Action potential patterns

In this video, I want to talk about action potential patterns. The information from inputs to a neuron is converted to the size, duration, and direction of graded membrane potentials in the dendrites and the soma, so that a small excitatory input to a dendrite, say, usually causes a small excitatory graded potential, also called a depolarization. And a larger excitatory input usually causes a larger excitatory potential. And the same goes for inhibitory inputs. A small inhibitory input usually causes a small hyperpolarization or inhibitory potential. And a larger inhibitory input usually causes a larger hyperpolarization or inhibitory potential. Neurons process that information by summation of the graded potentials at the trigger zone to determine if an action potential will be fired down the axon. Action potentials, however, are consistently the same size and duration for any given neuron, so that the information contained in the graded potentials is, instead, converted into a temporal pattern or a timing of action potentials being fired down the axon. So here I've drawn some lines to just represent time. And we'll look at the temporal patterns or the timing of action potentials that can happen to transmit different kinds of information down the axons of different types of neurons. Some neurons fire no action potentials until there is sufficient excitatory inputs. And then the size and duration of depolarization over threshold is converted into the frequency and duration of a series, which is also called a train of action potentials. So let's say this is one of these neurons that doesn't fire any action potentials at rest. But then if it gets sufficient excitatory input to depolarize the trigger zone over threshold right here, then we see a little train of action potentials. And I'll just write out one little line here that's often called a spike to represent one action potential. And then this neuron will fire a little train, a little series of action potentials for as long as that depolarization is over the threshold potential. And then when the depolarization ends or when it dips below the threshold at the trigger zone, the train of action potential stops, and then the neuron is quiet again. It's not firing any action potentials. So this is a very common method used by lots of neurons in the nervous system. For example, the motor neurons that synapse on skeletal muscle, they tend to fire very few or no action potentials until they're excited enough. And then they'll fire a train of action potentials, and then they're quiet again. Other neurons, however, actually fire action potentials at a regular rate in the absence of any input. And the reason they do this is that they have differences in their leak channels and/or their voltage-gated channels that actually spontaneously depolarize the membrane to threshold at a regular interval, which is very similar to how the pacemaker cells in the heart function. And with these types of neurons, excitatory input will cause them to fire action potentials more frequently during the period of time that they're excited. And then when that excitation goes away, they go back to their regular rate of firing. And inhibitory input will have the opposite effect. That will slow down their firing during the period of inhibition. And then when that goes away, they go back to their regular rate of firing again. And there are even more complicated neurons that, in the absence of input, fire little bursts of action potentials, followed by a little space. And then they have another regular little burst of action potentials. With these types of neurons, excitatory input can cause the little bursts to happen more frequently. It can cause changes within the burst, and it can cause changes to the spacing between the bursts. But then when the input goes away, they go back to their regular bursts. And the opposite happens with inhibitory input. That can slow down the frequency of these bursts. The advantage of these sorts of systems, where the neurons fire at regular rates spontaneously or in bursts, is that information passed along to the target cells can be fine-tuned in either direction, because with a neuron like this that's quiet at rest, the information can only go in one direction. It can only go from no action potentials being fired to trains of action potentials of different frequencies and durations. But if there's more inhibitory input to these types of neurons, that information can't be passed along. But with these types of neurons, information from both excitatory and inhibitory inputs can be passed along in a more fine-grained fashion. The different temporal patterns of action potentials are then converted to the amounts and temporal patterns of neurotransmitter release at the synapse. And target cells can be set up a lot of different ways to respond to these temporal patterns and amounts of neurotransmitter release.