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The Cerebrum
The cerebrum is divided into two major parts: the right and left cerebral hemispheres or halves at a fissure, the deep groove down the middle. The hemispheres communicate with each other through the corpus callosum which is a bundle of fibers between the hemispheres. Each hemisphere controls muscles and glands on the opposite side of the body (i.e. the right side of the brain or hemisphere controls the left side of the body.)

Comprising 85% of total brain weight, the cerebrum is the largest part of the brain and controls language, conscious thought, hearing, somatosensory or sense of touch functions, memory, personality development, and vision.

The cerebrum is a term often used to describe the entire brain. A fissure or groove that separates the two hemispheres is called the great longitudinal fissure. The two sides of the brain are joined at the bottom by the corpus callosum. The corpus callosum connects the two halves of the brain and delivers messages from one half of the brain to the other. The surface of the cerebrum contains billions of neurons and glia that together form the cerebral cortex.

The cerebral cortex, which is the most superficial part of the hemispheres and is only a few millimeters in thickness, is composed of gray matter, in contrast to the interior of the hemispheres, which is composed partly of white matter.

The exterior surface of the cerebrum, the cerebral cortex, is a convoluted (folded) grayish colored layer known as gray matter. The convolutions are made up of ridge like bulges (gyri) separated by small grooves called sulci and larger grooves called fissures. About 66% of the surface of the brain is hidden in the fold of the sulci with the total surface area being about 16 ft².

Gray matter which consists of unmyelinated nerve cell bodies, composes the cerebral cortex or outer portion of the cerebrum. Beneath the cortex lies the white matter or myelinated axons. During embryonic development, the cortex folds upon itself to form gyri (folds) and sulci (shallow grooves) so that more gray matter can reside within the skull cavity and giving the brain its distinctive look.

The cerebral cortex is the structure within the brain that plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. Gray matter is formed by neurons and their unmyelinated fibers, whereas the white matter below the gray matter of the cortex is formed predominantly by myelinated axons interconnecting different regions of the central nervous system (CNS). The human cerebral cortex is between 2 to 4 mm thick.

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The cerebral cortex is connected to various subcortical structures like the thalamus and the basal ganglia, sending information to them along efferent connections and receiving information from them via afferent connections. Most sensory information is routed to the cerebral cortex via the thalamus. Olfactory information, however, passes through the olfactory bulb to the olfactory cortex or piriform cortex. The vast majority of connections are from one area of the cortex to another rather than to subcortical areas.

Cortical regions known as associative cortex are responsible for integrating multiple inputs, processing the information, and carrying out complex responses. The cortex is commonly described as comprising three parts: Sensory Areas, Motor Areas, and Association Areas.

Sensory Areas are the areas that receive and process information from the senses. The parts of the cortex that receive sensory inputs from the thalamus are called primary sensory areas. The senses of vision, audition, and touch are served respectively by the primary visual cortex, primary auditory cortex and primary somatosensory cortex. In general, the two hemispheres receive information from the opposite (contralateral) side of the body.

Motor Areas are located in both hemispheres of the cortex. They are shaped like a pair of headphones stretching from ear to ear. The motor areas are very closely related to the control of voluntary movements, especially fine fragmented movements performed by the hand. The right half of the motor area controls the left side of the body, and vice versa.

Two areas of the cortex are commonly referred to as primary motor cortex, which executes voluntary movements, and supplementary motor areas and premotor cortex, which select voluntary movements.

Association Areas function to produce a meaningful perceptual experience of the world, enable us to interact effectively, and support abstract thinking and language. The parietal, temporal, and occipital lobes - all located in the posterior part of the cortex - organize sensory information into a coherent perceptual model of our environment centered on our body image. The frontal lobe or prefrontal association complex is involved in planning actions and movement, as well as abstract thought. Our language abilities are localized to the association areas of the parietal-temporal-occipital complex, typically in the left hemisphere. Wernicke's area relates to understanding language while Broca's area relates to its use.
The Cerebral Hemispheres
A longitudinal fissure or division separates the brain into two distinct cerebral hemispheres, connected by the corpus callosum. The sides resemble each other and each hemisphere's structure is generally mirrored by the other side. Yet despite the strong similarities, the functions of each cortical hemisphere are different.

Some areas of the cerebral hemispheres control muscular activity, and their nerve cells send processes to the brain stem and spinal cord, where they are connected with motor neurons, the processes of which leave by way of cranial nerves or ventral roots in the spinal cord. Other areas are sensory and receive impulses that have reached the spinal cord by way of peripheral nerves and dorsal roots, and have ascended in the spinal cord and brain stem by pathways that consist of a succession of nerve cells and their processes.

In general, the left hemisphere or side of the brain is responsible for language and speech. Because of this, it has been called the "dominant" hemisphere. The right hemisphere plays a large part in interpreting visual information and spatial processing. In about one third of individuals who are left-handed, speech function may be located on the right side of the brain. Left-handed individuals may need specialized testing to determine if their speech center is on the left or right side prior to any surgery in that area.

Research has determined that touching one side of the brain sends electrical signals to the other side of the body. Touching the motor region on the right side of the brain, would cause the opposite side or the left side of the body to move. Stimulating the left primary motor cortex would cause the right side of the body to move. The messages for movement and sensation cross to the other side of the brain and cause the opposite limb to move or feel a sensation. The right side of the brain controls the left side of the body and vice versa So if damage occurs on the right side of the brain that controls the movement of the arm, the left arm may be weak or paralyzed.


French neurosurgeon Pierre Paul Broca's research in 1861 gave one of the first indications of brain function lateralization. His research involved a man who suffered a speech deficit or aphasia. In the later autopsy of the man, Broca determined he had a syphilitic lesion in the left cerebral hemisphere. This left frontal lobe brain area, or Broca's Area, is an important speech production region. The motor aspects of speech production deficits caused by damage to Broca’s Area are known as Broca's aphasia. In clinical assessment of this aphasia, it's noted that the patient can't clearly articulate the language being employed.


German physician Karl Wernicke began pursuing his own research into the effects of brain disease on speech and language by studying language deficits unlike Broca aphasias. Wernicke noted that not every deficit was in speech production with some being linguistic. It was found that damage to the left posterior, superior temporal gyrus, or Wernicke's area, caused language comprehension deficits rather than speech production deficits, a syndrome known as Wernicke's aphasia.

The Brains Two Hemispheres
Left Brain Damage
The left half of the brain controls the right side of the body
Right Brain Damage
The right half of the brain controls the left side of the body
Left Side - Rational Side
Right Side - Intuitive Side
Understanding and use of language (listening, reading, speaking and writing)
Memory for spoken and written messages
Detailed analysis of information
Responds to verbal instructions
Solves problems by logically and sequentially looking at the parts
Looks at differences
Is planned and structured
Prefers established, certain information
Prefers talking and writing
Prefers multiple choice tests
Controls feelings
Prefers ranked authority structures
Judging the position of things in space
Knowing body position
Understanding and remembering things we do and see
Putting information together for the big picture
Responds to demonstrated instructions
Problem solves with hunches, looking for patterns and configurations
Looks at similarities
Is fluid and spontaneous
Prefers elusive, uncertain information
Prefers drawing and manipulating objects
Prefers open ended questions
Free with feelings
Prefers collegial authority structures
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