You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: spm_fx_bgc.m
    path of file: ./repos/bspm/thirdparty/spm12/toolbox/dcm_meeg
    
    the code of the file until where you have to start completion: function [f,J,Q] = spm_fx_bgc(x,u,P,M)
% state equations for a neural mass model of the basal ganglia circuit
% models the circuit between striatum, gpe, stn, gpi, and thalamus as a
% single source (no extrinsic connections)
%
% order           cells     states
% 1 = striatum  - ii        x(
    
function
% NB for more pronounced state dependent transfer functions use R  = 3/2;

if isfield(M,'BGC_G'); G = M.BGC_G; end
if isfield(M,'BGC_T'); T = M.BGC_T; end


% [specified] fixed parameters
%--------------------------------------------------------------------------
if isfield(M,'pF')
    try, E = M.pF.E; end
    try, G = M.pF.G; end
    try, T = M.pF.T; end
    try, R = M.pF.R; end
end

% input connections
%--------------------------------------------------------------------------
C    = exp(P.C);

% pre-synaptic inputs: s(V)
%--------------------------------------------------------------------------
R    = R.*exp(P.S);              % gain of activation function
F    = 1./(1 + exp(-R*x + 0));   % firing rate
S    = F - 1/(1 + exp(0));       % deviation from baseline firing (0)

% input
%==========================================================================
if isfield(M,'u')

    % end