  The analytical theory of our earlier study (Mortensen et al. (2021),
Mathematical Medicine and Biology, 38(1), pp. 106-131) is extended to address
the outstanding cases of fibroblast barrier distribution and myocyte strait
distribution. In particular, closed-form approximations to the resting membrane
potential and to the critical parameter values for propagation are derived for
these two non-uniform fibroblast distributions and are in good agreement with
numerical estimates.
