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Neuroanatomy in a Nutshellby Dr. Jay Vandersteenhoven The purpose of this overview is to provide a simple outline by which some of the basic functions of the brain as well as its complexities can be understood. This may be especially useful to a recently diagnosed brain tumor patient who may be overwhelmed by the many strange new terms and descriptions. This section may be helpful in understanding the terminology used by the neurosurgeon and neuroradiologist. Neuroanatomy Overview: The cerebrum is the largest part of the brain and the most advanced in terms of development. The cerebrum appears to have identical right and left halves, or "hemispheres" although, as we shall see, there are important differences between the hemispheres. The cerebrum is a coarsely wrinkled, tan-gray, roughly oval structure with a consistency of very firm jello. These coarse wrinkles are referred to as gyri and the deep indentations as sulci. The cerebrum is made up of two layers: a thin outer layer of "gray" matter, also called the cortex, which contains layers of the basic functional cells called neurons and a larger underlying layer of "white" matter, which contains the cells that produce the insulating substance around the fibers that connect the neurons, or axons (see fig.2) The two cerebral hemispheres are joined at the center by a broad band of white matter called the corpus callosum. The cerebrum is essentially divided into four lobes: the frontal, parietal, occipital and temporal lobes. The frontal lobe is involved in anticipated movement, concentration and behavior. It also contains an area referred to as the "motor" area which is involved in the processing and execution of voluntary movement. The parietal lobe contains an area referred to as "somato sensory" which deals with sensory signals from the body. The occipital lobe is primarily involved in visual processing and reception. The temporal lobe, which has the appearance of a thick blunted horn, has two parts. The "medial" part which is closer to the center of the brain, is involved in memory processing. The "lateral" part, or area closer to the surface, is involved with auditory and verbal processing. The cerebral cortex is comprised of nerve cells, called neurons, their supporting cells, called neuro glial cells, and include both astrocytes and oligodendroglial cells, and blood vessels. The neurons are organized into six layers and each layer has certain functional characteristics. The neurons represent the most important component of the brain and are responsible for generating electrical signals which are the essence of brain function including thought processing, memory, speech and muscle movement. There are three types of glial cells which include astrocytes, oligodendrocytes, and ependymalcells. Astrocytes are the cells which are responsible for providing for the nourishment of neurons. Oligodendrocytes are responsible for producing a substance called myelin which serves as an insulator for the axons. The myelin is also responsible for the white appearance of the "white" matter. Ependymal cells line much of the ventricular system which produces spinal fluid. Microglia, which despite the name, are not true glial cells, represent the immune cells of the central nervous system. The basal ganglia is a collection of neurons which have different yet specific functions and is centered deep within the cerebrum. This includes the caudate nucleus, the globus pallidus, the caudate nucleus, claustrumand amygdala. Another large collection of neurons is referred to as the diencephalon or thalamus. The thalamus is involved as a relay station for sensory input to and from the cerebrum. The ventricles are spaces within the brain that produce and circulate spinal fluid. There are four ventricles; two lateral ventricles, a third ventricle and a fourth ventricle. The ventricles are lined by ependymal cells. There are also specialized structures derived from ependymal cells present in the lateral and third ventricles called choroid plexus produces cerebrospinal fluid (CSF). The CSF slowly percolates through the ventricles and eventually exits, to then bathe the outside of the brain within the subarachoid space, a narrow space between the surface of the brain and its outer coverings. CSF is then reabsorbed by the arachnoid granulations directly into the veins of the bloodstream. The cerebral aqueduct, also called the aqueduct of sylvius, and is a narrow canal which connects the third and fourth ventricles. The limbic system, together with an area called the hypothalamus, controls basic functions including emotional reactions, hunger, thirst and biological rhythms. The hypothalamus, deep within the central portion of the cerebrum near the third ventricle, is responsible for water balance, sleep, temperature, blood pressure and appetite regulation. The brainstem is a vital control center for many basic functions including blood pressure, heart rate and breathing. It consists of several structures including the pons, reticular formation, and medulla oblongata. The reticular formation connects with all parts of the brain and brainstem and is responsible for conscious awareness, wake and sleep patterns, and attentiveness. The medulla oblongata is the origin of a number of cranial nerves and is responsible for breathing (respiration), heart beat and vomiting. Cranial nerves are major nerve roots that for the most part, originate as offshoots from the brain in contrast to the spinal nerves which are derived from the spinal cord. These cranial nerves are responsible for the movement and sensation of the head and neck, as well as vision, hearing, taste, and smell. The pons is a bulbous part of the mid brain which serves as a relay between the cerebrum and cerebellum. It also serves as the origin for numerous cranial nerves. The cerebellum is the second largest part of the brain, and like the cerebrum, has two hemispheres which are connected in the middle by an area called the vermis. This structure is connected to the mid brain. Together with the thalamus and cerebrum, the cerebellum is responsible for coordination of fine motor function which would include walking and speech. The spinal cord is nothing more than a large cable with many groups of axons to and from the brain. This extends down through a narrow canal formed by the vertebrae giving off and receiving fibers from different parts of the body. The signals transmitted within the spinal cord would include signals for controlling muscular movement and sensation/feeling. The pituitary gland (the "Master Gland") is a complex organ which is part of the endocrine system. A large number of critical hormones are produced by this gland which are involved in growth, sexual development, lactation, and metabolism/electrolyte control. The gland is attached to and receives input from the hypothalamus. It is situated at the base of the brain, adjacent to the optic nerves, and rests within a compartment of the base of the skull referred to as the sella tursica. The pineal gland, once referred to as the "seat of the soul", is a small gland tucked below the surface of the cerebrum. This gland has limited function in man in contrast to many animals, but is thought to have some role in regulation of diurnal rhythm (sensation of day and night). The brain is encased by a number of coverings including the dura mater, pia mater and the arachnoid. The dura mater is outermost covering and the toughest; it is continuous with the falx cerebri, which divides the two cerebral hemispheres. The pia mater and the arachnoid are together known as the leptomeninges. It is the narrow space between these layers (the subarachnoid space, mentioned above) that contains spinal fluid. |
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The
brain, since the time of the Greeks, has been recognized as essential for all
human thought and behavior. What makes us human--the ability to think and feel,
to remember, to create and to wonder--is our brain. The brain is the most
complex of all biological organs and even to the experienced students of
neuropathology and neuroanatomy, its study evokes a sense of awe and wonderment.