In Chapter 3 we discussed the input-output relations of synaptic connections, as measured by spike-train analysis. In this chapter we attempt to relate these phenomenological transmission curves to intracellular synaptic mechanisms. There appears to be no uniquely accurate way of translating the membrane potential changes into firing rates, though two extreme cases can be understood fairly well. The first case is that of a neuron whose membrane potential hyperpolarizes strongly after each action potential, and then begins to depolarize gradually until it hits the threshold and fires again. Firing times of such neurons are quasi-periodic. The second case is that of a neuron whose membrane potential fluctuates strongly around a constant mean level and whose excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) are small.

Both types of neurons can be found in the mammalian nervous system. We refer to them as the periodically firing neurons and the randomly firing neurons. Sections 4.1 and 4.2 deal with the analysis of firing times in these two types of neurons. Section 4.3 describes the autocorrelation function and shows how it can be used to distinguish between the two types of neurons. It also shows that cortical neurons behave like randomly firing neurons. Sections 4.4 and 4.5 describe the expected modulations of firing rates generated by a postsynaptic potential in these two types of neurons.