A Macro View Of The Brain - How The Brain Works: Stroke

Content provided by the Faculty of the Harvard Medical School

A macro view of the brain

The human brain is the product of millions of years of evolution. It has three interconnected levels: the brainstem and cerebellum, the limbic system, and the cerebral cortex. The brainstem, an extension of the spinal cord, is the oldest part of the brain in evolutionary terms. It connects the rest of the brain to the spinal cord and regulates blood pressure, breathing, chewing, swallowing, and eye movements. At the top of the brainstem is the thalamus, which relays sensory information to other parts of the brain. At the back, near the brainstem, lies the cerebellum, which is responsible for maintaining balance and posture and coordinating movement.

The cerebellum's role is crucial. For example, when you are learning a new, complex skill such as playing the violin, the larger, domed part of the brain known as the cerebral cortex helps you understand the process of playing the violin. But with practice, it is the cerebellum that helps you accomplish real mastery and makes the activity second nature.

The limbic system, a step up the evolutionary ladder from the brainstem, is a group of related structures that help regulate emotion, memory, and certain aspects of movement. One of these structures, the hippocampus, is vital to the storage of recently acquired information — one of the brain's most important functions. Damage to the hippocampus can destroy the ability to learn new information or cause the loss of recent knowledge. Emotions such as fear, anger, and pleasure are stored nearby in the amygdala; damage to this structure can erase emotion-charged memories, such as details of your wedding day or a family reunion. Deep in the brain, clusters of neurons called basal ganglia play an important role in movement.

At the top of the evolutionary ladder sits the cerebral cortex, which is involved in all forms of conscious experience. It is the large, dome-shaped mass of gray matter that most people imagine when they picture the human brain. The cerebral cortex covers the top and the outermost sides of the brain and is divided into left and right hemispheres. The two halves communicate through a vital superhighway of neurons called the corpus callosum.

Each hemisphere of the cerebral cortex consists of four lobes. Although there is some overlap among these sections, each has distinct functions. The front portion is the frontal lobe, which controls motor function, planning, organization skills, and the expression of language. Behind it lies the parietal lobe; the left side of it enables you to understand language and do mathematical calculations and the right side controls tasks that involve visual and spatial orientation. Farther back is the occipital lobe, which perceives and interprets vision. Wrapped around the sides is the temporal lobe, which is involved in hearing, long-term memory, and behavior (see Figure 3).

Figure 3: Functions of the brain Each hemisphere of the brain is divided into four specialized lobes. When stroke damages an area within one of these lobes, the following functions may be impaired: The frontal lobe controls motor function, planning, and speech, while the parietal lobe interprets sensation and coordinates movement. The occipital lobe perceives and interprets vision, and the temporal lobe is involved in hearing, long-term memory, and behavior.

The visual cortex, where vision is processed, is in the occipital lobe, in the back of the cerebral cortex. The right visual cortex controls the left visual field for both eyes, and the left visual cortex controls the right visual field. That's why defective vision or blindness in half the visual field of one or both eyes — a condition called hemianopia — is a common consequence of stroke.

The left side of the brain usually controls activity on the right side of the body, and the right side of the brain controls the left side of the body. Damage to the left hemisphere can produce paralysis or loss of sensation on the right side of the body, and vice versa. Such impairment on one side of the body is one of the hallmarks of stroke. Weakness that is limited to one side is called hemiparesis; paralysis on one side of the body is known as hemiplegia.

Speech centers are on the left side of the brain. Motor speech (the physical movements of the mouth, tongue, and lips) is formulated in Broca's area of the frontal lobe, while understanding written and spoken words occurs in Wernicke's area. These two regions are in constant communication through a dense bundle of neurons.

Last updated:	September 05, 2008