BISC 421 1st Edition Lecture 8 Outline of Current LectureI. Synaptic transmissionCurrent LectureSynaptic Transmission •More complex using neurotransmitters as a chemical •Released from a presynaptic cell and have an efect on the post synaptic cell •Voltage gated Ca2+ channels •Synaptic vesicles are balls of lipids with NT inside and release NT, which difuses across and binds to receptor sequence of events involved in transmission at a typical chemical synapse1. Transmitter is stored in synaptic vesicles.2. An AP enters the presynaptic nerve terminal.3. Depolarization opens voltage gated Ca2+ channels.4. The influx floods theterminal with Ca2+.5. The Ca2+ causes the synaptic vesicles to fuse with the membrane.6. Release the transmitters.know how calcium causes this to happen)7. Transmitter diffuses across the synaptic clef to the postsynaptic cellmembrane.8. Transmitters binds to their specific receptors.9. Postsynaptic ligand (transmitter) gated ion channels open or close.10.The vesicular membrane is retrieved from the plasma membrane andrecycled.A lot of energy required and process needed to store vesicles•Action potential reaches nerve table and open voltage gated channel•Influx of Ca2+ floods the cell•Causes synaptic vesicles to fuse with membrane (confusing issue we do not need to know how calcium causes this to happen)•Bind to specific receptors known as ligand gated channel which sense the concentration change of specific chemical•Induces a change in the post synaptic current-‐ get current flow across the membrane•Can be excitatory or inhibitory•Vesticular membrane retrieved and taken back into membrane and recycled•10 sequences of events involved in synaptic transmission across the chemical synapse•name chemical based on fact that release a chemical substance (NT)Looking at transmission from nerves to muscles•Heart has rhythmic contraction and will beat on its own-‐ neural activity speeds up and slows down contraction (comes from vagus nerve)•Over time contract at regular beat•Get slowing down of contractile rate and amplitude of contraction when stimulated the vagus nerve•Innervated heart connects to chamber with a heart not innervated•Stimulated first heart slowly but surely get reaction in the second heart•Something must be released into solution that builds up and difuses to afect other heartCriteria that define a Neurotransmitter #1Three criteria must be met for a substance to be a neurotransmitte 1. It must be present within the presynaptic neuron.Also the enzymes and precursors that facilitate synthesis must also be present. However, some are needed for protein synthesis. Glutamate, glycine, & aspartate.•3 specific criteria for NTCriteria that define neurotransmitter #3• #3: how many drugs work•many times derivatives of original chemicalSynaptic transmission at the neuromuscular junctionMost of what is known of transmitter releasehas come from study of the neuromuscularjunction.When the neuron is stimulated, a change in the muscle membrane potential occurs.End plate potental (EPP). Is usually large enough to evoke an AP.•How do we know this happens?•Much of study has come from the neuromuscular junction (muscle always post synaptic cell always recording from this)•Region where axon is innervating was given the name “end plate”•See change in membrane potential (EPP or end plate potential)Spontaneous changes in the muscle membrane potential also occur. Same shape as EPPs. Are muchsmaller. <1 mV vs. 40-50 mV. Can be blocked by drugs that block the postsynaptic receptors.•Started to see a lot of small spontaneous activity•Muscle depolarizing ever so briefly by itself•So small not enough to cause an action potential (>1mV)•Found adding antagonists for post synaptic receptors go block these•Call MEPP miniature end plate potentials•When action stimulated not enough to reach threshold look the same so perhaps it iscoming from the nerveSynaptic transmission at the neuromuscular junctionWhat are the MEPPs? Each individual MEPP depolarized the membrane ~0.4 mV. The amplitudes of the subthreshold EPPs are integer multiples of the MEPPs mean 0.4, 0.8, 1.2, 1.6, etc. Each MEPP represents the fusion of one vesicle releasing its neurotransmitter.•Idea is that these are actually release of NT (spontaneous release)•How much was it?•MEPP were all about the same size (.4 in amplitude, .8, 1.2, 1.6) were integers•Perhaps each MEPP is the fusion of one vesicle, it’s a quanta of NT•Explains why all were multiples of .4Quantal release of neurotransmittersFreeze fracture analysis of un-stimulated vs. stimulated presynaptic terminals. Dimples are seen in the stimulated sample. If the # of fusions are compared to the number of quanta released, a linear correlation is observed.•Membrane is a lipid bilayer•Can freeze cell and fracture them so that layer can be pulled apart•Bumps are the proteins within the membranes•Proteins localized in a sort of channel are calcium channels (localized-‐this is where the axon ends )•Stimulated found indentations (these are the vesicles fusing with the plasma membrane)•Stimulate tissue, measure how many quanta we get released then take tissue and count how many we see•Correlated when counted and got linear relationship•For every quanta released they could count a vesicle•Same amount is going to be within each vesicles (can only put so much NT that can go in because all about the same size)Role of Ca2+ in neurotransmitter releaseFirst evidence for a role of Ca2+ in release came from MEPP/EPP experiments. Later, inhibition of voltage- gated Na+-channels did not totally prevent APs. Current flowed through Ca2+-channels.- Taking away calcium can afect theseRole of Ca2+ in neurotransmitter release Voltage-clamp experiments showed the presence of voltage gated Ca2+-channels. Block Na+ and K+ currents. Residual current in Ca2+. When the Ca2+-channel blocker cadmium is added, the presynaptic currents and the postsynaptic depolarization is blocked.•Necessity and efficiency of calcium in this role•Voltage clamp to control membrane potential and recording post synaptic cell•Can see a calcium current coming into presynaptic membrane when depolarized(voltage gated calcium current)•Now record membrane potential of post synaptic cell and get depolarization just with calcium•Calcium block still get passive currents but no