How Does the Human Brain Work?
Zoom out: The big picture
Your brain is physically connected to the rest of your body. The brain and spinal cord make up the central nervous system, while the network of related cells in the rest of your body is the peripheral nervous system.
Your eyes, ears, nose, tongue, and skin constantly send information to your brain about what their sensory receptors detect in the world around you. In a similar process, nerve cells inside your muscles and organs report to your brain about internal conditions, such as what your muscles are doing, the rate of your pulse and breathing, and how warm, full, and hydrated you are. These nerve cells also communicate subtle indicators like your balance and your hormone and blood-sugar levels.
When you think, networks of cells send signals throughout your brain. These networks integrate new information from your senses with emotions, habitual thought processes, memories, and context to drive decisions. Your brain enacts those decisions consciously and subconsciously by communicating muscle signals and other directions to cells throughout your body.
For example, when you see a friend's face, networks of nerve cells get to work. Your brain uses a few quick measurements to check who the friend is, notes how your body involuntarily responds to seeing them, generates an emotional response, puts the sight of them in context with memories and current events, chooses a response, and, perhaps, instructs your arm and face to wave and smile.
Many processes happen unconsciously without our attention. Dreams and some thought processes go on without conscious direction. Your brain and spinal cord send signals that regulate processes in your organs, including pupil dilation and digestion. Reflexes are another example: When you step on something sharp, neurons in your lower spine signal your hip and thigh to retract and tell your other leg to hold you up.
As you grew from infancy to the person you are today, the things you sensed, your experiences, and your choices and reflections have changed your brain, developing its unique cellular pathways.
Primary parts of the brain
In some respects, your brain is unlike any other, with connections and other physical characteristics that reflect your own experiences. However, most human brains have major parts in common.
Your cerebrum is the largest part of your brain. It is made of two connected halves, or hemispheres. Scientists divide each side of the brain into four parts that are approximately located on the front, top, back, and side of the head: the frontal, parietal, occipital, and temporal lobes, respectively.
Each half sends and receives information related to the opposite side of your body. Networks of cells in the cerebrum help us recognize and process sights, sounds, smells, tastes, and touch. The cerebrum is also important in our use of language and in what we consider thinking—planning, association to memory and emotion, and decision-making.
Underneath your cerebrum is the cerebellum, which also has two connected sides. Researchers have estimated that more than half the neurons in the brain are in the cerebellum. It coordinates and corrects our movements, keeps us balanced, helps us learn and recall sequences of movements (such as those involved in walking or playing a sport), and contributes to other areas of thought such as language processing.
The wrinkled outermost portion of the cerebrum and the cerebellum are most of the brain's cortex, called the cerebral cortex and cerebellar cortex. Cortices are major hubs for processing information. The cortex and cerebellum also transmit signals to other parts of the brain and to the spinal cord.
Deep inside your brain are several parts that have long been described as the limbic system. It helps govern our hormones, appetite, thirst, desire, fear, feelings of reward, emotions, learning, memory, navigation, and sleep rhythms. Components of this system include the hippocampus, amygdala, hypothalamus, cingulate gyrus, and hypothalamus. Some researchers would also include parts of the thalamus and basal ganglia.
At the base of the brain is the brain stem, which connects to the spinal cord. The brain stem helps regulate essential survival functions such as breathing, pulse, swallowing, digestion, and sleep.
A little more than half a cup of cerebrospinal fluid fills hollows in and under the brain and surrounds it and the spinal cord. This slowly refreshed fluid lowers pressure on the bottom of the brain, absorbs shocks to prevent injuries, washes the brain in nutrients and chemicals brought by blood vessels, and flushes away waste from neurons' metabolism.
Cells specialized for defense line the walls of most incoming blood vessels in the brain and spinal cord. The cells let oxygen and nutrients into the cerebrospinal fluid and the brain but keep out bacteria, viruses, and toxic chemicals that may be circulating in the blood. This defense, called the blood–brain barrier, is hardly perfect: caffeine, alcohol, psychoactive drugs, and some medicines, viruses, bacteria, and toxins can get through it.
Zoom in: How the brain works at the cellular level
Two main types of cells work together in our brains, spinal cords, and peripheral nervous systems: neurons and glia.
Neurons
Neurons use chemicals and electricity to receive and send signals throughout the nervous system. You have tens of billions of these cells. The typical neuron has a cell body, dendrites that branch off the cell body and act as receivers, and a wire-like projection called an axon. Axons range in length from millimeters to a meter, and they also have branches.
Neurons work in groups called neural circuits, each related to specific functions. These groups and their organization evolve over time to support our changing lives. You can read more in this explainer about neurons and how they communicate.
Trivia bonus: White matter gets its color from myelin sheaths that surround axons. The myelin sheath allows signals to travel faster along the axon. White matter is found where longer axons make connections inside the brain and in the outer regions of the spinal cord. Most of the axons in white matter are part of cells whose bodies and dendrites are in gray matter (which is rosy tan when you are alive). Gray matter contains more cell bodies and dendrites, and axons that communicate within the gray matter are often shorter and have less myelin coating. Gray matter is made up of layers, like the layers of cerebral cortex, and clusters of neurons. The clusters are called nuclei and are present inside the cerebrum and the brain stem.
Glia
Glia, or glial cells, outnumber neurons in our brains, spinal cords, and peripheral nervous systems. Glia do not send electrical signals as neurons do. That may be why researchers once thought glia were a form of glue or packing material.
Scientists are learning that glia do far more. Here are just a few of their functions:
- Ependymal cells make cerebrospinal fluid and beat their cilia in rhythm to stir it.
- Microglia prune neural circuits, refine their function, check for and ingest pathogens and dying cells, and respond to injuries, strokes, and inflammation.
- Schwann cells and oligodendrocytes form the sheaths on neurons' axons that help them signal faster.
- Astrocytes bring nutrients from blood vessels to the brain, make substances that neurons need to survive, balance pH in the brain, regulate blood flow to the brain by making chemicals that dilate or constrict blood vessels, and transfer waste into the blood for disposal.
- Satellite glial cells surround neurons outside of the central nervous system to protect them and control their immediate environment.
- And, after all, glia do serve as packing material for neurons.
