The root NeuroML element. Description of neuronal models, including biophysics and channel mechanisms (NeuroML Level 2). Collection of all Level 2 cells. A single cell specified in MorphML extended to include channel density info. Unit of all length measurements. Usually has the value <b>micrometer</b>. Note: length_units will be the preferred form in v2.0 Unit of all length measurements. Usually has the value <b>micrometer</b>. Note: length_units will be the preferred form in v2.0 Unit of all volume measurements. Cell with extensions for biophysics.