For many of us, our memory is an essential part of our lives. Without it, we would not be able to remember our family, the past, or even basic things such as motor skills. Unfortunately, for people with Alzheimer's or brain injuries, this can be their reality. Take H.M. for example: after undergoing surgery to cure his seizures, he was unable to form long term memories. He could remember things from the past, had the same personality, and retained his motor skills but he was unable to remember things that just happened. If someone walked into a room and then left, he would not remember that meeting moments later. Today, we’re going to explore exactly how memories are made, different types of memories, and where they are stored.
Types of Memories
Declarative memories are your explicit memories that allow you to remember names, places and other facts and can either be semantic or episodic. You can consciously decide to recall these memories. Semantic memories are facts such as your vocabulary, knowledge of history or sciences, or concepts such as math skills. These are your general facts, things you would learn in school or absorb in daily life. Often these memories are found in cortical regions outside the hippocampus. Episodic memories are related to your personal everyday experiences and often have sensory details associated with them. An example could be your first day of college or a surprise birthday party you threw. Semantic and episodic memories are long term memories; on the other hand, working memories are short-term memories. Temporary memories can only be used for a short period of time and include things such as a phone number or what you had for breakfast yesterday. Spatial memories are another aspect of declarative memories and use place cells to create a mental map of a place, such as your house or town.
Nondeclarative memories are memories that you use unconsciously such as motor skills (walking or speaking). This is often referred to as “muscle memory” and often involves the cerebellum, prefrontal cortex, and the basal ganglia.
Memory Formation
Memories are represented by changes in strength and number of synapses - which include changing the neurotransmitter used, neurotransmitter release, increasing or decreasing the number of axons, and dendrites. For short term memories, the changes are not permanent whereas, in permanent memories, the changes are fixed.
Long term potentiation is one such change to a synapse, mostly found in the hippocampus and cerebral cortex. It is a permanent (or longer-lasting) increase in strength of a synapse. LTP results in the change of NMDA (N-methyl-d-aspartate) receptors. In order to stabilize these changes, cAMP (cyclic adenosine monophosphate) is activated as a result of releasing calcium ions into the synapse. This triggers other enzymes that make the synapse more sensitive to neurotransmitters and activates CREB (cAMP-response element-binding protein) to produce neurotrophins. These neurotrophins help grow the synapse and increase response to stimulation. This stabilization sequence is essential for maintaining long term memories, not short term memories.
Long term depression is another synaptic change that results in the opposite of long term potentiation. Although decreasing the effectiveness of a synapse in "holding" a memory may sound negative, it is essential for learning new things by making space for them.
Important Structures
Memory is not isolated to a certain part of the brain. The four lobes of the brain process all of the sensory information that they receive and these inputs are then associated with a certain memory. Depending on the type of memory, the memories are stored in different locations. These structures work together to create the cohesive memory system we use on a daily basis.
The prefrontal cortex (which handles decisions, planning, etc.) coordinates memories from various areas and is particularly active when you are trying to remember something (such as a phone number) for a brief period of time.
The parahippocampal cortex is important in spatial memory and provides context to memories as well as in retrieval of memories.
The amygdala is essential in “attaching” the emotional significance to memories.
The dentate gyrus, a part of the hippocampus, is responsible for remembering certain events, learning, and memory. It is also one of the few areas in the brain where new neurons are made (neurogenesis).
Notable Researchers: Eric Kandel
Eric Kandel is one of the most influential researchers in the field of memory. He discovered the role synapses play in memory and learning and later won a Nobel Prize for his work. He used the sea slug Aplysia to study how memories are formed as it has only 20,000 large neurons that are easy to study. Kandel showed how differences in the changes in the synapse lead to short term and long term memories. This formed the basis for understanding long term potentiation.
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