Cerebral hemisphere

The cerebral hemispheres are the opposite halves of the human brain. The two sides are approximately equal in size and each controls the opposite side of the body. The right hemisphere controls the muscles on the body's left side, and the left hemisphere controls the muscles on the body's right side. Each hemisphere also has different functions. In general, the left hemisphere handles language processing, science, math, logic, and memory retrieval. The right hemisphere controls creativity, spatial perception, artistic ability, and musical skills. The hemispheres are connected to each other by a bundle of nerve fibers that allows the two sides to communicate. Despite their different functions, the cerebral hemispheres work together in most of the body's processes. Each side also has the ability to adjust and take over functions of the other if that hemisphere is damaged.

Background

The average human brain weighs about three pounds. It is a gelatinous organ that makes up about 2 percent of a person's body weight. The brain functions as the control center for the human nervous system, which consists of the brain, spinal cord, and peripheral nerves. The brain oversees both the voluntary and involuntary functions of the body, such as regulating blood pressure and heart rate, coordinating physical movement, and processing information through the senses. The brain contains about 100 billion nerve cells called neurons. Neurons transmit electrochemical signals that act as instructions to the body, telling it how to function.

The brain is divided into several sections. The brainstem is the oldest section of the brain, first evolving in the distant ancestors of humans. The brainstem connects the upper sections of the brain to the spinal cord and consists of the medulla, pons, and midbrain. This section is responsible for many automatic body functions such as making the heart beat, breathing, regulating sleep cycles, sneezing, swallowing, and digestion. The limbic system is a series of structures within the brain that controls basic memory, learning, and emotional responses. The cerebellum is the second largest section and is located near the back of the brain. It is often called the "little brain" and coordinates muscle movements, motor control, and balance.

Overview

The cerebrum is the topmost and largest section of the brain. It contains about 70 percent of the brain's neurons and makes up about 85 percent of its weight. The outer layer of the cerebrum is a folded expanse of neurons called the cerebral cortex. Its grayish color is why it is often referred to as gray matter. The cerebrum is responsible for higher brain functions such as vision, speech, reasoning, emotional control, learning, and fine motor skills. A deep fissure runs through the center of the cerebrum, separating the brain into right and left hemispheres, or halves. These hemispheres are roughly symmetrical; they oversee different functions to make the brain more efficient and capable of performing simultaneous tasks. The process, called lateralization, gives the left side of the brain control over the actions of the right side of the body and vice versa. If a person raises his or her right hand, the action is dictated by the left hemisphere of the brain. If one of the hemispheres is damaged, such as by a stroke or a brain tumor, the opposite side of the body is affected.

While the cerebral hemispheres share many tasks, they also have different functions, coordinating with each other through a thick network of nerve tissue called the corpus callosum. The right hemisphere is generally the more creative side, focusing on art and music, humor, intuition, nonverbal skills, three-dimensional recognition, and visual comprehension. The left hemisphere tends to be more logic-based, handling tasks such as math, science, writing, reasoning, language skills, and analytic thought. The idea that individuals proficient in math are "left brained" and artists are "right brained" has been proven by scientists to be a myth. People who gravitate toward an artistic or scientific field tend to do so because of complex psychological and environmental reasons, not brain chemistry.

The specialized features of both hemispheres work in tandem to accomplish any task. For example, in the process of reading a sentence, the left hemisphere is responsible for recognizing word order and grammar while the right hemisphere interprets its meaning. In other words, the left side may perceive the phrase "I could eat a horse" as having the proper words and sentence structure; the right side perceives it as a figure of speech implying hunger and not a literal desire to eat a horse. The right side also interprets changes in tone, allowing people to recognize when a sentence is a question or a command.

Each hemisphere is also further divided into four sections, or lobes, each with its own specialized functions. The frontal lobes behind the forehead handle voluntary motor skills, personality, and judgment. The parietal lobes near the top of the head sense pain and interpret spatial and language signals. The temporal lobes near the temples process hearing, memory, and language comprehension. The occipital lobes at the back of the cerebrum process visual information such as light and color. As with the right and left hemispheres, the four lobes use their specializations to work together to help the body function.

In cases where the brain is severely damaged either by injury, illness, or developmental changes, it has shown the ability to "rewire" itself by altering its neural connections. This process, called neuroplasticity, allows the brain to compensate and reroute tasks from one section to another. For example, if a stroke damages the language center of the left hemisphere, the tasks normally associated with that area may be taken over by the right hemisphere. The effectiveness of the neuroplasticity depends on the severity of the injury. In some cases, people have managed to live relatively normal lives missing a whole section of their brain or even an entire hemisphere.

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