The brain is one of the largest and certainly the most complex organ in the human body. It consists of brain cells and more than 100 billion nerve cells (neurons), estimated to be equal to the number of stars in the Milky Way! Neurons in the brain are linked and communicate with other neurons through trillions of connections. Modern medical technology (such as MRI scans, CT scans, MRA scans, and brain angiograms) has made it possible for researchers and scientists to have a better understanding of the functioning of the brain, but much is still unknown. Thinking, learning and memory are closely related as part of the functioning of the brain. While learning refers to acquiring information that would make it possible to change an individual’s behavior as a result of experience, memory refers to the retention and storage of the particular information that was acquired.
Explicit and implicit memory:
Memory as such is divided into explicit and implicit forms. Both explicit memory and many forms of implicit memory are involved in short term memory (lasting from seconds to hours) and long term memory (which stores memories for years). Working memory is a form of short term memory that keeps information available for very short periods, while the person plans what action to take with the acquired information.
Explicit memory:
• Explicit memory is associated with consciousness and awareness.
• Retention of explicit memory takes place in the hippocampus, located in the brain’s temporal lobe; and in the neocortex, the largest part of the cerebral cortex that forms the outside surface of the brain; as well as in the amygdala, an almond shaped area in the temporal lobe.
• Explicit memory is divided into memory of events that happened to you (episodic memory), (for example learning to ride a bicycle) as well as general facts and information (semantic memory) (for example words, rules and language).
• Episodic memories are formed in the hippocampus and indexed to be accessed later. These memories are autobiographical in nature for specific events that took place in your life. Episodic memory refers to the ability to remember the details of specific events.
• Semantic memories (the way we understand words, meanings and concepts) are formed in the anterior temporal lobe – a region just in front of the ears. Semantic memory refers to the ability to learn facts and acquire general knowledge.
• Working memory is the ability of the brain to maintain information temporarily about a task you are involved in, it is short-term in nature and is connected to the hippocampus.
• While short-term explicit memories are encoded in the hippocampus, long-term memories are stored in various parts of the neocortex, for example the different locations of visual, auditory and olfactory (sense of smell) memories.
Implicit memory:
Implicit memory is long-term in nature and the two main areas in the brain involved with implicit memory are the basal ganglia and the cerebellum. There are four forms of implicit memory:
• Procedural memory includes skill and habits that, once required, would become automatic and unconscious.
• Priming memory facilitates the recognition of words or objects as a result of prior exposure to them.
• Non-associative learning refers to learning about a single stimulus.
• Associative learning refers to learning about the relation of one stimulus to another.
How memory is formed in the brain:
The forming of memories starts by sensory input to the brain, which is sent via electrochemical signals between nerve cells, called neurons, to different parts of the brain. These complex electrochemical signals form the basis of the brain’s essential functions, such as forming memories and thoughts, producing and/or initiating actions, as well as assisting to interpret the world around us. Neurons are the basic building blocks of the body’s nervous system.
In the developing fetal brain, neurons are created in special zones that contain stem cells. Once a neuron is in place, the DNA “recipe” in the nucleus triggers it to grow and extend a number of tiny “feelers” called dendrites and a single more substantial “feeler” called the axon into the area around it. The axons sprout and navigate through the brain until their tips, guided by studded chemical receptors, reach their specific targets and form synaptic connections with other neurons. The junction between two neurons is called a synapse, which consists of a miniscule gap (20-40 nanometers) where the axons and dendrites meet. In this way a synaptic matrix is generated between neurons in the brain.
Neurons are basically electric devices that communicate with each other via electric events, called action potentials, across synapses. This can be viewed as the electric part of the electrochemical signals between neurons. The chemical part of this communication process starts when an action potential reaches the synaptic cleft (“gap”) and causes neurotransmitters (”chemicals”) to be released from the neuron’s axon into the cleft. After crossing the cleft, the neurotransmitter attach to receptors on the dendrites of the receiving neuron. The synapse can be seen as the junction between the axon of one neuron and the dendrite of another neuron, where the electrochemical communication process takes place.
At cellular level, the brain is primarily composed of two types of cells – the neurons as discussed above and glial cells. (Glia is the Greek word for glue, although the glia cells are more than just connective tissue.) The glial cells provide structural and metabolic support, provide insulation, plays a role in the forming of the protective sheath (the myelin) around the axons, as well as playing a role, along with neurons, in the communication processes in the central nervous system.
Memory refers to the process of encoding and storing knowledge and experiences, while memory recall refers to the retrieving of knowledge and experience, by the reactivation of a specific group of neurons. The connections between brain cells can be made stronger or weaker, depending on when and how often they have been activated in the past. Memories occur when specific groups of neurons are activated. A good night’s sleep (8 hours) provides the brain with downtime to do important maintenance and to consolidate memories. The hippocampus replays recent events, which helps to update the neocortex on what needs to be stored.
Researchers believe that memory is a brain-wide process that is made up of a group of systems that each play a role in creating, storing and recalling memories. It is a complex process of reconstruction, for example when you think of an object such as a book, your brain would retrieve information from different areas of the brain, such as the name of the object, its shape, its function, the sound it makes as you turn the pages, and even the smell associated with a new book. You are never aware of these different processes taking place instantaneously in different functional areas of the brain.
A fairly new field of study is trying to understand how the brain forgets. Research in the past have mainly focused on memory, but scientists believe that there are also a collection of processes that determine how the brain forgets, and this aspect have been overlooked in the past. Forgetting is an important function that helps us to move forward.
Conclusion:
Memory is what transforms us from helpless newborn babies into capable adults that are able to function in the world around us.
The extremely complex functioning of the brain means that scientists still do not fully understand exactly how you remember or exactly what happens in the brain during recall.
An indication of the complex nature of the neural connections in the brain and the intricate way the brain works, comes from a study by neurobiologists at the Allen Institute, where they mapped the neurons and synapse-level connections in a cubic millimeter of mouse brain, a sample the size of a grain of sand. Microscopes collected more than 100 million images of 25 000 slices of the mouse’s visual cortex, with each slice only 40 nanometers thick. The complete data set, which took three months to assemble into a 3D model, took up 2 petabytes of computer memory, equal to 2 million gigabytes! (Source: How to map the brain, published in Nature Outlook.) Studies like these are attempts by scientists to understand how neural circuits in the brain encode, store and retrieve information. Equally important is trying to understand how the brain forgets.
The following detailed image of the anatomy of the brain shows where the functional areas of the brain are located:
Sources:
Learning, memory, language and speech. Chapter 21. Medical Physiology. A systems approach. (Physiology Handbook). By Hershell Raff, Michael Levitzky, et al.
Where are memories stored in the brain? Published online and updated 23 July 2018. Queensland Brain Institute. The University of Queensland. (www.qbi.uq.edu.au)
Brain functions. Published online. Queensland Brain Institute. The University of Queensland. (www.qbi.uq.edu.au)
How your brain works: myths and facts. Published 29 September 2017. WebMD. (www.webmd.com)
Semantic momory pinpointed in the brain. Published 7 September 2007. New Scientist. (www.newscientist.com)
Neuroscientists identify brain circuits necessary for memory formation. Published 6 April 2017. MIT News. Massachusetts Institute of Technology. (www.news.mit.edu)
How human memory works. Published online. HowStuffWorks. (www.howstuffworks.com)
The forgotten part of memory. Published 24 July 2019. Nature Outlook. International Journal of Science. (www.nature.com)
How to map the brain. Published 24 July 2019. Nature Outlook. International Journal of Science. (www.nature.com)
Images provided by Google.
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