The Three Main Parts of the Brain | What Are the Core Components That Define Our Thoughts, Actions, and Life?

The cerebrum is the largest and most developed part of the human brain, accounting for approximately 85% of its total weight. It is responsible for the highest-order cognitive functions, including thought, language, memory, and voluntary action. The cerebrum is divided into two distinct cerebral hemispheres, the left and the right, which are connected by a thick bundle of nerve fibers called the corpus callosum. Its outer layer is the cerebral cortex, a highly folded sheet of neural tissue responsible for conscious thought and processing sensory information. This cortex is further organized into four primary lobes, each with specialized functions. The frontal lobe governs executive functions such as reasoning, planning, and problem-solving. The parietal lobe processes sensory information related to touch, temperature, and pain. The temporal lobe is crucial for auditory processing, memory formation, and language comprehension. Finally, the occipital lobe is dedicated almost exclusively to processing visual information from the eyes.

The cerebellum, whose name is Latin for "little brain," is located at the posterior base of the skull, beneath the cerebrum. While much smaller than the cerebrum, it contains more than half of the brain's total neurons. Its primary and most well-understood function is the coordination of voluntary motor movements, posture, balance, and speech, resulting in smooth and balanced muscular activity. The brainstem is the posterior part of the brain that connects the cerebrum and cerebellum to the spinal cord. It is the most primitive region of the brain and is critical for regulating essential, involuntary life functions. These include heartbeat, respiration, blood pressure, and sleep-wake cycles. The brainstem itself consists of three parts: the midbrain, the pons, and the medulla oblongata, each managing specific autonomic and relay functions.

This action requires seamless communication between the cerebrum, cerebellum, and brainstem. The initial decision to pick up the cup originates in the cerebrum's frontal lobe. The motor cortex then designs the plan for movement and sends signals down through the brainstem and spinal cord to the arm and hand muscles. Simultaneously, it sends a copy of this motor plan to the cerebellum. The cerebellum receives real-time sensory feedback from the body about the position of the arm and hand. It compares the intended movement with the actual movement, calculates any discrepancies, and sends corrective signals back to the motor cortex to ensure the action is smooth, accurate, and coordinated. The brainstem's role is to relay these signals and maintain the body's overall posture and stability during the action.

Survival and the resulting deficits depend entirely on which part is damaged. Significant damage to the cerebrum, for instance from a stroke, can lead to specific functional losses like paralysis, speech impairment (aphasia), or memory loss, but it is not typically fatal. Damage to the cerebellum does not cause paralysis but leads to a condition called ataxia, which involves a loss of coordination, balance, and fine motor control. Survival is the norm, but with significant disability. In contrast, severe damage to the brainstem is almost always catastrophic. Because the brainstem controls the most fundamental functions for life—breathing, heart rate, and consciousness—any major trauma or lesion in this area will result in a rapid cessation of these functions, leading to what is medically defined as brain death.

The brainstem is fundamental to our daily cycles of sleep and wakefulness through a network of neurons called the Reticular Activating System (RAS). The RAS is located within the brainstem and extends upwards to project signals throughout the cerebral cortex. It acts as the brain's gatekeeper for consciousness, filtering the constant stream of sensory information and determining which signals are important enough to be brought to our conscious attention. When the RAS is highly active, it stimulates the cerebrum, keeping us awake, alert, and focused. As its activity decreases, the cortex is no longer stimulated in the same way, leading to drowsiness and eventually sleep. Damage to the RAS can have profound effects on arousal, ranging from chronic lethargy to a persistent vegetative state or coma, demonstrating its critical role in enabling conscious experience.