All the display equations/centered equations are no need to convert to MathML or PNG and need to retain in LaTeX coding only.
If it is possible convert to HTML tag in-between text part only convert to HTML tag.
But no need to convert to same positions of Superscript and Subscript, Same positions of Limits Superscript and Subscript and so on in the text part.
\documentclass{book}
\usepackage{amssymb,amsmath}
\begin{document}
Schematic representation of the model, including the following populations: $e$ vaue is $e=2$ and `%e and e=2 convert to html tag`
thalamic reticular nucleus; $s^2_3$ `%s^2_3 should be in latex as its tag`
primary and secondary thalamic relay nuclei themselves with gains $\pi + 2\pi n (n=0,1,\ldots)$ `%\pi + 2\pi n (n=0,1,\ldots) convert to html tag`
and $G_{ii}$ `%G_{ii} convert to html tag. Schematic to and $G_{ii}$ is a text part position`
\end{document}