Regeneration and Degeneration (1897–1904)

 “If he is so determined,” Cajal said, “man can become the sculptor of his own brain.”

Illustration of Calyx of Held drawing by Dawn Hunter, 11" x 14," marker and pen on paper.

Regeneration and Degeneration (1897–1904), Santiago Ramón y Cajal demonstrated that after nerve injury, the severed distal portion of the axon degenerates, while regeneration (when it occurs) depends on active growth from the surviving proximal axon rather than passive reconnection. He showed that regenerating fibers extend exploratory, amoeboid terminals guided by both proliferating Schwann cells and attraction from the target tissue. Cajal concluded that successful regeneration occurs primarily in the peripheral nervous system, where Schwann cells are present, and not in the central nervous system. Crucially, he argued that the failure of regeneration in the CNS reflects an inhospitable environment rather than an intrinsic inability of neurons to regrow.