I was wondering what is the minimum value that can be assigned to a section in NEURON without inducing computational problems?

I want to model a kind of injury in neurons. I have a model of an axon with three (types) sections in internodal regions like this:

**Type 1**: A section with user-defined properties of a

*normal*internodal region (Length = A)

**Type 2**: A section with user-defined properties of an

*injured*internodal region (Length = B)

**Type 3**: A section with user-defined properties of a

*normal*internodal region (Length = C)

I assume when my axon is intact, B value should be equal to 0. When Injury level increases, I increase B value and reduce A and C values each by B/2. Therefore, the total length of my axon remains constant. However, since it is not possible to to assign L=0 to a section, I assigned 0.000000001 to the injured region.

To test my model and make sure that I am not inducing any computational error because of the very small section, I designed a simple experiment as follow:

I assigned the same properties to all internodal sections. Basically other than section's length,

**Type 1**,

**Type 2**and

**Type 3**are exactly the same components.

However, I see slightly different results (Conduction velocity) when I run my simulations for these two cases:

**Case 1:**

1- B= 0.000000001

2- Total length between the nodes which are used for conduction velocity measurement = X + 0.000000001

**Case 2:**

1- B= 0.5

2- Total length between the nodes which are used for conduction velocity measurement = X - 0.000000001;

In both cases, when I measured the distance between two nodes that are used for conduction velocity measurement using

*distance(x)*, I got the same results which was equal to X. In theory, these two axonal models, are exactly the same and they should generate the same results. However, conduction velocity varies by about 5% in my simulations.

Interestingly, if I set B=0.5 and then increase B value (B=1.5 or etc.), I do not see any difference in my results. It seems that there is a problem with very small values of B.

Thank you in advance for your help!