With your code:
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix,chains,positioning,decorations.pathreplacing,arrows}
\begin{document}
\begin{figure}[htp]
\centering
\begin{tikzpicture}[
plain/.style={
draw=none,
fill=none,
},
net/.style={
matrix of nodes,
nodes={
draw,
circle,
inner sep=8.5pt
},
nodes in empty cells,
column sep=0.6cm,
row sep=-11pt
},
>=latex
]
\matrix[net] (mat)
{
|[plain]| \parbox{1cm}{\centering Input\\layer} & |[plain]| \parbox{1cm}{\centering Hidden\\layer} & |[plain]| \parbox{1cm}{\centering Output\\layer} \\
|[plain]| & \\
& |[plain]| \\
|[plain]| & & \\
& |[plain]| \\
|[plain]| & & \\
& |[plain]| \\
|[plain]| & \\
};
\foreach \ai [count=\mi ]in {3,5,7}
\draw[<-] (mat-\ai-1) -- node[above] {Input \mi} +(-2cm,0);
\foreach \ai in {3,5,7}
{\foreach \aii in {2,4,...,8}
\draw[->] (mat-\ai-1) -- (mat-\aii-2);
}
\foreach \ai in {2,4,...,8}
\draw[->] (mat-\ai-2) -- (mat-6-3);
\foreach \ai in {2,4,...,8}
\draw[->] (mat-\ai-2) -- (mat-4-3);
\foreach \ai [count=\mi ]in {4,6}
\draw[->] (mat-\ai-3) -- node[above] {Output \mi} +(2cm,0);
\foreach \ai in {3,5,7}
{\foreach \aii in {2,4,...,8}
\draw[->] (mat-\ai-1) -- (mat-\aii-2);
}
\end{tikzpicture}
\end{figure}
\end{document}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Or:
See: http://www.texample.net/tikz/examples/neural-network/ (Kjell Magne Fauske)
Modification:
\documentclass{article}
\usepackage{tikz}
\usepackage{verbatim}
% Basis: http://www.texample.net/tikz/examples/neural-network/
\begin{document}
\pagestyle{empty}
\def\layersep{2.5cm}
\begin{tikzpicture}[shorten >=1pt,->,draw=black!100, node distance=\layersep]
\tikzstyle{every pin edge}=[<-,shorten <=1pt]
\tikzstyle{neuron}=[circle,fill=black!25,minimum size=17pt,inner sep=0pt]
\tikzstyle{input neuron}=[neuron, fill=white!100,draw=black];
\tikzstyle{output neuron}=[neuron, fill=white!100,draw=black];
\tikzstyle{hidden neuron}=[neuron, fill=white!100,draw=black];
\tikzstyle{annot} = [text width=4em, text centered]
% Draw the input layer nodes
\foreach \name / \y in {1,...,3}
% This is the same as writing \foreach \name / \y in {1/1,2/2,3/3,4/4}
\node[input neuron, pin=left:Input \y] (I-\name) at (0,-\y) {};
% Draw the hidden layer nodes
\foreach \name / \y in {1,...,4}
\path[yshift=0.5cm]
node[hidden neuron] (H-\name) at (\layersep,-\y cm) {};
% Draw the output layer node
\node[output neuron,pin={[pin edge={->}]right:Output}, right of=H-2] (O1) {};
\node[output neuron,pin={[pin edge={->}]right:Output}, right of=H-3] (O2) {};
% Connect every node in the input layer with every node in the
% hidden layer.
\foreach \source in {1,...,3}
\foreach \dest in {1,...,4}
\path (I-\source) edge (H-\dest);
% Connect every node in the hidden layer with the output layer
\foreach \source in {1,...,4}
\path (H-\source) edge (O1);
\foreach \source in {1,...,4}
\path (H-\source) edge (O2);
% Annotate the layers
\node[annot,above of=H-1, node distance=1cm] (hl) {Hidden layer};
\node[annot,left of=hl] {Input layer};
\node[annot,right of=hl] {Output layer};
\end{tikzpicture}
% End of code
\end{document}
aiandaiiare loop variables.