McNair/Gateway Scholars Program USC

Danielle Mascarenas

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Major and Classification

Biological Sciences

Faculty Mentor

  • Albert Herrera, Ph.D.

Department

  • Biological Sciences

McNair Project

Use It or Lose It: Synapse Elimination and the Plasticity of the Nervous System

By studying fundamental mechanisms of the nervous system, one can better understand experience-dependent learning, paralysis, and recovery from disease and injury due to the long-term alterations of activity on synaptic efficacy. Each muscle fiber is innervated by several axons that are collaterals of different motoneurons. During development, the functional neuromuscular junctions (NMJs) that form immediately and abundantly result in innervation by axons from several motoneurons and in an overlap of motor units known as polyneuronal innervation. However, mononeuronal innervation eventually results with the separation of motor units and the elimination of the excess synaptic connections, known as synapse elimination. Synapse elimination is an activity-dependent process, in which competition among synaptic connections results in the strengthening of one set of axonal inputs and the withdrawal of the rest. The timing of the reduction of the overabundant synaptic connections is not clear, however. Therefore, the aim of this research was to determine the time course of synapse elimination in the pectoral muscle of the frog Xenopus laevis. Using light microscopy and neurophysiology, the change in concentration of acetylcholine receptors (AChRs) and motor axons were visualized with fluorescent dye, in which synapse elimination appeared to occur between stages 58 and 66. Additionally, special staining techniques were improved to allow for the appearance of selective tissues such as AChRs and axons, using Alpha-bungarotoxin and DiI, respectively.