Interspike interval
One of the most important ways by which neurons in the brain carry information is by means of electrical signals. At rest, neurons have a ‘resting membrane potential’ of typically about -70 mV: this is the potential difference between the inside of the cell and the external environment. However, this membrane potential is continually being disturbed by the chemical signals (neurotransmitters) released by other neurons. In the brain a neuron might be receiving inputs from as many as ten thousand other neurons, these inputs usually consist of brief excitations (EPSPs; excitatory post-synaptic potentials) or of brief inhibitions (IPSPs; inhibitory post-synaptic potentials) that either make the membrane less negative (depolarisations) or more negative (hyperpolarisations). If a neuron becomes sufficiently depolarised, perhaps because of a flurry of EPSPs, then the membrane potential may reach the critical threshold for triggering an action potential (commonly called a ‘’spike’’). An action potential is a very large and rapid rise in the cell membrane potential, that lasts for only about one millisecond before the membrane potential returns again to around its resting potential. These spikes are propagated along the axons of a neuron, to reach the nerve terminals, where they can trigger the release of chemical messengers to affect other neurons.
Thus spikes are a very important way by which neurons in the brain carry information. The spike itself is an all-or-none phenomenon, so information is coded not in the amplitude of a spike but in the timing of spikes. Accordingly, electrophysiologists, who study the electrical behaviour of neurons, are interested in the patterning of spikes in particular neurons.
The patterning of spike activity is influenced by three things: a) the intrinsic properties of the neuron, especially the properties of its membrane. The neurons in the brain are very diverse; there are very many different subpopulations of neurons that have quite different intrinsic properties b) network interactions, because spike activity in one neuron might have feedback effects on that neuron because of the changes that it produces in reciprocally connected neurons and c) the nature of the inputs to that neuron.