Firing pattern · Lobeworks/17

A neuron's firing pattern is the shape of the spike train it produces under a steady input (steady, slowing, in bursts or a single spike), and two neurons given exactly the same current can answer with completely different trains, because the pattern is set mostly by which ion channels, above all which potassium channe

Firing pattern. A neuron's firing pattern is the shape of the spike train it produces under a steady input (steady, slowing, in bursts or a single spike), and two neurons given exactly the same current can answer with completely different trains, because the pattern is set mostly by which ion channels, above all which potassium channels, each one expresses.

The membrane equation is the same for every neuron, Cm dV/dt=−∑Iion+IinputC_m, dV/dt = -\sum I_{\text{ion}} + I_{\text{input}}Cm​dV/dt=−∑Iion​+Iinput​; what differs is the list of currents and the size of each conductance. McCormick and colleagues sorted cortical cells by their answer to a step of current in 1985, and the names stuck:

Regular spiking cells adapt. Most pyramidal neurons fire a train that slows under a constant input, through the calcium-activated and M-currents of spike-frequency adaptation.

Fast-spiking cells barely slow down. Parvalbumin interneurons carry Kv3 channels (genes KCNC), which open and close very fast and do not inactivate, so each spike is narrow, the membrane is reset almost at once and the cell can hold several hundred hertz without adapting.

Intrinsically bursting cells fire clusters. Some layer 5 pyramidal neurons answer with a burst of spikes close together and then single ones, driven by calcium and sodium currents that outlast the first spike.

The genes behind the potassium channels come in families: KCNA for Kv1, KCNB for Kv2, KCNC for Kv3, KCNQ for Kv7 (the M-current), KCNMA1 for the BK channel. Cell identity sets most of the mix, but two cells of the same type are never clones: they differ in channel numbers, in where the channels sit (soma, dendrites, axon), in phosphorylation and in their history, and neurons adjust their own channels to keep their activity in range, which is homeostatic plasticity of excitability.

The potassium channels a neuron expresses are a large part of its firing personality.

Give four cells the same push and one fires at 300 Hz without tiring, one starts at 100 Hz and slows to 20, one fires once, and one answers in bursts.

Questions: Is spike-frequency adaptation good or bad for a neuron? Neither: it is a computational property, close to a filter in time. A cell that does not adapt holds 200, 200, 200 Hz for as long as the stimulus lasts, which suits a signal that must be sustained and precise. A cell that adapts falls from 200 towards 40 Hz under the same stimulus, so it signals mostly the onset or a change and quiets down while the stimulus stays, much like a high-pass filter. What are the classic firing patterns of cortical neurons called? Regular spiking, fast spiking and intrinsically bursting, the names McCormick and colleagues gave in 1985 to how cortical cells answer a step of current. Regular spiking cells, most pyramidal neurons, fire a train that slows down; fast-spiking cells, the parvalbumin interneurons, fire at hundreds of hertz without slowing; intrinsically bursting cells, some layer 5 pyramidal neurons, answer with a tight cluster of spikes first. Other cells fire only once at the start of the step. Why can a fast-spiking interneuron fire at hundreds of hertz while a pyramidal cell slows down? Because of the potassium channels each one expresses. Parvalbumin interneurons carry Kv3 channels (genes KCNC), which open and close very fast and do not inactivate: the spike is narrow, the membrane is reset at once and the next spike can follow almost immediately, with little adaptation. Most pyramidal cells instead carry slow calcium-activated potassium channels and the M-current, whose growing outward current IadaptI_{\text{adapt}}Iadapt​ makes each new spike harder, so their trains slow down. Are two neurons of the same type identical in how they fire, and which genes set the potassium channels behind it? No. Cell identity sets most of the channel mix through gene families (KCNA for Kv1, KCNB for Kv2, KCNC for Kv3, KCNQ for Kv7, KCNMA1 for the BK channel), but two cells of a type differ in channel numbers, in where the channels sit on soma, dendrites and axon, in phosphorylation, age and history. Neurons also retune their own channels to keep their activity in range, which is homeostatic plasticity of excitability, so firing patterns drift with what a cell has been doing.