Final answer:
Long-term Potentiation (LTP) is a persistent strengthening of a synaptic connection, while Long-term Depression (LTD) is a long-term weakening of a synaptic connection. The mechanisms behind both LTP and LTD involve the activation or removal of glutamate receptors in the postsynaptic membrane, resulting in an increase or decrease in the strength of synaptic transmission.
Step-by-step explanation:
Long-term potentiation (LTP) is a persistent strengthening of a synaptic connection. LTP is based on the Hebbian principle: cells that fire together wire together. One known mechanism involves a type of postsynaptic glutamate receptor, called NMDA (N-Methyl-D-aspartate) receptors. These receptors are normally blocked by magnesium ions, but when the postsynaptic neuron is depolarized by multiple presynaptic inputs, the magnesium ions are forced out, allowing calcium ions to pass into the postsynaptic cell. This initiates a signaling cascade that causes a different type of glutamate receptor, called AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors, to be inserted into the postsynaptic membrane. The insertion of additional AMPA receptors strengthens the synapse and makes the postsynaptic neuron more likely to fire in response to presynaptic neurotransmitter release.
Long-term depression (LTD) is a long-term weakening of a synaptic connection. One mechanism known to cause LTD involves the removal of AMPA receptors from the postsynaptic membrane. Calcium that enters through NMDA receptors initiates a signaling cascade that leads to the removal of AMPA receptors. The decrease in AMPA receptors makes the postsynaptic neuron less responsive to glutamate released from the presynaptic neuron. While it may seem counterintuitive, LTD is important for learning and memory because it allows for the pruning of unused synapses and strengthens the synapses that have undergone LTP by comparison.