions into or out of the cellThe permeability of the membrane is determined by the state of these channels – i.e. whether they are open or closed
13 Neurons have the ability to “gate” their ion channels (permeability of the membrane to selected ions)3 basic types of ion channels:(i) Passive – each passive channel is identified by the specific ion it allows through it(e.g. K+, Na+ channel) resting membrane potential (ii) Voltage gated – open or close based on the membrane potential action potentials (AP) (iii) Chemically gated – transmitters binding on sites (receptors) on the channels. These channels are important in synaptic transmission
14 How do we get the cell to depolarize? Depolarize the CellMake more positiveHow do we get the cell to depolarize?
15 How do we get the cell to depolarize? EPSP: excitatory post synaptic potentialIncreases the likelihood that postsynaptic cell will fireNT binds to chemical gated channelIon Channel opens (NA+)Flood of Na rushes in…why?Cell becomes depolarize (–70mV)If reaches “threshold” (-65 mV)Action Potential (AP)! Will be triggered!!!!
16 NT binds to chemical gated channel Ion Channel opens (NA+)Flood of Na rushes in…why?Cell becomes depolarize (–70mV)If reaches “threshold” (-65 mV)Action Potential (AP)! Will be triggered!!!!
17 How do we reach threshold? Cell “adds” up signals over space & time Neural Integration:Cell “adds” up signals over space & timeGRADED RESPONSESpatial Summation:Many synaptic inputs adds upto thresholdTemporal Summation: OneInput that fires quickly in timeserving to build on each otherTo reach thresholdAP triggered
18 Integration of Signals Figure 8-25: Locations of synapses on a postsynaptic neuron
20 Postsynaptic and Action Potentials Excitatory Postsynaptic Potentials (EPSP) Graded depolarizationsInhibitory Postsynaptic Potentials (IPSP) Graded hyperpolarizationsRelationship between EPSP’s, IPSP’s and AP????All postsynaptic potentials are added together and if enough EPSP’s occur to cause cell to cross threshold, an action potential occurs
21 Hits Threshold All-or-Nothing! Action Potential: firing of a neuron massive momentary change in the membrane potential from –70mV to ~ +50mV
22 Figure 3.3 Recordings from a postsynaptic neuron during synaptic activation
29 Properties of the action potential AP is triggered by depolarization Depolar. must exceed threshold value to trigger APAP is all-or-noneAP propagates without decrementAP involves reversal ("overshoot") of membrane potentialAP is followed by refractory period
30 Voltage Gated Channels in axon open serving to “Propogate” the AP down the axon