Electrical impulses, generated by the nervous system, cannot reach every cell in the animal body due to the absence of direct innervation. While neurons can transmit signals to muscle cells, glands, and certain sensory cells, not all cells have direct connections to nerves. Additionally, many cells lack excitable membranes and ion channels necessary for responding to electrical signals. Moreover, cells in deeper tissues or isolated regions may not have direct neural access. Instead, signaling molecules like hormones or local chemical mediators are often used for communication with cells that cannot be reached by direct electrical impulses.