Conceived and designed the experiments: JP RB. Performed the experiments: JP. Analyzed the data: JP RB. Wrote the paper: JP RB. Abstract In central neurons, the threshold for spike initiation can depend on the stimulus and varies between cells and between recording sites in a given cell, but it is unclear what mechanisms underlie this variability. Author Summary Neurons communicate primarily with stereotypical electrical impulses, action potentials, which are fired when a threshold level of excitation is reached.
Introduction Spike initiation in neurons follows the all-or-none principle: a stereotypical action potential is produced and propagated when the neuron is sufficiently excited, while no spike is initiated below that threshold.
Results What is the spike threshold? Spike threshold in vitro. Download: PPT. Spike threshold in vivo. Spike threshold in models. The threshold equation Sodium channel activation.
Figure 3. Influence of Na activation characteristics on spike threshold. Sodium channel inactivation and other conductances. Figure 4. Threshold dynamics To derive the threshold equation, we made a quasi-static approximation, assuming that all mechanisms that modulate the threshold are slow processes compared to the timescale of spike initiation.
Sodium inactivation. Voltage-dependent conductances. Synaptic conductances. Spike initiation site Effect of neuronal morphology. Na channel density in the AIS. Accuracy of the threshold equation Threshold dynamics in a single-compartment model.
Figure 7. Threshold variability and Na channel inactivation in a single-compartment model. Threshold prediction in a realistic multicompartmental model of spike initiation. Figure 8. Accuracy of the threshold equation in a multicompartmental model of spike initiation [ 54 ]. Discussion The spike threshold differs between cells and for different types of stimulations [2] , [15] , [32] , [33] , [35].
Mechanisms for threshold modulation and variability Since Na channels are responsible for the generation of action potentials, the threshold is firstly determined by their activation characteristics. Approximations in the threshold equation To derive the threshold equation, we made several simplifying assumptions. Figure 9. Fitting the Na activation curve to a Boltzmann function. Figure Experimental difficulties in the measurement of Na activation curves. Sharpness of spikes and threshold variability Spikes look sharper in the soma than in the AIS, presumably because they are initiated in the AIS and back-propagated to the soma [10] , [13] , [14].
Materials and Methods Membrane equation We consider a single-compartment neuron model with voltage-gated sodium channels and other ion channels voltage-gated or synaptic , driven by a current I. Dynamic threshold The effect of Na inactivation on the threshold can be seen in the exponential model above, neglecting other conductances thus.
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Neurophysiology 71 1 — Download references. You can also search for this author in PubMed Google Scholar. Reprints and Permissions. Koch, C. Do neurons have a voltage or a current threshold for action potential initiation?.
J Comput Neurosci 2, 63—82 Download citation. Received : 01 September Revised : 29 November Accepted : 12 December Issue Date : March Anyone you share the following link with will be able to read this content:.
Sorry, a shareable link is not currently available for this article. Gov't, Non-P. Research Support, U. Gov't, P. Much of what we know about how neurons work comes from experiments on the giant axon of the squid. This giant axon extends from the head to the tail of the squid and is used to move the squid's tail. How giant is this axon? It can be up to 1 mm in diameter - easy to see with the naked eye.
Neurons send messages electrochemically. This means that chemicals cause an electrical signal. Chemicals in the body are "electrically-charged" -- when they have an electrical charge, they are called ions. There are also some negatively charged protein molecules. It is also important to remember that nerve cells are surrounded by a membrane that allows some ions to pass through and blocks the passage of other ions.
This type of membrane is called semi-permeable. When a neuron is not sending a signal, it is "at rest. Although the concentrations of the different ions attempt to balance out on both sides of the membrane, they cannot because the cell membrane allows only some ions to pass through channels ion channels.
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