Spectrins form a submembranous cytoskeleton proposed to confer strength and flexibility to neurons and to participate in ion channel clustering at axon initial segments (AIS) and nodes of Ranvier. Neuronal spectrin cytoskeletons consist of diverse subunits and II spectrin. Although II spectrin is found in neurons in both axonal and somatodendritic domains, using proteomics, biochemistry, and superresolution microscopy, we show that II and IV spectrin interact and form a periodic AIS cytoskeleton. To determine the role of spectrins in the nervous system, we generated Sptan1 f/f mice for deletion of CNS II spectrin. We analyzed II spectrin-deficient mice of both sexes and found that loss of II spectrin causes profound reductions in all spectrins. II spectrin-deficient mice die before 1 month of age and have disrupted AIS and many other neurological impairments including seizures, disrupted cortical lamination, and widespread neurodegeneration. These results demonstrate the importance of the spectrin cytoskeleton both at the AIS and throughout the nervous system.