The Anatomy of the Brain

The brain is a unique organ that is responsible for many functions such as problem-solving, thinking, emotions, controlling physical movements, and mediating the perception and responses related to the five senses. The many nerve cells of the brain communicate with each other to control this activity.

Each area of the brain has one or more functions. The skull, which is composed of bone, protects the brain. A number of different health conditions can affect the brain, including headaches, seizures, strokes, multiple sclerosis, and more. These conditions can often be managed with medical or surgical care.

Anatomy

The brain is primarily composed of nerve cells, which are also called neurons. Blood vessels supply oxygen and nutrients to the neurons of the brain. Cerebrospinal fluid (CSF), a fluid that provides nourishment and immune protection to the brain, flows around the brain and within the ventricular system (spaces between the regions of the brain).

The brain and the CSF are protected by the meninges, composed of three layers of connective tissue: the pia, arachnoid, and dura layers. The skull surrounds the meninges.

Structure

The brain has many important regions, such as the cerebral cortex, brainstem, and cerebellum. The areas of the brain all interact with each other through hormones and nerve stimulation.

The regions of the brain include:

• Cerebral cortex: This is the largest portion of the brain. It includes two hemispheres (halves), which are connected to each other—physically and functionally—by the corpus callosum. The corpus callosum runs from the front of the cerebral cortex to the back of the cerebral cortex. The outer part of the cerebral cortex is often described as gray matter, and the deeper areas are often described as white matter due to their microscopic appearance.
• Lobes of the cerebral cortex: Each hemisphere of the cerebral cortex is composed of four lobes. The frontal lobes are the largest, and they are located at the front of the brain. The temporal lobes are located on the sides of the brain, near and above the ears. The parietal lobes are at the top middle section of the brain. And the occipital lobes, which are the smallest lobes, are located in the back of the cerebral cortex.
• Limbic system: The limbic system is located deep in the brain and is composed of several small structures, including the hippocampus, amygdala, thalamus, and hypothalamus.
• Internal capsule: This area is located deep in the brain and is considered white matter. The frontal regions of the cerebral cortex surround the left and right internal capsules. The internal capsule is located near the lateral ventricles.
• Thalamus: The left and right thalami are below the internal capsule, above the brainstem, and near the lateral ventricles.
• Hypothalamus and pituitary gland: The hypothalamus is a tiny region of the brain located directly above the pituitary gland. The pituitary gland is a structure that extends directly above the optic chiasm, where the optic nerves meet.
• Brainstem: The brainstem is the lowest region of the brain and is continuous with the spinal cord. It is composed of three sections: the midbrain, pons, and medulla. The cranial nerves emerge from the brainstem.
• Cerebellum: The cerebellum is located at the lower back of the brain, under the occipital lobe and behind the brainstem. It has two hemispheres (left and right) that are connected by a middle structure called the vermis.
• Blood vessels: The blood vessels that supply your brain include the anterior cerebral arteries, middle cerebral arteries, posterior cerebral arteries, basilar artery, and vertebral arteries. These blood vessels and the blood vessels that connect them to each other compose a collection of blood vessels described as the circle of Willis.
• Ventricular system: CSF flows in the right and left lateral ventricles, the third ventricle, the cerebral aqueduct, the fourth ventricle, and down into the central canal in the spinal cord.

Function

The brain has a number of functions, including motor function (controlling the body’s movements), coordination, sensory functions (being aware of sensations), hormone control, regulation of the heart and lungs, emotions, memory, behavior, and creativity.

These functions often rely on and interact with each other. For example, you might experience an emotion based on something that you see and/or hear. Or you might try to solve a problem with the help of your memory. Messages travel very quickly between the different regions in the brain, which makes the interactions almost instantaneous.

Functions of the brain include:

• Motor function: Motor function is initiated in an area at the back of the frontal lobe called the motor homunculus. This region controls movement on the opposite side of the body by sending messages through the internal capsule to the brainstem, then to the spinal cord, and finally to a spinal nerve through a pathway described as the corticospinal tract.
• Coordination and balance: Your body maintains balance and coordination through a number of pathways in the cerebral cortex, cerebellum, and brainstem.
• Sensation: The brain receives sensory messages through a pathway that travels from the nerves in the skin and organs to the spine, then to the brainstem, up through the thalamus, and finally to an area of the parietal lobe called the sensory homunculus, which is directly behind the motor homunculus. Each hemisphere receives sensory input from the opposite side of the body. This pathway is called the spinothalamic tract.
• Vision: Your optic nerves in your eyes can detect whatever you see, sending messages through your optic tract (pathway) to your occipital lobes. The occipital lobes put those messages together so that you can perceive what you are seeing in the world around you.
• Taste and smell: Your olfactory nerve detects smell, while several of your cranial nerves work together to detect taste. These nerves send messages to your brain. The sensations of smell and taste often interact, as smell amplifies your experience of taste.
• Hearing: You can detect sounds when a series of vibrations in your ear stimulate your vestibulocochlear nerve. The message is sent to your brainstem and then to your temporal cortex so that you can make sense of the sounds that you hear.
• Language: Speaking and understanding language is a specialized brain function that involves several regions of your dominant hemisphere (the side of the brain opposite your dominant hand). The two major areas that control speech are Wernicke’s area, which controls the understanding of speech, and Broca’s area, which controls the fluency of your speech.
• Emotions and memory: Your amygdala and hippocampus play important roles in storing memory and associating certain memories with emotion.
• Hormones: Your hypothalamus, pituitary gland, and medulla all respond to the conditions of your body, such as your temperature, carbon dioxide level, and hormone levels, by releasing hormones and other chemicals that help regulate your body’s functions. Emotions such as fear can also have an influence on these functions.
• Behavior and judgment: The frontal lobes control reasoning, planning, and maintaining social interactions. This area of the brain is also involved in judgment and maintaining appropriate behavior.
• Analytical thinking: Mathematical problem solving is located in the dominant hemisphere. Often, this type of reasoning involves interaction with the decision-making regions of the frontal lobes.
• Creativity: There are many types of creativity, including the production of visual art, music, and creative writing. These skills can involve three-dimensional thinking, also described as visual-spatial skills. Creativity also involves analytical reasoning and usually requires a balance between traditional ways of thinking (which occurs in the frontal lobes) and "thinking outside the box."

Associated Conditions

There are many conditions that can affect the brain. You may experience self-limited issues, such as the pain of a headache, or more lasting effects of brain disease, such as paralysis due to a stroke. The diagnosis of brain illnesses may be complex and can involve a variety of medical examinations and tests, including a physical examination, imaging tests, neuropsychological testing, electroencephalography (EEG), and/or lumbar puncture.

Common conditions that involve the brain include:

• Headaches: Head pain can occur due to chronic migraines or tension headaches. You can also have a headache when you feel sleepy, stressed, or due to an infection like meningitis (an infection of the meninges).
• Traumatic brain injury: An injury to the head can cause damage such as bleeding in the brain, a skull fracture, a bruise in the brain, or, in severe cases, death. These injuries may cause vision loss, paralysis, or severe cognitive (thinking) problems.
• Concussion: Head trauma can cause issues like loss of consciousness, memory impairment, and mood changes. These problems may develop even in the absence of bleeding or a skull fracture. Often, symptoms of a concussion resolve over time, but recurrent head trauma can cause serious and persistent problems with brain function, described as chronic traumatic encephalopathy (CTE).
• Transient ischemic attack (TIA): A temporary interruption in the blood supply to the brain can cause the affected areas to temporarily lose function. This can happen due to a blood clot, usually coming from the heart or carotid arteries. If the interruption in blood flow resolves before permanent brain damage occurs, this is called a TIA. Generally, a TIA is considered a warning that a person is at risk of having a stroke, so a search for stroke causes is usually necessary—and stroke prevention often needs to be initiated.
• Stroke: A stroke is brain damage that occurs due to an interruption of blood flow to the brain. This can occur due to a blood clot (ischemic stroke) or a bleed in the brain (hemorrhagic stroke). There are a number of causes of ischemic and hemorrhagic stroke, including heart disease, hypertension, and brain aneurysms.
• Brain aneurysm: An aneurysm is an outpouching of a blood vessel. A brain aneurysm can cause symptoms due to pressure on nearby structures. An aneurysm can also bleed or rupture, causing a hemorrhage in the brain. Sometimes an aneurysm can be surgically repaired before it ruptures, preventing serious consequences.
• Dementia: Degenerative disease of the regions in the brain that control memory and behavior can cause a loss of independence. This can occur in several conditions, such as Alzheimer’s disease, Lewy body dementia, Pick’s disease, and vascular dementia (caused by having many small strokes).
• Multiple sclerosis (MS): This is a condition characterized by demyelination (loss of the protective fatty coating around nerves) in the brain and spine. MS can cause a variety of effects, such as vision loss, muscle weakness, and sensory changes. The disease course can be characterized by exacerbations and remissions, a progressive decline, or a combination of these processes.
• Parkinson’s disease: This condition is a progressive movement disorder that causes tremors of the body (especially the arms), stiffness of movements, and a slow, shuffling pattern of walking. There are treatments for this condition, but it is not curable.
• Epilepsy: Recurrent seizures can occur due to brain damage or congenital (from birth) epilepsy. These episodes may involve involuntary movements, diminished consciousness, or both. Seizures usually last for a few seconds at a time, but prolonged seizures (status epilepticus) can occur as well. Anti-epileptic medications can help prevent seizures, and some emergency anti-epileptic medications can be used to stop a seizure while it is happening.
• Meningitis or encephalitis: An infection or inflammation of the meninges (meningitis) or the brain (encephalitis) can cause symptoms such as fever, stiff neck, headache, or seizures. With treatment, meningitis usually improves without lasting effects, but encephalitis can cause brain damage, with long-term neurological impairment.
• Brain tumors: A primary brain tumor starts in the brain, and brain tumors from the body can metastasize (spread) to the brain as well. These tumors can cause symptoms that correlate to the affected area of the brain. Brain tumors also may cause swelling in the brain and hydrocephalus (a disruption of the CSF flow in the ventricular system). Treatments include surgery, chemotherapy, and radiation therapy.

Tests

If you have a condition that could be affecting your brain, there are a number of complex tests that your medical team may use to identify the problem. Most important, a physical exam and mental status examination can determine whether there is any impairment of brain function and pinpoint the deficits. For example, you may have weakness of one part of the body, vision loss, trouble walking, personality or memory changes, or a combination of these issues. Other signs, such as rash or fever, which are not part of the neurological physical examination, can also help identify systemic issues that could be causing your symptoms.

Diagnostic tests include brain imaging tests such as computerized tomography (CT), magnetic resonance imaging (MRI), or functional magnetic resonance imaging (fMRI). These tests can identify structural and functional abnormalities. And sometimes, tests such as CT angiography (CTA), MRI angiography (MRA), or interventional cerebral angiography are needed to visualize the blood vessels in the brain.

Another test, an evoked potential test, can be used to identify hearing or vision problems in some circumstances. And a lumbar puncture may be used to evaluate the CSF surrounding the brain. This test can detect evidence of infection, inflammation, or cancer. Rarely, a brain biopsy is used to sample a tiny area of the brain to assess the abnormalities.