The Human Brain (II)
Brain's divisions
The most important divisions of the human brain are the stem, the cerebellum, and the cerebral hemispheres. The stem is the lowest part and serves as connection between the brain and the spinal cord. All afferent and efferent nerves pass trough the stem, and the majority cross over at it. This means that motor nerves that originate in one hemisphere actually control muscles in the other side of the body. For example, a lesion in the left hemisphere may affect the right-side limbs.
Once they pass the stem, nerves reach the thalamus, which distributes signals to the cortical areas. Sensations of pain, heat, and cold, as well as the sensation of movement, thus reach the cerebral cortex.
The cerebellum, which accounts for about ten percent of the brain's weight, coordinates reflex and voluntary body movements. Fibers connect the cerebellum to other brain structures and the spinal cord. The cortex is also connected to the cerebellum. Lesion of this structure leads to disorders of movement.
The reticular formation, a group of cells located in the brain stem, is responsible of consciousness. A damage to the reticular formation almost always results in a state of coma. The hypothalamus, located below the thalamus, controls hormone production and many functions belonging to the autonomic nervous system, like temperature, blood pressure, and pulse rate. A part of the hypothalamus is the pituitary gland.
The cerebral cortex accounts in humans for about eighty-five percent of the brain's cells. It is formed by two hemispheres (the cerebrum), each of which may be further divided in the temporal, frontal, parietal, and occipital lobes. Sensations and movements are governed by the motor cortex in the frontal lobe and the sensory cortex in the parietal lobe, respectively.
The Broca's area, in the left frontal lobe of right-handed people, is in charge of the production of speech. The Wernicke's area, in the left temporal lobe, is in charge of understanding speech. Left-handed people may have these areas in the opposite side. A damage to the Broca's area or the Wernicke's area results in impaired communication ability.
The senses of smell, sight, and hearing are managed by neurons at the limbic system, the occipital lobe, and the temporal lobe, respectively. The temporal lobe has functions that affect memory; this was confirmed by the brain surgeon W. G. Penfield when he operated on people who had vivid hallucinations of past events. Penfield found that the hallucinations could be artificially produced by stimulating the surface of the temporal lobe. A structure known as the hippocampus may be involved in long-term retention.
Diagnostic and research techniques
The electroencephalogram (EEG) consists in the recording of electrical brain waves by the affixing of electrodes to the scalp. It became extensively used since 1935 to diagnose epilepsy and to study the brain workings. The EEG waves originate mainly at the cerebral cortex, and are classified according to their frequency. There are alpha, beta, delta, and theta waves corresponding to as many frequency bands. Each type has been associated with certain type of mental or bodily activity.
In the early 1960s a new tool in brain research was invented: the computerized tomograph, which using an X-ray beam draws images of brain slices that permitted for the first time to uncover brain tumors without need of surgery. Since then, other radiology techniques have been developed to perform diagnosis using brain images. Magnetic resonance is one of them; it involves the use of magnetic fields and radio-frequency pulses. Another one is tomography by positron emission. This one recurs to injecting the patient with a solution of glucose that has been tagged with a radioactive isotope. The emission of the isotope is recorded by a scanner and abnormal levels of glucose metabolism can be identified that signal the location of a tumor. Still another technique for producing brain images uses ultrasonic waves.
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