The four main parts of the human brain are the cerebrum, the cerebellum, the brainstem, and the diencephalon. Each has a distinct location, structure, and set of responsibilities, from conscious thought and movement to automatic functions like breathing and heart rate. Together they account for roughly three pounds of tissue that controls everything your body does.
The Cerebrum
The cerebrum is the largest part of the brain by a wide margin, making up about 77% of total brain volume. It’s the wrinkled, folded structure that most people picture when they think of a brain. The cerebrum handles your highest-level functions: thinking, planning, language, memory, personality, and voluntary movement. It’s divided into two halves, called hemispheres, connected by a thick band of nerve fibers that lets them communicate.
Each hemisphere is further divided into four lobes, each with its own specialties:
- Frontal lobe: The largest lobe, sitting behind your forehead. It controls decision-making, planning, problem-solving, short-term memory, and movement. It also contains a region critical to producing speech.
- Parietal lobe: Located in the upper middle area of the brain. It processes touch, pain, temperature, and taste, and helps you understand where your body is in relation to objects around you. A key language-comprehension area sits here as well.
- Temporal lobe: Found on each side of the brain, roughly behind your temples. These lobes handle sound processing, aspects of smell and taste, and play a major role in storing memories.
- Occipital lobe: Positioned at the back of the head. It processes everything you see, matching incoming visual information with images stored in memory so you can recognize faces, objects, and text.
Left and Right Hemispheres
The two halves of the cerebrum aren’t identical in how they work. Left-hemisphere regions tend to communicate more strongly within their own side, which supports functions like language and fine motor control of the hands and mouth. Right-hemisphere regions connect more broadly across both sides of the brain, supporting spatial awareness and attention to your visual surroundings. Research published in PNAS found that the degree of this specialization between hemispheres directly predicted how well people performed on verbal and spatial tasks, suggesting that having distinct roles for each side genuinely sharpens cognitive ability.
The Cerebellum
The cerebellum sits at the lower back of the brain, tucked beneath the cerebrum. Despite being much smaller (about 10% of total brain volume), it contains an enormous number of nerve cells packed tightly together. For centuries scientists thought its only job was coordinating muscle movements, but its role turns out to be much broader.
The cerebellum fine-tunes your balance, posture, and the precision of physical movements. When it’s damaged, people have trouble walking steadily or reaching for objects accurately, often overshooting or undershooting their target. It also governs your sense of timing. People with cerebellar damage struggle with rhythmic tasks like tapping their fingers to a beat, consistently hitting too early or too late.
More recently, researchers discovered the cerebellum also contributes to learning new words and skills, judging the size and distance of objects, controlling eye movements, processing emotions, and making decisions. It’s no longer considered a purely “motor” structure.
The Brainstem
The brainstem is the stalk-like structure at the bottom of the brain that connects the cerebrum to the spinal cord. It makes up only about 6% of brain volume (combining the midbrain and a region called the hindbrain), but it controls functions you literally cannot survive without. It has three segments: the midbrain at the top, the pons in the middle, and the medulla oblongata at the bottom.
Your brainstem manages the body’s automatic processes: breathing rhythm, heart rate, blood pressure, swallowing, and the gag reflex that protects your airway. It also adjusts your pupil size in response to light changes and helps regulate balance. None of these require conscious thought. Your brainstem sends constant signals to the rest of your body, keeping these systems running whether you’re awake, asleep, or under anesthesia. Because it controls so many life-sustaining functions, injuries to the brainstem tend to be far more immediately dangerous than damage to other brain regions of similar size.
The Diencephalon
The diencephalon is the least familiar of the four parts, but it plays a central role in how your brain processes information and keeps your body in balance. It sits deep in the center of the brain, nestled between the cerebrum above and the brainstem below, and accounts for roughly 4% of total brain volume. Its two most important structures are the thalamus and the hypothalamus.
The thalamus works as the brain’s relay station. Nearly every type of sensory information (sight, sound, touch, taste) passes through it before reaching the cerebrum for conscious processing. Smell is the one exception, bypassing the thalamus entirely. Different clusters of cells within the thalamus handle different senses: one group relays what you see to the visual processing area, another sends what you hear to the auditory area, and others route touch and temperature signals from your limbs, trunk, and face to the appropriate regions. Without the thalamus, sensory information would never reach the parts of the brain that make sense of it.
The hypothalamus, located just below the thalamus, regulates your body’s internal environment. It controls hunger, thirst, body temperature, sleep-wake cycles, and the release of hormones. While tiny, it acts as the command center that keeps your body’s basic conditions stable, adjusting systems constantly in response to what’s happening inside and around you.
How the Four Parts Work Together
No part of the brain operates in isolation. When you catch a ball thrown in your direction, your occipital lobe processes the visual image, your thalamus relays that information at high speed, your frontal lobe plans the movement, your cerebellum fine-tunes the timing and trajectory of your hand, and your brainstem keeps your heart pumping and lungs breathing the entire time. All four parts communicate through dense networks of nerve fibers, with signals traveling between them in milliseconds.
Damage to any one part produces distinct symptoms. Cerebrum injuries affect thinking, language, or sensation depending on the lobe involved. Cerebellar damage causes coordination and balance problems. Brainstem injuries disrupt breathing, consciousness, or heart function. Diencephalon damage can distort sensory perception or throw off hormonal regulation. Understanding which part does what helps explain why two brain injuries in different locations can produce completely different outcomes, even if they’re similar in size.