Final answer:
LTP occurs due to the insertion of AMPA receptors into the postsynaptic membrane following the expulsion of magnesium from NMDA receptors during postsynaptic depolarization. LTD is the removal of AMPA receptors, leading to synapse weakening. Both processes are central to synaptic plasticity and neural transmission.
Step-by-step explanation:
Long-term potentiation (LTP) and long-term depression (LTD) are crucial processes for neural plasticity and the transmission of impulses across synapses. LTP occurs when there is a persistent strengthening of the synaptic connection, often associated with learning and memory. This strengthening is the result of increased postsynaptic responsiveness to neurotransmitters due to the insertion of additional AMPA receptors into the postsynaptic membrane. During postsynaptic depolarization, NMDA receptors, which are normally blocked by magnesium ions, allow calcium ions to pass into the postsynaptic cell once the magnesium is expelled. This then triggers a cascade of signaling events leading to the insertion of AMPA receptors, thereby enhancing synaptic efficiency.
LTD, on the other hand, is associated with the weakening of synapses due to the removal of AMPA receptors from the postsynaptic membrane. This process is important for the 'pruning' of less-used synapses, optimizing the efficiency of neural networks. Both LTP and LTD play significant roles in the dynamic adjustment of synaptic transmission, underlying the brain's ability to adapt and learn.