With constant current pulses (lasting several hundred milliseconds), do neurons fire trains of action potentials or just one single action potential? Since voltage-gated Na+ channels can only remove their inactivation block following repolarization, does this constant current pulse prevent Nav channels from opening multiple times during the pulse? Or can Nav channels locally repolarize and get activated again, allowing for trains of action potentials?
It depends on intrinsic properties of the neuron's membrane. Some types of cells fire bursts of action potentials during the whole depolarizing pulse (for example, pyramidal cells of subiculum) while other show a specific property called spike-frequency adaptation (also called accommodation) after one, two or several action potentials (for example, superior cervical ganglion neurons, see Romero et al., 2004 Eur J Neurosci). It is believed that this adaptation is due to the activation of muscarinic activated K-current (M-current) and after-hyperpolarization (AHP) currents.
During an action potential, Na+ channels inactivate after its opening and a different composition of K+ currents activate to repolarize and further hyperpolarize the cell membrane, thus allowing the deinactivation of Na+ channels, making them ready to open again after Vm reaches the AP threshold. For this reason, some cells types can still fire during constant current pulses.
Some examples of AP firing depending on the type of neurons or interneurons are shown in the link below (for an exhaustive list; see also reviews and other references cited inthere). I also attached a picture from a review by Henry Markram et al. (2004) to give you a quick overview of AP firing as Diego Fernandez pointed out. All the best, Fouad L-C.
http://jn.physiology.org/content/jn/98/5/2517.full.pdf
You also have to consider that if you inject a current to elicit repetitive firing for let's say 1 second then this current is usually very small (e.g. sth. between 10 - 100 pA) compared to the ionic currents flowing through ion channels during an AP.
The repolarisation due to the opening of K+ channels is strong enough to hyperpolarize the membrane potential again and by that, Na+ channels can recover from inactivation and a new AP can be fired.
As the others already mentioned this behavior is strongly dependent on the cell type you are looking at and, in fact, each individual neuron of even the same type of neuron. And even if the neuron is capable of repetitive firing you won't see APs if you inject too little current or you will see a significant decline of AP amplitude if you inject too much current.
As it was said, it depends on the neuron type. In any case, try several current pulses with increasing amplitude to see, which is the optimal for your cells.
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