\documentclass{standalone}
\usepackage[utf8]{inputenc}
\usepackage{amsmath,amsfonts,amssymb}

\usepackage{tikz}
\usetikzlibrary{shapes,positioning}
\usetikzlibrary{narrow}

\tikzset{block/.style={draw, rectangle, line width=2pt,
     minimum height=3em, minimum width=3em, outer sep=0pt}}
\tikzset{sumcircle/.style={draw, circle, outer sep=0pt, 
     label=center:{{$\sum$}}, minimum width=2em}}
\tikzset{every picture/.style={auto, line width=1pt,
          >=narrow,font=\small}}
          
\begin{document}
\begin{tikzpicture}
\node[block](GP2){$G_{P2}$};
\node[block, left=12mm of GP2](GP1){$G_{P1}$};
\node[block, left=12mm of GP1](GR1){$G_{R1}$};
\node[sumcircle, left=8mm of GR1](sum2){};
\node[block, left=12mm of sum2](GR2){$G_{R2}$};
\node[sumcircle, left=8mm of GR2](sum1){};
\coordinate[left=8mm of sum1](input);
\coordinate[right=12mm of GP2](output);
\draw[->](input)--node[xshift=-2mm](R2){$R_2$}(sum1);
\draw[->](sum1)--(GR2);
\draw[->](GR2)--node[](R1){$R_1$}(sum2);
\draw[->](sum2)--(GR1);
\draw[->](GR1)--node[](U){$U$}(GP1);
\draw[->](GP1)--node[](Y1){$Y_1$}(GP2);
\draw[->](GP2)--node[](Y2){$Y_2$}(output);
\node[block, below=10mm of U](M1){$MA$};
\node[block, below=6mm of M1](M2){$MB$};
\draw[->](Y2)|-(M2);
\draw[->](M2)-|(sum1);
\draw[->](Y1)|-(M1);
\draw[->](M1)-|(sum2);
\end{tikzpicture}
\end{document}