The pathophysiology of myelin basic protein-induced acute experimental allergic encephalomyelitis in the Lewis rat

Histological and electrophysiological studies were performed on Lewis rats with acute experimental allergic encephalomyelitis (EAE) induced by inoculation with guinea-pig myelin basic protein (MBP) and Freund's adjuvant. The histological studies showed demyelination in the lumbar, sacral and coccygeal dorsal and ventral spinal roots and to a lesser extent in the spinal cord, including the dorsal root entry and ventral root exit zones. The electrophysiological studies demonstrated reduced conduction velocities between the lumbar ventral roots and sciatic nerve. Conduction block was demonstrated at the ventral root exit zone of the lumbar spinal cord but was less severe than in rats with whole spinal cord-induced acute EAE. Recordings of the M wave and H reflex elicited in a hindfoot muscle by sciatic nerve stimulation showed a normal M wave, indicating normal peripheral nerve motor conduction, but a markedly reduced H reflex. The reduction in the H reflex is accounted for by demyelination-induced nerve conduction block in the dorsal and ventral spinal roots, intramedullary ventral roots and at the dorsal root entry and ventral root exit zones of the spinal cord. Demyelination and nerve conduction abnormalities were well established in the relevant lumbar segments on the day of onset of hindlimb weakness. It is concluded that demyelination in the lumbar ventral roots and to a lesser extent in the lumbar spinal cord, including the ventral root exit zone, is an important cause of hindlimb weakness in myelin basic protein-induced acute EAE in the Lewis rat.