In the code snippet you have provided, the pixel values are sampled along only one line segment in image I. The endpoints of this line segment are I(0, size(I,1)/2) and I(size(I,2), size(I,1)/2). Hence, the output variable M is giving a single vector of pixel values along the segment.
To get the profile along multiple horizontal line segments in the image, you will have to generate the profile for all the line segments in a loop and combine their output in a matrix. For example, to get a profile along 3 horizontal lines at the top, middle and bottom, the code would be as follows.
I = imread('Test-0.jpg');
for yi = [1 size(I,1)/2 size(I,1)]