Boundary conditions

Dear COMSOL Community,

I am quite new in COMSOL world and I would like to ask some help for my problem.

I am trying to simulate neuron behaviour.
I started with a really simple model to see if everything works, so my model has 2 sphere domains:
- internal sphere is a neuron;
- external sphere is the extracellular space.
I implemented the Hodgkin-Huxley equations to simulate the active properties of the neuron.

At this moment to make firing the neuron I'm giving a distributed current pulse to all the neuron's boundaries (A/m^2). However, I would like to set the current pulse only in a portion of my sphere, so only in 1 boundary, and give the current pulse only in that boundary. Yet, this condition doesn't work, because comsol says that I might have reached a singularity.
I also tried to add a current point source inside my neuron, but the simulation took too long time and I stopped because I thought I did something wrong.

Is there someone that can help me or has some suggestion to solve/bypass this problem?

I hope I made clear.
Thank you!