poli/o is a combining form that refers to “gray matter”.
Gray matter is a brain tissue which has a pinkish-gray color, is host to neural cell bodies, axon terminals, dendrites, and nerve synapses. A large amount of this brain tissue can be found in the cerebellum, cerebrum, and brain stem. Also, it is the part of the central spinal cord that is shaped like a butterfly. It is the gray matter that actually processes information while the white matter provides the communication between different gray matter areas.
