How is memory formed and maintained? We explore the principles of how memory is formed and maintained by synaptic connections of neurons and long-term potentiation.
Many studies in neuroscience explain memory formation as “long-term potentiation”. According to this theory, nerve cells in the brain share information by transmitting electrical and chemical signals across synapses, the gaps between cells. The phenomenon in which the signal is strong and the synaptic connection is maintained for a long time is called long-term potentiation, and this is how memory is formed.
To better understand the process of memory formation, it is necessary to understand the basic structure and function of nerve cells. Nerve cells are generally composed of three main parts: the cell body, the axon, and the dendrite. The cell body contains the nucleus and most of the organelles of the nerve cell, and the axon is responsible for transmitting electrical signals over long distances. Dendrites receive signals from other neurons. These structures work together to enable the transmission and processing of information.
Synaptic connections are based on the activity of ions in nerve cells. Ions move in and out of the nerve cell membrane due to their tendency to diffuse and move from areas of high concentration to areas of low concentration. This movement of ions changes the state of the nerve cell. First, without external stimuli, there are more cations outside the cell membrane and more anions inside, resulting in a polarization in which the inside and outside of the cell membrane are divided into positive and negative charges, respectively. The nerve cells are in a stable state in this process. However, when external stimuli, such as new information, are received, positively charged Na⁺ (sodium ions) diffuse from the outside to the inside, causing depolarization, which is the accumulation of positive charges in the cell. Depolarization excites the nerve cell, forming an action potential, which is an electrical signal. When the nerve cell is excited, Ca²⁺ (calcium ions) from outside the cell diffuse into the cell. This Ca²⁺ then releases various neurotransmitters, including glutamate, which are chemical signals. These signals combine with other nerve cells to form synaptic connections. The cell that releases the chemical signal is called the presynaptic cell, and the cell that receives the chemical signal is called the postsynaptic cell.
The role of glutamic acid and Ca²⁺ is to strengthen these synaptic connections in the long term. Glutamate secreted by the excited presynaptic cell stimulates the AMPAR and NMDA receptors on the postsynaptic cell. First, the AMPAR channel opens when stimulated by a large amount of glutamic acid. When Na+ diffuses into this channel, the postsynaptic cell is also depolarized and excited. This removes Mg²⁺ (magnesium ion) from the NMDA receptor channel stimulated by glutamate and opens the channel. And Na⁺ and Ca²⁺ diffuse into the open NMDA receptor channel. The diffused Ca²⁺ activates the proteins in the cell, and the activated proteins create new AMPARs. As a result, the postsynaptic cell receives more Na⁺ to enhance depolarization, and the influx of Ca²⁺ continues to maintain the excited state for a long time.
In addition, the excited postsynaptic cells send signals to the presynaptic cells to increase the amount of glutamate secreted by the presynaptic cells, further strengthening the synaptic connection. This allows the synaptic connection to be maintained for up to three hours, which is called early long-term potentiation. In contrast, the synaptic connection can last for more than 24 hours, which is called late long-term potentiation. The difference between late and early long-term potentiation is that new proteins are synthesized. AMPAR receptors have a short lifespan, so new AMPAR receptors must be made to maintain synaptic connections, but this cannot be done by using only the proteins in the cell as in early long-term potentiation. Therefore, new proteins are synthesized to continue making AMPAR receptors. Neuroscientists believe that short-term memory is formed by early long-term potentiation and long-term memory is formed by late long-term potentiation.
Interestingly, this process is not limited to biological mechanisms, but is also closely related to the psychological aspects of learning and memory. For example, repetitive learning and experience strengthen synaptic connections, making certain information or skills last longer and be more easily recalled. This explains why repetition and practice are important when learning new information. In addition, stress and emotional state can also affect synaptic strengthening, which plays an important role in memory formation and retrieval. This integrative approach shows that neuroscience is not just a biological study, but is linked to various fields such as psychology and education.
