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radiology

The Stroke Center at University Hospital

Anatomy of the Brain

In order to understand how stroke occurs and the damage it can cause, it is useful to understand the basic anatomy of the brain. The signs and symptoms of a stroke depend on which region of the brain is affected and how severely.

The brain has three primary components. Each is responsible for different functions:

The Cerebrum: The cerebrum is the largest and most developmentally advanced portion of the brain. It controls a number of higher functions, including speech, emotion, the integration of sensory stimuli, initiation of the final common pathways for movement, and fine control of movement.

It is divided into a left and a right hemisphere. The left hemisphere controls the majority of functions on the right side of the body, while the right hemisphere controls most of functions on the left side of the body. Thus, injury to the left cerebral hemisphere produces sensory and motor deficits on the right side, and vice versa.

The cerebrum is composed of the frontal, parietal, temporal, and occipital lobes:

  • The frontal lobe is involved in planning, organizing, problem solving and selective attention. The portion known as the prefrontal cortex controls personality and various higher cognitive functions such as behavior and emotions. The back of the frontal lobe consists of the pre-motor and motor areas, which produce and modify movement.
  • The left and right parietal lobes contain the primary sensory cortex, which controls sensation (touch and pressure), and a large association area that controls fine sensation (judgment of texture, weight, size, and shape). Damage to the right parietal lobe can cause visuo-spacial deficits, making it hard for the patient to find his/her way around new or even familiar places. Damage to the left parietal lobe may disrupt a patient’s ability to understand spoken and/or written language.
  • The left and right temporal lobes, located around ear level, allow a person to differentiate smells and sounds. They also help in sorting new information and are believed to be responsible for short-term memory. The right lobe is primarily involved in visual memory (i.e., memory for faces and pictures). The left lobe is primarily involved in verbal memory (i.e., memory for words and names).
  • The occipital lobe processes visual information. It is mainly responsible for visual reception and contains association areas that help in the visual recognition of shapes and colors. Damage to this lobe can cause visual deficits.

The Cerebellum: The cerebellum is the second largest area of the brain. It controls reflexes, balance and certain aspects of movement and coordination.

The Brain Stem: The brain stem is responsible for a variety of automatic functions that are critical to life, such as breathing, digestion and heart beat – as well as alertness and arousal (the state of being awake).

Symptoms Point to Stroke Location

A stroke can occur anywhere in the brain or just outside it. The symptoms that a stroke victim experiences depend on which area(s) of the brain are involved.

When a stroke occurs in the right hemisphere of the cerebrum, the result may be paralysis on the left side of the body, difficulty reasoning or thinking out solutions to even the simplest problem. A stroke in the left hemisphere can result in paralysis of the right side of the body and may disrupt the ability to speak.

A stroke involving the cerebellum may result in a lack of coordination (ataxia), clumsiness and balance problems, shaking, or other muscular difficulties. This can interfere with a person’s ability to walk, talk, eat and perform other self-care tasks.

Brain stem strokes are the most devastating and life threatening because they can disrupt the involuntary functions essential to life. People who survive may remain in a vegetative state or be left with severe impairments.

Blood Flow to the Brain

The heart pumps oxygen- and nutrient-laden blood to the brain, face, and scalp via two major sets of vessels: the carotid arteries and the vertebral arteries. The jugular and other veins bring blood out of the brain.

The carotid arteries run along the front of the neck – one on the left and one on the right. They are what you feel when you take your pulse just under your jaw. The carotid arteries split into external and internal arteries near the top of the neck.

The external carotid arteries supply blood to the face and scalp. The internal carotid arteries supply blood to the front (anterior) three-fifths of cerebrum, except for parts of the temporal and occipital lobes.

The vertebral arteries travel along the spinal column and cannot be felt from the outside. They join to form a single basilar artery (hence the name vertebrobasilar arteries) near the brain stem at the base of the skull. The arteries supply blood to the posterior two-fifths of the cerebrum, part of the cerebellum, and the brain stem.

Because the brain relies on only two sets of major arteries for its blood supply, it is very important that these arteries are healthy. Often when an ischemic stroke occurs, the carotid or vertebral artery system is blocked with a fatty buildup called plaque, allowing little or no blood to flow to the brain. During a hemorrhagic stroke, an artery in or on the surface of the brain has ruptured or is leaking, causing bleeding and damage in or around the brain.

These arteries that conduct blood to the brain — the internal-carotid and vertebral arteries — connect through the Circle of Willis, which loops around the brainstem at the base of the brain. From this circle, other arteries — the anterior cerebral artery (ACA), the middle cerebral artery (MCA), and the posterior cerebral artery (PCA) — arise and travel to all parts of the brain.

Because the carotid and vertebrobasilar arteries form a circle, if one of the main arteries is blocked, the smaller arteries that the circle supplies can receive blood from the other arteries. This phenomenon is called collateral circulation.

Collateral circulation is a process in which small (normally closed) arteries open up and connect two larger arteries or different parts of the same artery. They can serve as alternate routes of blood supply. Sometimes when an artery in the brain is blocked due to ischemic stroke or transient ischemic attack (TIA), open “collateral” vessels can allow blood to "detour" around the blockage, restoring blood flow to the affected part of the brain. Everyone has collateral vessels, at least in microscopic form. These vessels normally aren't open. However, they grow and enlarge in some people with coronary heart disease or other blood vessel disease. While everyone has collateral vessels, they don't open in all people.

The Circle of Willis has a downside, however. Cerebral aneurysms tend to occur at the junctions between the arteries that make up the Circle.