The Forebrain
The forebrain is the most developed part of the human brain (See Diagram 2). Many structures are contained in the forebrain, but only the cerebrum, thalamus, hypothalamus, and basal ganglia are discussed here. The large cerebrum is divided in two by the great longitudinal fissure into two cerebral hemispheres (right and left brain). The longitudinal fissure is a cleft that runs from the forehead to the back of the head through the cerebrum, dividing it in two. The surface of the cerebrum is highly convoluted and grey in appearance and is specifically known as the cerebral cortex. The gray color is due to the presence of cell types (neuronal cell bodies, neuropil and glial cells) found in the nervous system (the cellular region of the brain is sometimes called). Glial cells are non-neuronal cells that maintain and provide nutrition and support for neurons. Neuropil lie between neuronal cell bodies consisting of axon cell bodies, dendrites, and glial cell bodies. The dense folds of the cerebral cortex extend over two square meters and act to increase the surface area, thus increasing the volume of cells in the forebrain. The more cells of the cerebral cortex, the better the ability to perceive, remember, and perform all higher cognitive functions, including concentration, reasoning and abstract thought, which are all attributed to the cerebral cortex.
Below the gray surface cerebral cortex is white matter, which is composed of myelinated or insulated neurons.
The two hemispheres of the cerebrum are united by the corpus callosum, which can be identified as a white arch-like structure (made of white matter) that links the two cerebral hemispheres. White matter is composed of myelinated or insulated neurons. The white matter nerve fibers of the corpus callosum connect the grey matter of the two cerebral hemispheres to allow the communication between the right and left brain. It has been suggested that the difference in size of the corpus callosum between men and women (with a women's being larger) may explain the lay belief that woman are more intuitive. The increase in "cross-talk" between the right brain (associated with creativity) and the left brain (associated with linear thinking) may be a basis for some gender differences.
Each of the cerebral hemispheres can further be divided into four lobes: the frontal, parietal, occipital and temporal lobes. The frontal lobe is at the front of the head, parietal lobe is on top of the head, occipital lobe is at the rear of the head, and temporal lobe is on the sides of the head (See Diagram 1).
The organization of functions within the cerebral hemisphere is highly localized. Language functions (speech, reading, writing and calculation) are organized in the frontal, parietal and temporal lobes. Vision is organized in the occipital lobes and the perception or recognition of objects and human faces is found within the temporal lobes. The ability to maneuver the limbs and body in space, known as the visuospatial function, are found within the parietal lobes.
Areas of the frontal lobes are also involved in learned movements of the head, neck, and limbs and other motor functions, problem solving, and goal-directed behavior. The temporal lobes are also involved in learning new information and recollecting from experience (memory).
Regions of brain responsible for the special senses (vision, hearing, taste, touch, and smell) can also be found within the forebrain. Within the parietal lobe is the somatosensory region, which is responsible for touch, temperature, proprioception (recognition of body position), and pain. Sound is processed within the temporal lobe and vision within the occipital lobe.
Three other major structures of the forebrain are the thalamus and hypothalamus. The thalamus functions include relaying sensation and special sense signals to the cerebral cortex, relaying motor signals from the cerebral cortex, and the regulation of consciousness, sleep and alertness. Since the thalamus is centrally located within the brain, it is ideally positioned to relay information to different regions of the brain. The hypothalamus is the smaller structure lying below the thalamus. The hypothalamus controls body temperature, hunger, thirst, fatigue, and circadian cycles. The basal ganglia, which is also found in the forebrain, facilitates useful, purposeful movements and inhibits unwanted movements initiated by the substantia nigra, which will be discussed below. In addition to the structures mentioned, there is a pathway in the forebrain known as the reward pathway that is associated with addiction.
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