Which of the following must be true about the structures of axons and muscle cells in order for nerve impulses to stimulate muscle contraction? . a- axons and muscle cells must both have structures that make and secrete the same small molecules.. b- axons and muscle cells must both have receptors that recognize the same small molecules.. c- axons must have structures that make and secrete molecules, and muscle cells must have receptors that recognize these molecules.. d-muscle cells must have structures that make and secrete molecules, and axons must have receptors that recognize these molecule