The brain as a physical organ
An adult human brain is a soft, wrinkled organ weighing roughly three pounds. It sits inside the skull, cushioned by cerebrospinal fluid and wrapped in three membrane layers called the meninges. The tissue has a somewhat firm gelatin-like consistency. It is sometimes described as being made of gray matter and white matter: gray matter is rich in nerve cell bodies, while white matter consists mainly of the insulated fibers that connect them. The cells themselves are covered in the article on neurons and glia.
Anatomists often organize the brain into three broad parts: the cerebrum, the cerebellum and the brainstem. The divisions are conventions, and the parts are densely interconnected.
The cerebrum and the cortex
The cerebrum is the largest part, divided into left and right hemispheres joined by a thick bundle of fibers called the corpus callosum. Its wrinkled outer layer, the cerebral cortex, is only a few millimeters thick, and its folds greatly increase surface area within the skull. The cortex is typically divided into four lobes, named for the skull bones above them.
- Frontal lobe. Located at the front, it is associated with planning, decision making, movement control and aspects of personality and language production.
- Parietal lobe. Toward the top and back, it is associated with touch, spatial awareness and integrating information from the senses.
- Temporal lobe. At the sides, it is associated with hearing, language comprehension and memory.
- Occipital lobe. At the back, it is primarily involved in processing vision.
These associations are general guides. Most complex tasks recruit several lobes at once, a point developed in the discussion of brain regions.
Deep structures
Beneath the cortex lie structures that work closely with it.
- The hippocampus, in the temporal lobe, is involved in forming new memories, as described in memory formation.
- The amygdala, an almond-shaped cluster nearby, contributes to emotional processing, including responses to threat, though it does much more than generate fear.
- The thalamus acts as a major relay station, passing sensory information to the cortex.
- The hypothalamus, small but important, helps regulate temperature, hunger, thirst, sleep cycles and hormone release.
- The basal ganglia are involved in movement and in learning habits.
Together some of these are sometimes grouped as the limbic system, a term that is useful but loosely defined, and whose boundaries scientists debate.
The cerebellum
Tucked beneath the back of the cerebrum is the cerebellum, whose name means little brain. It is traditionally linked to coordination, balance and the fine-tuning of movement. Its role is not limited to movement: research also associates it with some aspects of timing, language and attention. Despite its small size, it is often reported to contain a large proportion of the brain’s neurons, which is a reminder that volume does not equal cell count.
The brainstem
The brainstem connects the brain to the spinal cord and has three main sections: the midbrain, the pons and the medulla. It controls many functions that are vital and mostly automatic, including breathing, heart rate regulation and wakefulness. Nearly all signals traveling between brain and body pass through it, and many of the nerves that serve the head and face begin here. Because these functions are essential, damage to the brainstem can be extremely serious.
Blood supply and protection
The brain relies on a constant supply of oxygen and glucose. Interruption of blood flow, as in a stroke, can quickly injure tissue. A specialized barrier, the blood-brain barrier, regulates which substances pass from the bloodstream into brain tissue, protecting it from many harmful agents while also complicating drug delivery. Fluid-filled spaces called ventricles produce and circulate cerebrospinal fluid, which cushions the brain and helps clear waste.
Reading anatomy with care
Anatomical labels are helpful, but they can give the false impression of neat compartments, each with a single job. A few cautions apply.
- Regions overlap in function, and a single region often participates in many tasks.
- Individual brains vary in size and shape of structures.
- Much of what is known about specific structures comes from a mix of injury studies, imaging and animal research, each with limitations.
- Functions such as attention and emotion involve widespread networks.
Putting it together
A helpful way to remember the layout is from the bottom up. The brainstem keeps the body running, the cerebellum refines movement, deep structures link emotion, memory and motivation, and the cortex supports perception, thought and voluntary action. The structure reflects, in part, the long history of vertebrate evolution, as touched on in the article on brain size evolution. At the same time, no part works alone, and the most interesting questions concern how the pieces cooperate.