ALTERATIONS IN SYNAPTIC STRENGTH PRECEDING AXON WITHDRAWAL

Permanent removal of axonal input to postsynaptic cells helps shape th e pattern of neuronal connections in response to experience, but the p rocess is poorly understood. Intracellular recording from newborn and adult mouse muscle fibers temporarily innervated by two axons showed a n increasing disparity in the synaptic strengths of the two inputs bef ore one was eliminated. The connection that survived gained strength b y increasing the amount of neurotransmitter released (quantal content) , whereas the input that was subsequently removed became progressively weaker, because of a reduction in quantal content and a reduction in quantal efficacy associated with reduced postsynaptic receptor density . Once the synaptic strengths of two inputs began to diverge, complete axonal withdrawal of the weaker input occurred within 1 to 2 days, Th ese experiments provide a link between experience-driven changes in sy naptic strength and long-term changes in connectivity in the mammalian nervous system.