NSC 4352 1nd Edition Exam 2 Study Guide Lectures 7 10 Lecture 7 Electrotonic spread of potentials vs active spread of the action potential Electrotonic refers to the passive spread of charge inside a neuron Passive means that voltage dependent changes in membrane conductance do not contribute Electrotonic spread of the potential describes the attraction of opposite and reflection of likecharged ions within the cell Neurons produce two types of electrical potentials 1 a non propagated local potential called an electrotonic potential due to a local change in ionic conductance e g channel opening due to synaptic response When it spreads along the membrane it becomes exponentially smaller 2 a propagated impulse called an action potential Electrotonic potentials from multiple sources i e different synapses can sum spatially ortemporally Because the ionic charge enters in one location and dissipates to others losing intensity as it spreads electrotonic spread is a graded response Electrotonic spread and summation of many inputs is responsible for depolarizing the voltage of the soma sufficiently to threshold and trigger the action potential Electrotonic potentials are conducted faster than action potentials but attenuate rapidly thus they are unsuitable for long distance signaling In contrast the action potential is an all or none propagation down the axon of the neuron during which the current is regenerated Input Resistance A Increases in outward or inward current pulses A1 produce proportional and symmetrical changes in membrane potential Vm A2 Note that the potential changes more slowly than the current steps B An I V curve is obtained by plotting the steady state voltage against the injected current The slope of the I V curve defines the input resistance of the neuron The I V here is linear 10 mV for change in curve shown Vm changes by every 1 nA current Passive Membrane Properties Capacitance capacitance unit farad is the ability of a body to hold an electrical charge a capacitor is a device that stores energy in the electric field created between a pair of conductors on which equal but opposite electric charges have been placed A capacitor consists of two conductors plates separated by an insulator dielectric In a parallel plate capacitor capacitance is directly proportional to the surface area of the conductor plates and inversely proportional to the separation distance between the plates If the charges on the plates are Q and Q and V gives the voltage between the plates then the capacitance is given by C Q V The length constant lambda The length constant is a measure of the ef ficiency of the passive spread of voltage changes along the neuron how far Time constant tau The time constant is defined as the time when the voltage response Vt rises to 1 1 e or 63 of the steady state membrane response V Lecture 8 Properties of single Na channels 1 Currents carried by Na Inward at potentials more negative than ENa and reverse their polarity above ENa The amplitude of current depends on Na concentration 2 Time course of opening closing and inactivation matches macroscopic current C vs D stochastic events averaged many times 3 Opening and closing of channels are voltage dependent E 4 Tetrodotoxin blocks both microscopic and macroscopic Na currents K Channels Similarly properties of single channel K currents also reflect macroscopic currents 1 Outward currents 2 Do not inactivate during brief depolarizations 3 Are voltage dependent depolarization increases probability of opening hyperpolarization closes 4 Are blocked by drugs that affect the macroscopiccurrent K channel blockers Nearly 100 K channel genes are known Form distinct groups that differ in their activation gating and inactivation properties as well as their pharmacological profile Lecture 10 Criteria that define Neurotransmitters 1 The substance must be present within the presynaptic neuron Problems Transmitters like glutamate glycine and aspartate have also other functions in cellular metabolism and or function as precursor for other transmitters e g dopamine 2 The substance must be released in response to presynaptic depolarization and the release must be Ca2 dependent 3 Specific receptors for the substance must exist on the postsynaptic cell Release of neuropeptides requires high frequency stimulation Importance of calcium levels in the presynaptic terminal The Neuromuscular junction NMJ Large postsynaptic cell One axon but about 100 synapses per muscle cell Highly reliable Most pre synaptic action potentials lead to a post synaptic action potential Chemical signaling is simple Only one type of ion channel Key variables that characterizequantal vesicular release the number of release sites N the probability of a quantal release p the size of the quantal response q the distribution of amplitudes of the postsynaptic response can be fitted to a binominal distribution From this the best fitting values of N p and q can be extracted quantal analysis Ca2 affects the probability of release not the quantal size At the NMJ an action potential normally triggers the release of 150 quanta which each generate synaptic potentials of 0 5mV Normal spontaneous level of release is one quanta per second Ca2 influx increases probability of release 100 000fold At central synapses an action potential normally triggers the release of only 1 10 quanta Lecture 10 Synapsin keeps vesicles tethered in the reserve pool cross links vesicles to cytoskeletal filaments f actin regulated by PKA and Ca2 Calmodulin dependent kinase CaMKII Phosphorylation frees vesicles to move Rab Proteins Small GTP binding protein Rab GTPases regulate many steps of membrane traffic including vesicle formation vesicle movement along actin and tubulin networks and membrane fusion more than 60 different Rab proteins are involved in vesicle transportmark transport vesicles interact with v SNAREs to initiate fusion Calcium Sensor Synaptotagmin 1 Has two C2 domains membrane targeting domains Binds phospholipids in a calcium dependent manner binding of Ca2 allows it to interact with syntaxin vSNARE Calcium Sensor Releases clamp on release or facilitates release Involved in docking and vesicle fusion Also aids in recycling binds clathrin