Which statement on the nervous system is incorrect? a. Neurons are the fundamental units of the brain and nervous system. b. Ganglia are clusters of hormonal cell bodies in the peripheral nervous system. c. The myelin sheath that surrounds some axons in the peripheral nervous system is formed by Schwann cells. d. Glial cells are non-neuronal cells in the peripheral nervous system that do not produce electrical impulses. e. Synapses are the sites where electric nerve impulses are transmitted between two nerve cells.