How is memory formed and sustained over time? This explores the principles by which memory is created and maintained through synaptic connections between neurons and the process of long-term potentiation.
Many studies in neuroscience explain memory formation through ‘long-term potentiation’. According to this theory, neurons in the brain form synaptic connections by transmitting electrical and chemical signals across the synapse, the gap between cells, thereby sharing information. Long-term potentiation occurs when these signals become stronger, leading to the sustained maintenance of synaptic connections, and it is through this process that memory is formed.
To better understand the process of memory formation, it is necessary to examine the basic structure and function of neurons. Neurons are generally composed of three main parts: the cell body, the axon, and the dendrites. The cell body contains the neuron’s nucleus and most of its organelles. The axon transmits electrical signals over long distances. The dendrites receive signals from other neurons. These structures work together organically to enable the transmission and processing of information.
Synaptic connections are based on the activity of ions within the neuron. Ions move across the inner and outer surfaces of the nerve cell membrane due to properties such as diffusion from areas of high concentration to low concentration. This movement of ions alters the state of the nerve cell. Initially, without external stimulation, the outside of the cell membrane predominantly contains cations, while the inside contains anions, creating a polarization where the inside and outside carry positive and negative charges, respectively. During this process, the nerve cell is in a stable state. However, when a new stimulus, such as new information, occurs, positively charged Na⁺ (sodium ions) diffuse from outside to inside, causing a buildup of positive charge within the cell—a process called depolarization. Depolarization excites the nerve cell, forming an action potential, which is an electrical signal. When the neuron becomes excited, Ca²⁺ (calcium ions) from outside the cell diffuse inward. This Ca²⁺ then triggers the release of various neurotransmitters, including glutamate, which are chemical signals released outward. These signals bind to other neurons, forming synaptic connections. The cell that secretes the chemical signal is called the ‘presynaptic cell’, and the cell that receives the chemical signal is called the ‘postsynaptic cell’.
This synaptic connection leads to long-term potentiation due to the roles of glutamate and Ca²⁺. Glutamate secreted by the excited presynaptic cell stimulates the AMPA receptors and NMDA receptors on the postsynaptic cell. First, the AMPA receptor channel opens when stimulated by a large amount of glutamate. When Na⁺ diffuses inward through this channel, the postsynaptic cell also depolarizes and becomes excited. This then causes Mg²⁺ (magnesium ions) to be removed from the NMDA receptor channel being stimulated by glutamate, opening the channel. Then, Na⁺ and Ca²⁺ diffuse inward through the open NMDA receptor channel. The incoming Ca²⁺ activates proteins within the cell, and these activated proteins produce new AMPA receptors. As a result, the postsynaptic cell takes in more Na⁺, strengthening the depolarization, and the continued influx of Ca²⁺ allows the excited state to be sustained for a longer period.
Furthermore, the excited postsynaptic cell sends a signal back to the presynaptic cell, increasing its glutamate release and further strengthening the synaptic connection. This maintains the synaptic connection for up to 3 hours, termed early long-term potentiation (LTP). In contrast, synaptic connections can persist for over 24 hours, termed late long-term potentiation (LTP). The key difference between late-phase and early-phase long-term potentiation is the synthesis of new proteins. AMPA receptors have a short lifespan, so to maintain the synaptic connection, new AMPA receptors must be created. Relying solely on proteins already present within the cell, as in early-phase potentiation, is insufficient for sustained maintenance. Therefore, new proteins are synthesized to continuously produce AMPA receptors. Neuroscientists believe that short-term memory is formed through early LUTP, while long-term memory is formed through late LUTP.
Interestingly, these processes are not limited to purely biological mechanisms; they are also deeply connected to the psychological aspects of learning and memory. For example, repeated learning and experience strengthen synaptic connections, making specific information or skills last longer and be more easily recalled. This explains why repetition and practice are crucial when learning new information. Furthermore, stress or emotional states can also influence synaptic strengthening, playing a significant role in both memory formation and recall. This integrated approach demonstrates how neuroscience extends beyond simple biological research, connecting with diverse fields such as psychology and education.
