Endocrine function of the thymus affecting neuromuscular transmission.

Neuromuscular transmission was studied with micro-electrodes in thymus-deficient Lewis rats thymectomized within 3 days of birth and in Lewis rats bearing increased thymic tissue in the form of multiple isogeneic thymic grafts. The amplitude of miniature end-plate potentials (MEPPS) was increased in thymus-deficient animals and decreased in animals with increased thymic tissue. These data suggest that a substance we have previously termed thymin is being secreted by the normal thymus in physiologically active amounts which inhibit transmission at the neuromyal synapse. The proposed substance thus appears to act like a hormone. It is suggested that in myasthenia gravis autoimmune thymitis causes the release of toxic amounts of thymin which produce the characteristic myasthenic neuromuscular block.