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What is the brain?

The brain is an incredibly intricate organ responsible for managing a wide range of bodily functions, including thinking, memory, emotions, movement, sensory perception, breathing, temperature control, and hunger. Together with the spinal cord, which extends from it, the brain forms the central nervous system (CNS), the control center of the body.

What is the brain made of?

In an average adult, the brain weighs roughly three pounds and is composed of about 60% fat. The rest is a mixture of water, proteins, carbohydrates, and minerals. Unlike muscles, the brain consists of blood vessels, nerve cells called neurons, and support cells known as glial cells.

Gray matter vs. white matter

The central nervous system consists of two main types of tissue: gray matter and white matter. In the brain, gray matter forms the outer layer and appears darker, while white matter is found deeper inside and looks lighter. In the spinal cord, the arrangement is reversed, with white matter on the outside and gray matter inside.

Gray matter mainly contains the cell bodies of neurons — the round centers of nerve cells. White matter mostly consists of long nerve fibers called axons, which are coated in a protective layer called myelin that speeds up signal transmission. This difference in structure is what causes their distinct appearances on brain scans.

Gray matter’s main role is to process and interpret information, while white matter acts like a highway, sending signals between different brain regions and other parts of the nervous system.

How does the brain function?

The brain communicates with the rest of the body by sending and receiving electrical and chemical signals. These signals regulate various functions — some make you feel tired, others alert you to pain, or control muscle movements. Some signals stay within the brain, but many travel through the spinal cord and a vast network of nerves to reach limbs and organs. This communication depends on billions of neurons transmitting information rapidly and efficiently.

Major parts of the brain and their roles

Broadly, the brain is divided into three key areas: the cerebrum, the brainstem, and the cerebellum.

• Cerebrum: The largest section of the brain, located at the top, includes the cerebral cortex (gray matter on the surface) and white matter beneath. The cerebrum is responsible for initiating movement, regulating body temperature, and governing higher functions like speech, reasoning, emotions, learning, and sensory perception such as vision and hearing.
  
  
• Cerebral Cortex: This outer layer of gray matter, named for its bark-like appearance, is folded to increase its surface area and makes up about half the brain’s weight. It is split into two halves (hemispheres), each controlling the opposite side of the body. The two hemispheres connect via a thick band of white matter called the corpus callosum, which allows them to communicate.
  
  
• Brainstem: Located beneath the cerebrum, the brainstem links the brain to the spinal cord and consists of three parts: the midbrain, pons, and medulla.
  
  
  • The midbrain handles functions like hearing, movement, and environmental awareness.
    
    
  • The pons acts as a bridge connecting parts of the brain and helps control facial expressions, eye movement, balance, and hearing.
    
    
  • The medulla manages vital involuntary functions such as heartbeat, breathing, and reflexes like sneezing and swallowing.
    
    
• Cerebellum: Often called the “little brain,” the cerebellum sits at the back of the head beneath the cerebrum. It has two hemispheres and helps coordinate voluntary movements, maintain posture, balance, and equilibrium. Recent research also suggests it may play a role in cognitive functions and emotional regulation.

Protective layers around the brain: the meninges

The brain and spinal cord are protected by three layers called the meninges:

• The dura mater is the tough, outermost layer that lines the inside of the skull.
  
  
• Beneath it lies the arachnoid mater, a thin, web-like layer without blood vessels or nerves.
  
  
• The innermost layer, the pia mater, closely wraps the brain’s surface, following every contour and containing blood vessels.
  
  

Between the arachnoid and pia mater flows cerebrospinal fluid, which cushions the brain and spinal cord while removing waste.

Brain lobes and their functions

Each cerebral hemisphere is divided into four lobes, each controlling specific functions:

• Frontal lobe: Located at the front of the brain, it governs personality, decision-making, movement, and parts of speech and smell.
  
  
• Parietal lobe: Positioned near the center, it processes sensory information like touch, pain, and spatial awareness.
  
  
• Occipital lobe: Located at the back, it handles visual processing.
  
  
• Temporal lobe: Found on the sides, it’s involved in hearing, short-term memory, language comprehension, and recognizing smells.

Deeper brain structures

• The pituitary gland, a small but vital structure deep in the brain, regulates hormones that control other glands in the body.
  
  
• The hypothalamus, sitting above the pituitary, manages body temperature, hunger, thirst, sleep, and emotional responses.
  
  
• The amygdala, almond-shaped clusters beneath each hemisphere, play a key role in emotions, memory, stress, and the fight-or-flight response.
  
  
• The hippocampus, shaped like a seahorse, supports memory, learning, and navigation.
  
  
• The pineal gland controls sleep-wake cycles by releasing melatonin in response to light changes.

Ventricles and cerebrospinal fluid

Inside the brain are four connected cavities called ventricles, which produce cerebrospinal fluid (CSF). This fluid circulates around the brain and spinal cord, cushioning them and removing waste.

Blood supply

The brain receives blood through two main artery groups: the carotid arteries (supplying the front) and vertebral arteries (feeding the back). These vessels form a circular network at the brain’s base, ensuring continuous blood flow.

Cranial nerves

There are 12 pairs of cranial nerves that originate either in the cerebrum or brainstem. These nerves control functions like smell, vision, eye movement, facial sensation, hearing, balance, taste, swallowing, and tongue movement.