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George E. Carvell, PhD, PT   University of Pittsburgh
 
NEUROTRANSMITTERS III: NMDA RECEPTORS AND SYNAPTIC PLASTICITY
Two Types of Glutamate Receptors
    Non-NMDA (AMPA) receptors bind glutamate and open ion channels for NA+ influx, and K+ efflux resulting in a rapid, short duration depolarization of the postsynaptic spine (EPSP)

   NMDA Receptors bind glutamate and when sufficiently depolarized, eject Mg++ from the ion channel allowing passage of not only Na+ and K+ but also the influx of Ca++. Ca++ is a critical regulatory ion that sets in motion a number of secondary responses (2nd messengers) in the postsynaptic spine and perhaps, via a retrograde messenger, modifies the presynaptic terminal's release of glutamate. NMDA receptors have ionotropic & metabotropic properties. Metabolic and structural alterations are set in motion at the facilitated synapse.
KS&J, Fig. 63-10 p. 1261
Ca2+ + Calmodulin = 2nd Messenger Cascade Trigger