Epsps And Ipsps

As has been seen, when an excitatory chemical synapse is stimulated by a pre-synaptic spike, it releases the NT contents of a number of vesicles into the cleft. The NT molecules diffuse rapidly to receptor sites on large protein molecules embedded in the subsynaptic membrane, where they combine and initiate either directly or by a second messenger cascade reaction the opening (or closing) of specific ionic channels in the subsynaptic membrane. The channels remain open for a short time then close again as enzymes in the cleft rapidly destroy the free and bound NT molecules.

In the simplest case, the NT is ACh. Two ACh molecules bind with a nicotinic receptor embedded in the SSM, causing a center channel in the receptor protein complex (containing five protein subunits arranged around a common center) to open. The open channel permits Na+ to flow inward and K+ to flow outward at the same time. The net result is that the transmembrane potential of the SSM depolarizes (i.e., goes positive) as shown in Figure 1.3-4, creating an excitatory postsynaptic potential (epsp). Epsps are characterized by the delay between arrival of the presynaptic spike and the initiation of the epsp, the time to the peak, the peak height in millivolts above the resting potential, and the decay time constant. Since many excitatory synapses are made on the thin branching dendrites of the postsynaptic neuron, it is generally not possible to record a single epsp at the synapse because the dendrite is too thin to accommodate a glass micropipette electrode. Recording of the epsp is generally done from the soma of the PSN, or thick, basal portions of dendrites. See the diagram of epsp recording procedure in Figure 1.3-5. Note that as the presynaptic stimulus intensity increases, it recruits more and more axons to fire, increasing the height of the soma epsp as more and more synapses become active. At some high stimulus voltage, all presynaptic axons fire, a maximum epsp height is reached. There is a slight extra delay and attenuation of the epsp as it propagates from the site of the synapse to the recording site. Table 1.3-2, adapted from Eccles (1964), lists comparative figures for epsps. The peak voltage for a mammalian motoneuron epsp is about 5 to 7 mV above the normal resting potential for the postsynaptic neuron.

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