Parts of the brain

The brain is about 2% of body weight but uses about 20% of the oxygen the body takes in.

A three-pound organ with a large appetite

NINDS describes the brain as a three-pound organ, about 1.4 kilograms, lying in its bony shell and washed by protective fluid. It is small next to the rest of the body, roughly 2% of body weight, yet it accounts for about 20% of the oxygen the body consumes. The cells doing most of that work are neurons, about 86 billion of them. The OpenStax anatomy textbook divides the adult brain into four major regions: the cerebrum, the diencephalon, the brain stem and the cerebellum. The model above shows all four, with the inner parts visible when you turn on See inside.

The cerebrum and its cortex

The cerebrum is the large wrinkled part that most people picture when they think of a brain. A deep groove, the longitudinal fissure, splits it into left and right hemispheres, and a thick band of fibres at the bottom of that groove, the corpus callosum, carries signals between them. The outer layer is the cerebral cortex, gray matter about as thick as a stack of two or three dimes, according to NINDS. Most information processing happens here. The ridges are called gyri and the grooves sulci; folding packs more cortex into the skull. Underneath lies white matter, and deeper still are groups of neurons called the basal nuclei, which help plan and start movements.

Four lobes

Landmark grooves divide each hemisphere into four lobes. The central sulcus separates the frontal lobe in front from the parietal lobe behind it. The lateral sulcus runs along the side, with the temporal lobe below it. The occipital lobe sits at the back and has no sharp border with its neighbours on the outer surface, a hint that their jobs overlap.

In the early 1900s the German neuroscientist Korbinian Brodmann studied the microscopic structure of the cortex and divided it into 52 areas. His numbers are still used, and they match function surprisingly well: area 17 in the occipital lobe handles primary vision, while area 18 is a secondary visual area, and areas 41 and 42 in the upper temporal lobe handle primary hearing.

Deep inside: the diencephalon

Below the cerebrum and above the brain stem is the diencephalon, made mostly of the thalamus and the hypothalamus. The thalamus is a relay. Every sense except smell passes through it before reaching the cortex, and it filters as it relays, helping decide which signals get attention. The hypothalamus, about the size of a pearl, regulates the inside of the body: it directs the autonomic nervous system and controls hormones through the pituitary gland attached beneath it. NINDS adds that it helps wake you up in the morning. Tucked into each temporal lobe is the hippocampus, which works like a memory index, sending memories out for long-term storage and helping retrieve them.

The brain stem and the cerebellum

The brain stem connects the brain to the spinal cord. From top to bottom it is the midbrain, the pons and the medulla oblongata. The midbrain takes part in eye movements and some reflexes. The pons is the main link with the cerebellum, and together the pons and medulla regulate breathing and the heart and blood vessels. The cranial nerves attach here, and all the major pathways between the cerebrum and the spinal cord pass through. Behind the brain stem sits the cerebellum, the little brain. It compares the movement commands from the cerebrum with feedback from the body and sends corrections, which is why it matters for coordination, balance and practised movements like playing the piano or hitting a tennis ball.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.