In this blog post, we’ll examine the basic structure and functions of the brain—which plays a crucial role in human identity, thought, and emotion—and explore the complex mechanisms of the brain as revealed by modern neuroscience.
“I think, therefore I am.” It is no exaggeration to say that the development of human culture has gone hand in hand with the history of brain development; as the “organ responsible for thought,” the brain is one of the most crucial elements in understanding human identity. For a long time, many scientists and philosophers have strived tirelessly to uncover the secrets of the brain, which is considered the source of thought and emotion. However, using the “brain” to uncover the secrets of the “brain” has never been an easy task. The human brain contains approximately 86 billion neurons, and countless synaptic connections are formed between them. While the number of synapses varies depending on the research and estimation methods used, it is estimated to be vast—ranging from several tens of trillions to over 100 trillion. Because the brain’s structure is so intricate and finely organized, even research conducted on the brains of deceased individuals faced numerous limitations. Furthermore, since properly understanding brain function required observing the actual activity occurring in a living brain, such research was long considered an extremely difficult challenge. However, with the remarkable advancements in brain imaging technology, neuroscience, and medical technology from the late 20th century through the 21st century, the secrets of the brain—once shrouded in mystery—began to be revealed one by one. In particular, advancements in various brain imaging technologies—such as magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI)—have played a crucial role in studying the structure and activity of the living human brain. The brain is often nicknamed the “small universe.” Just as the universe is an unknown world where it is difficult to discern its boundaries and where there is so much we do not know, the brain, too, presents us with endless questions and research challenges the deeper we delve into it. Furthermore, as these trends converge with artificial intelligence, brain-computer interfaces, and research on neurodegenerative diseases, global interest in neuroscience is growing even more. We are now entering the “Age of Neuroscience” in earnest. This article explains the basic knowledge about the brain that is essential for understanding these trends in neuroscience. Let’s explore the brain’s basic structure and functions.
Although the human brain can be classified anatomically in various ways, this article will examine it broadly divided into the brainstem, diencephalon, cerebellum, and cerebrum. The brain forms its complex structure as various regions of the neural tube differentiate during early embryonic development, and in adulthood, these structures form a tightly interconnected network. In this article, we will examine each part in the following order: cerebrum, diencephalon, cerebellum, and brainstem. Before classifying the brain’s structure and explaining the functions of each part, we must note that although we divide the brain into distinct sections for convenience, these parts are intricately interconnected; therefore, it is difficult to view any single region as responsible solely for one specific function. In particular, functions that involve multiple brain regions and networks—such as language—do not depend on any single area. This neural plasticity can also be observed in cases where, following brain damage, some functions are restored as the functions of other areas or remaining neural networks reorganize. In fact, it has been observed that in patients with aphasia resulting from left-hemisphere damage, both the undamaged areas of the left hemisphere and certain areas of the right hemisphere are involved in the process of language recovery. Therefore, when understanding the various parts of the brain, rather than establishing a one-to-one correspondence between specific regions and functions, we must also consider that multiple areas are interconnected to perform functions.
The cerebrum is the most highly developed part of the human brain, accounting for approximately 80% of the total brain mass. In particular, the cerebral cortex is a region of critical importance for understanding human higher-order cognitive functions. While the cerebrum consists of numerous regions, it can be divided into the cerebral cortex and various subcortical structures based on function and location. Deep within the cerebrum, various structures—such as the thalamus, hypothalamus, hippocampus, and amygdala—are interconnected. Among these, the structures involved in emotions, memory, motivation, and homeostasis regulation are functionally grouped together and referred to as the limbic system. However, the limbic system is not so much a single, distinct anatomical organ as it is a functional concept that describes a network of interconnected structures. The limbic system and related circuits, including the hypothalamus, are involved not only in emotional responses such as fear, anger, and pleasure but also in basic survival behaviors like appetite and sexual behavior, as well as in the regulation of the autonomic nervous system and homeostasis. Therefore, physical responses such as an increased heart rate or sweating when feeling fear involve not just a single specific region but multiple brain areas and the autonomic nervous system working together. The cerebral cortex, which covers the outer surface of the cerebrum, forms a complex structure of folds to accommodate a large number of neurons within the confined space of the skull. The average thickness of the cerebral cortex is approximately 2.6 mm, though this varies by region. When the cortices of both cerebral hemispheres are unfolded, their total surface area reaches approximately 1,600–2,000 cm². The number of neurons in the cerebral cortex is estimated to be approximately 16 billion, accounting for about 20% of all neurons in the brain. The cerebral cortex is involved in various higher-order cognitive functions, including language, perception, decision-making, and motor planning. Because of the complex interactions among its numerous neurons and their connections, the cerebral cortex plays a central role in understanding human thought, behavior, and experience.
The cerebellum is located at the rear of the brainstem and consists of a pair of hemispheres, accounting for approximately 10% of the brain’s total mass. The human cerebellum is known to weigh around 150 grams and contains a very high density of neurons relative to its size. In particular, it is estimated that about 80% of all brain neurons are located in the cerebellum, due to the very high density of small granule cells there. The cerebellum, with its deeply furrowed surface, contains circuits that are crucial for maintaining balance and precisely controlling posture and movement. In addition, it is involved in motor learning, the temporal control of movement, motor accuracy, and coordination, and is known to play a role not only in motor function but also in cognitive and emotional processes. While the cerebellum was historically understood primarily as an organ that regulates movement, recent research has increasingly focused on its connections to various cognitive functions. Both animal experiments and human studies have reported that cerebellar damage does not merely affect motor control but can also have various effects on cognition and behavior. Therefore, the function of the cerebellum extends beyond simply controlling walking and balance; it has become an important subject of research for understanding how motor and cognitive functions are interconnected.
Located beneath the cerebrum and deep between the two cerebral hemispheres, the diencephalon plays a crucial role in various physiological functions as well as in the transmission and regulation of sensory information. The diencephalon includes several structures, such as the thalamus and the hypothalamus. The thalamus is a gray matter structure densely packed with neuronal cell bodies and has a shape resembling a pair of eggs. Serving as a crucial “relay station” for the brain, the thalamus receives and processes most sensory information—excluding olfactory input—and transmits the necessary information to the appropriate areas of the cerebral cortex. It is also involved in the transmission and regulation of various signals related not only to sensory information but also to motor function, arousal, and consciousness. The hypothalamus is a small structure that plays a crucial role in maintaining homeostasis, including body temperature, appetite, the sleep-wake cycle, and fluid balance. It also regulates the autonomic nervous system and the neuroendocrine system, and is closely connected to the pituitary gland, through which it controls hormone secretion. Since the hypothalamus influences various endocrine functions through the pituitary gland, it plays a vital role in maintaining the body’s homeostasis and physiological state. Therefore, the relationship between the hypothalamus and the pituitary gland is central to understanding the body’s hormonal system.
The brainstem is located deep within the lower part of the brain and is a crucial structure that connects the cerebrum and cerebellum to the spinal cord. The brainstem consists of the midbrain, pons, and medulla oblongata; although these three parts are anatomically distinct, they are closely interconnected and function in unison. The medulla oblongata, located at the lowest part of the brainstem, contains neural circuits that regulate vital functions such as respiration and cardiovascular activity; it is also involved in blood pressure regulation and reflexes such as coughing, sneezing, and vomiting. The pons, situated above the medulla oblongata, acts as a relay station for signal transmission between the cerebrum, cerebellum, and spinal cord, and is involved in various functions including respiratory control and sleep. The midbrain, located immediately above the pons, is involved in eye movements, visual and auditory reflexes, and motor control. Since the brainstem is involved in functions essential for sustaining life—such as respiration, heart rate, blood pressure, and consciousness—any damage to it can result in severe neurological disorders and pose a direct threat to life. However, since the outcome varies greatly depending on the extent and location of the damage, it should not be assumed that even a minor brainstem injury always leads directly to death.
We have explored the basic structure and functions of the brain, which can be considered the foundation of neuroscience. This is a very general overview; modern neuroscience has advanced to the stage of elucidating not only the fine structure and function of individual brain regions but also how different regions are connected and the operating principles of neural circuits. To understand the brain’s structure and function, it is necessary to examine not only its anatomical structure but also neurons, synapses, neural circuits, cerebral blood vessels, and mechanisms at the genetic and molecular levels. Furthermore, neuroscience is not limited to a single branch of biology. Knowledge from various fields—including physics, chemistry, medicine, psychology, computer science, and artificial intelligence—is increasingly integrated into neuroscience research. In particular, modern neuroscience is increasingly focusing on studying interconnected networks and the information processing that occurs within them, rather than analyzing specific brain regions in isolation. To understand the future advancements in neuroscience, we must first acquire knowledge of the brain’s basic structure and function and then integrate this with our expertise in our respective fields. By doing so, humanity will be able to gain a deeper understanding of the secrets of the brain—often referred to as a “miniature universe”—step by step.
