PSL300H1 Lecture Notes - Ion Channel, Depolarization, Acetylcholine

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2 May 2012

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Excitatory post-synaptic potential (epsp) - ion channel may be specific for cations (na+, Inhibitory post-synaptic potential (ipsp) - on channel specific for cl- or k+ ion, hyperpolarizing. 2 ach binds to pocket, cation channel open. Psps are generated in inexcitable membrane of neuronal dendrites and cell bodies. Nearest excitable membrane is initial segment of axon. -> passive conduction of psps to initial segment where they must summate to depolarize membrane to threshold levels (initial 1mv signal too weak) Spines isolate individual synaptic inputs so that they can be independently modified in learning situations. Preferentially generated near the soma - advantageous because excitatory signals must pass through soma to get to initial segment. Inhibition in nervous system is far more important in signal transduction than excitatory. Types of psp summation: spatial summation: minimum of 10-30 synchronous (simultaneous) epsps in dendritic tree, each generated at a different synapse, temporal summation:

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Above is a schematic representation of a neuron with a synapse.Switch A is a synapse. R1 and R3 are the resistances of theextracellular space 2 microns from the plasma membrane. R2 is theextracellular resistance 1 mm from the plasma membrane. R5 is theintracellular resistance. When switch A is closed (synapseactivated) draw: (1) the direction of current flow in the circuit;(2) the polarity of potential changes across resistors R1-R6. (3)What are the relative magnitudes and polarities of theextracellular field potentials produced by the synaptic responserecorded at R1 and R3?

Membrane Capacitance and Conductance. For cells having simplemorphological shapes, the
intracellular voltage is nearly uniform throughout the cell suchthat any change in the membrane
potential acts equally on all parts of the membrane. If wehyperpolarize the cell or chemically block
excitable ionic conductances, the membrane potential vm ispassively related to the incoming current i
by:
Cm
dvm
dt
+Gm (vm - El ) = i (1)
where Cm is the whole-cell membrane capacitance, Gm is thewhole-cell membrane conductance, and
El is the leak potential. Most biological membranes have acapacitance of about 1 ÂµF per square
centimeter of membrane area, which allows one estimate cell size bymeasuring capacitance.
a) One way to measure the capacitance is to inject a sinusoidalcurrent i = Ie j?t and to measure the
sinusoidal perturbation in the membrane potential vm = Ve j?t . Inthe frequency domain, both the
current I and the voltage V are complex-valued numbers. Derive aformula for the cell capacitance in
terms of I and V.
b) Another way to measure cell capacitance and conductance is touse a step current injection. In this
case, the current is suddenly increased to a constant i = Io andthe voltage responds with an exponential
form. Measurement of the voltage for a known current impulseprovides a means to estimate the
capacitance and conductance. Toward this aim, find the solution ofEq. 1 for a step increase in
current. Once you have this solution, how would you use the voltageand current measurements to
estimate the passive capacitance and conductance?

PSL300H1 Lecture Notes - Ion Channel, Depolarization, Acetylcholine

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