Nested function not recognizing variable assigned in larger function

Two functions essentially copy-pasted from my professor, just need to pass the outputs of the main function as inputs of the subfunction. The variables "t" and "V" are outputs of my larger @IandF function, but won't pass into my subfunction @integrate at the end. (Yes, I know global variables are not optimal–I think the scope of my code is small enough for it not to be an issue.) Looking for the fastest answer possible as the assignment is due tonight and I have been working on it for HOURS (new to Matlab so this is a struggle for me)! Code below. Any help is much appreciated.

function varargout =  IandF(timespan,varargin)

% setting neuron parameters (unchanged)
    V_rest = -60; % resting voltage in mV
    V_peak = 20; %peak voltage in mV
    V_reset = -70; % reset voltage in mV
    V_thresh = -50; % threshold voltage in mV
    tref = 10; %refractory period in ms
    R = 100; % membrane resistance in mega-ohms
    C = 0.2; % membrane capacitance in nF
    tau = R*C; %time constant in ms

% defining start and end times based on input timespan vector 
    t_beg = timespan(1);
    t_end = timespan(end); 
    
% setting global variables for specific simulation conditions
    global current % current generating function
    global simCond % either f (sineCurrent) or t_step (stepCurrent)
    global Imax % amplitude of current
    global t_change % t_on or t_off

% assigning global variables to function inputs
    current = varargin{1};
    simCond = varargin{2};
    Imax = varargin{3};
    t_change = varargin{4};
    
if ~exist('timespan') 
error('IandF requires time vector timespan');
end

% performing integrate and fire step 

% setting ODE to search for fire event with maximum step of 0.1ms
% function @andFire defined elsewhere, same as function provided in lecture
    options = odeset('Events',@andFire,'MaxStep',0.1); 

    % initial values for loop
    V = V_rest;
    ts = [t_beg t_end];
    t = t_beg;
    
    while t(end) < t_end
        [t_new, V_new, tsp] = ode15s(@integrate,ts,V(end),options)
        t = [t; t_new];
        V = [V; V_new];
        if t(end) < t_end
            t = [t; tspan + [eps(tspan); eps(tspan)*2; tref]];
            V = [V; V_peak; V_reset; V_reset];
                ts = [t(end) t_end];
        end
    end 
    

   
    % defining subfunction to integrate voltage depending on current
    integrate     
    function dVdt = integrate(t,V)
        V_rest = -60; % resting voltage in mV
        R = 100; % membrane resistance in mega-ohms
        C = 0.2; % membrane capacitance in nF
        tau = R*C; %time constant in ms   
        
    % setting global variables for specific simulation conditions
    global current % current generating function
    global simCond % either f (sineCurrent) or t_step (stepCurrent)
    global Imax % amplitude of current
    global t_change % t_on or t_off
    
    % generating current for each individual time value based on inputs
    I = current(t,simCond,Imax);
    
            if 0 < t < t_change
                IR = I*R;
            else 
                IR = 0;
            end
           
        dVdt = (V_rest+V-IR)/tau;
    end

% assigning outputs to time vector and calculated voltage
varargout{1} = t;
varargout{2} = V;
end

        

[Walter Roberson]

if 0 < t < t_change

That needs to be

if 0 < t && t < t_change

Answer 1

[Walter Roberson]

sub-functions need to be of the general form

function outer_function
   initialize any shared variable, MUST be before inner function is defined
   main code that calls inner_function is usually here but not always
   function inner_function
        code that uses shared variables is here
   end
   main code that calls inner_function could go here
end

If you are going to use sub-functions, then do not use global! Use shared-variables instead.