I needed some figures that displayed a variety of different negative feedback loops so I created these using Tikz. Nothing particularly special. There are some absolute distances in the code which perhaps could be removed to make it more generic.

\documentclass{article} \usepackage{amsmath} \usepackage{tikz} \usetikzlibrary{arrows} \usetikzlibrary{calc} \begin{document} \begin{tikzpicture}[>=latex', node distance=2cm] \node (Xo) {}; \node [right of = Xo] (S1) {\Large $x$}; \node [right of = S1] (S2) {}; \draw [->,ultra thick,blue] (Xo) -- node[above, black] {$v_1$} (S1); \draw [->,ultra thick,blue] (S1) -- node[above, black] {$v_2$} (S2); % Lets draw a line with a blunt end, -| \draw [-|,ultra thick,blue] % start in the middle of S2, and move down 2.75 mm % ($ ... $) notation is used to add the coordinates ($ (S1) + (0mm,-2.75mm) $) % Now draw the line down by 3mm % -- means draw to, + means move by -- +(0,-3mm) % Now move back to the left of S2 % The symbol -| means draw horizontal then vertical. % If we used -- instead the line would be drawn % diagonally to the reaction edge. % (S1) is the center of the node. But we want the % blunt end to end below the S1 line and % half way to the left. The 10mm is half the node % distance of 2cm, and 1mm is slightly below % the reaction line. -| ($ (S1) - (10mm,1mm) $); \end{tikzpicture} \vspace{1cm} \begin{tikzpicture}[>=latex', node distance=2cm] \node (Xo) {}; \node [right of = Xo] (x1) {\Large $x_1$}; \node [right of = x1] (x2) {\Large $x_2$}; \node [right of = x2] (x3) {}; \draw [->,ultra thick,blue] (Xo) -- node[above, black] {$v_1$} (x1); \draw [->,ultra thick,blue] (x1) -- node[above, black] {$v_2$} (x2); \draw [->,ultra thick,blue] (x2) -- node[above, black] {$v_3$} (x3); % Lets draw a line with a blunt end, -|, using the following coords \draw [-|,ultra thick,blue] % start in the middle of x2, and move down 2.75 mm % ($ ... $) notation is used to add the coordinates ($ (x2) + (0mm,-2.75mm) $) % Now draw the line down by an additional 3mm % -- means draw to, + means move by -- +(0,-3mm) % Now move back to the left of x2 % The symbol -| means draw horizontal then vertical. % If we used -- instead the line would be drawn % diagonally to the reaction edge. % (S1) is the center of the node. But we want the % blunt end to end below the S1 line and % half way to the left. The 10mm is half the node % distance of 2cm, and 1mm is slightly below % the reaction line. -| ($ (x1) - (10mm,1mm) $); \end{tikzpicture} \vspace{1cm} \begin{tikzpicture}[>=latex', node distance=2cm] \node (Xo) {}; \node [right of = Xo] (x1) {\Large $x_1$}; \node [right of = x1] (x2) {\Large $x_2$}; \node [right of = x2] (x3) {\Large $x_3$}; \node [right of = x3] (x4) {}; \draw [->,ultra thick,blue] (Xo) -- node[above, black] {$v_1$} (x1); \draw [->,ultra thick,blue] (x1) -- node[above, black] {$v_2$} (x2); \draw [->,ultra thick,blue] (x2) -- node[above, black] {$v_3$} (x3); \draw [->,ultra thick,blue] (x3) -- node[above, black] {$v_4$} (x4); % Lets draw a line with a blunt end, -| \draw [-|,ultra thick,blue] ($ (x3) + (0mm,-2.75mm) $) -- +(0,-3mm) -| ($ (x1) - (10mm,1mm) $); \end{tikzpicture} \vspace{1cm} \begin{tikzpicture}[>=latex', node distance=2cm] \node (Xo) {}; \node [right of = Xo] (x1) {\Large $x_1$}; \node [right of = x1] (x2) {\Large $x_2$}; \node [right of = x2] (x3) {\Large $x_3$}; \node [right of = x3] (x4) {\Large $x_4$}; \node [right of = x4] (x5) {}; \draw [->,ultra thick,blue] (Xo) -- node[above, black] {$v_1$} (x1); \draw [->,ultra thick,blue] (x1) -- node[above, black] {$v_2$} (x2); \draw [->,ultra thick,blue] (x2) -- node[above, black] {$v_3$} (x3); \draw [->,ultra thick,blue] (x3) -- node[above, black] {$v_4$} (x4); \draw [->,ultra thick,blue] (x4) -- node[above, black] {$v_5$} (x5); % Lets draw a line with a blunt end, -| \draw [-|,ultra thick,blue] ($ (x4) + (0mm,-2.75mm) $) -- +(0,-3mm) -| ($ (x1) - (10mm,1mm) $); \end{tikzpicture} \vspace{1cm} \begin{tikzpicture}[>=latex', node distance=2cm] \node (Xo) {}; \node [right of = Xo] (x1) {\Large $x_1$}; \node [right of = x1] (x2) {\Large $x_2$}; \node [right of = x2] (x3) {\Large $x_3$}; \node [right of = x3] (x4) {}; \draw [->,ultra thick,blue] (Xo) -- node[above, black] {$v_1$} (x1); \draw [->,ultra thick,blue] (x1) -- node[above, black] {$v_2$} (x2); \draw [->,ultra thick,blue] (x2) -- node[above, black] {$v_3$} (x3); \draw [->,ultra thick,blue] (x3) -- node[above, black] {$v_4$} (x4); % Lets draw a line with a blunt end, -| \draw [-|,ultra thick,blue] (10mm, -8mm) -- +(0,6mm); \node (x) at (10mm,-11mm) {\Large $x$}; \end{tikzpicture} \end{document}