Question

As I understand it, if a subthreshold current of unlimited duration is injected in a neuron, a passive response is observed, like an RC circuit. The membrane potential is depolarized by some arbitrary amount, before gradually stabilizing due to more and more current flowing through the resistive branch of the circuit.

So yesterday, I was looking at some HH simulators when I noticed that increasing the duration of a normally subthreshold current pulse would allow it to depolarize the membrane to threshold. How is that? Can someone please point out the error in my logic here?

Also to clarify: I am not familiar with the dynamics of the HH model, so please take it as such.

Answer 1

Graded potentials in neurons can lead to depolarization if they add up and reach a positive threshold. Increasing the duration of a normally subthreshold current pulse can allow more depolarizing graded potentials to occur, ultimately reaching the threshold for an action potential. In the HH model, depolarization occurs when positive ions enter the neuron through ion channels in response to a stimulus.

Step-by-step explanation:

Graded potentials occur in the dendrites of a neuron. These potentials can be either depolarizing or hyperpolarizing, depending on the size of the stimulus. If the changes in voltage add up and reach a positive threshold, the neuron will depolarize and reach the action potential.

The Hodgkin-Huxley (HH) model is a mathematical model that describes the generation of action potentials in neurons. By increasing the duration of a normally subthreshold current pulse, more depolarizing graded potentials can occur, which can eventually reach the threshold for an action potential.

In the HH model, the depolarization occurs when positive ions, such as sodium (Na+), enter the neuron through ion channels. These ion channels open in response to a stimulus, and if the depolarization reaches the threshold potential, an action potential is generated.