The three main parts of the human brain are the cerebrum, the cerebellum, and the brainstem. Together, they account for roughly 3 pounds of tissue and control everything from abstract thought to your heartbeat. Each part handles a distinct set of jobs, and understanding what they do helps explain why damage to different areas of the brain produces such different symptoms.
The Cerebrum: The Largest Part
The cerebrum makes up about 82% of total brain mass, filling most of your skull. It is split down the middle into left and right hemispheres, connected by a thick band of nerve fibers that lets the two sides communicate. This is the part of the brain responsible for everything you think of as “thinking”: speech, memory, reasoning, judgment, personality, and decision-making. It also processes input from your five senses and initiates voluntary movement.
Each hemisphere is divided into four lobes, and each lobe has its own specialties.
- Frontal lobe: The largest lobe, sitting behind your forehead. It handles planning, decision-making, voluntary movement, and personality. It also contains a region critical for producing speech. When the frontal lobe is damaged, people often lose the ability to plan and carry out multi-step actions, like solving a problem or crossing a busy street. Damage to the front of this lobe can cause a striking loss of social inhibition, making a person inappropriately blunt, vulgar, or emotionally flat.
- Parietal lobe: Located in the upper middle portion of the brain. It processes touch and pain, helps you understand where your body is in space, and lets you recognize objects by feel. Damage here can make it impossible to distinguish left from right or to sense where your own limbs are without looking at them.
- Temporal lobe: Sits on either side of the brain, roughly behind your temples. It plays a key role in hearing, language comprehension, and memory formation.
- Occipital lobe: Found at the very back of the head. It processes visual information. If both sides of the occipital lobe are damaged, a person can become cortically blind, unable to recognize objects by sight even though the eyes themselves still work. Some people with this condition are completely unaware they cannot see.
One important detail: each hemisphere controls movement and sensation on the opposite side of the body. Damage to the left hemisphere causes weakness or numbness on the right side, and vice versa.
The Cerebellum: Coordination and Balance
The cerebellum is a fist-sized structure tucked beneath the back of the cerebrum, just above the brainstem. Despite its relatively small size (about 10% of brain mass), it contains a huge number of neurons and plays a critical role in making your movements smooth and accurate.
The cerebellum does not start movements on its own. Instead, it fine-tunes the motor commands that originate in the cerebrum. Think of it as an editor: when the cerebrum sends a signal to reach for a glass of water, the cerebellum adjusts the timing and force of every muscle group involved so the motion is fluid rather than jerky. It coordinates not just different joints working together but also opposing muscles around the same joint, ensuring they fire in precise sequence.
It does this using two strategies. The first is real-time feedback: the cerebellum constantly compares what your body is actually doing with what it intended to do, making corrections mid-movement. The second is feedforward learning, a trial-and-error process. If a movement produces an error (say, your hand overshoots the glass), the cerebellum registers that mistake and calibrates the movement more accurately next time. Over many repetitions, this is how you develop skilled, automatic motor abilities like typing, riding a bike, or playing an instrument.
The cerebellum also receives information from balance sensors in your inner ear and from stretch receptors in your muscles and joints. It uses this input to adjust your posture and compensate for shifts in body position or changes in the load on your muscles, like when you catch something heavy that’s tossed to you.
The Brainstem: Automatic Life Support
The brainstem is the lowest part of the brain, a narrow stalk that connects the cerebrum and cerebellum to the spinal cord. It handles the automatic functions you never have to think about: breathing, heart rate, blood pressure, swallowing, and the cycle of sleeping and waking. Because every nerve signal traveling between the brain and the body must pass through the brainstem, it also serves as a major communication highway.
The brainstem has three sections stacked on top of one another:
- Midbrain: The topmost section. It helps control eye movements and processes visual and auditory information.
- Pons: The middle section. It coordinates facial movements, facial sensation, hearing, and balance. It also relays signals between the cerebrum and the cerebellum.
- Medulla oblongata: The bottommost section, blending into the spinal cord. It regulates breathing, heartbeat, blood pressure, and swallowing. Because these functions are essential for survival, damage to the medulla is among the most life-threatening brain injuries.
How the Three Parts Develop
Early in embryonic development, the brain starts as a simple tube that expands into three bulges: the forebrain, midbrain, and hindbrain. As the embryo grows, the forebrain develops into the cerebrum (along with deeper structures like the thalamus and hypothalamus that relay sensory information and regulate hormones). The hindbrain becomes the cerebellum, pons, and medulla. The midbrain stays relatively compact and becomes the top portion of the brainstem. This three-part origin is the reason the adult brain is still organized into three main divisions, even though each one contains multiple specialized substructures.
What Happens When Each Part Is Injured
Because the three parts handle such different functions, damage to each one produces very different symptoms. Cerebrum injuries tend to affect thinking, sensation, or voluntary movement in ways that depend on which lobe is involved. A frontal lobe injury might make someone apathetic and slow to respond, or it might strip away social filters and make them impulsive. A parietal lobe injury might leave a person unable to dress themselves, not because of paralysis, but because they can no longer understand how objects relate to their body in space.
Cerebellum damage typically produces problems with coordination rather than outright paralysis. Movements become clumsy and poorly timed, balance deteriorates, and fine motor tasks like writing or buttoning a shirt become difficult. Speech may sound slurred because the muscles of the mouth and tongue lose their precise timing.
Brainstem damage is often the most immediately dangerous because it can disrupt breathing, heart rate, or consciousness. Even small injuries in this area can cause double vision, difficulty swallowing, or dizziness, because so many critical pathways are packed into a very small space.