Final answer:
LTP strengthens synaptic connections by adding AMPA receptors to the postsynaptic neuron, thus enhancing response to neurotransmitters. LTD weakens connections by removing AMPA receptors, making the neuron less responsive. Both are vital for synaptic plasticity associated with learning and memory.
Step-by-step explanation:
The differences in cellular mechanisms between Long-term Potentiation (LTP) and Long-term Depression (LTD) relate to how they impact synaptic strength. LTP involves the insertion of more AMPA receptors into the postsynaptic membrane, which increases the membrane's sensitivity to neurotransmitters, enhancing synaptic communication. In contrast, LTD is characterized by the removal or endocytosis of AMPA receptors from the postsynaptic membrane, which leads to a decrease in synaptic responsiveness.
During LTP, calcium and CaMKII-dependent cascades are initiated when calcium enters through NMDA receptors due to strong and repeated synaptic activity. This leads to more AMPA receptors being added to the postsynaptic neuron, making it more responsive to the presynaptic neurotransmitter glutamate. On the other hand, calcium that enters through NMDA receptors during lower frequency firing initiates a different cascade, involving calcineurin and protein phosphatase 1, which results in the internalization of AMPA receptors and thus, LTD. While LTP and motivation work to strengthen synaptic connections through the enhancement of signal transmission, LTD is also critical for learning and memory as it enables the brain to eliminate weaker synapses, making room for more significant ones.