|
The Brain | Laberal Brain diagram | Sagittal Brain diagram |
| The brain develops from the anterior end of the neural tube. (See Figure 12.2) . The central cavity of
the tube is seen as the central canal of the spinal cord, and as cavities of the brain called
ventricles.
These ventricles contain cerebrospinal fluid (See Figure
12.22 and later discussion for production and
circulation of cerebrospinal fluid). The brain is described in sections based on its embryological
development, each of which contains certain adult brain structures. (See Figure
12.3). In linear order
they are:
The telencephalon consists of the cerebral hemispheres. Each hemisphere has within it the cortex, the white matter, and the basal nuclei. The diencephalon contains the thalamus, hypothalamus, and epithalamus. The mesencephalon contains the midbrain. The metencephalon includes the pons and cerebellum. The myelencephalon is the medulla oblongata. The midbrain, pons, and medulla are collectively called the brainstem. Due to their confinement in the cranium the linear nature of this arrangement is contorted as shown in Figure 12.4. |
| Arrangement of gray and white matter: (See Figure
12.5)
In the spinal cord the gray matter is found only as an H-shaped area in the center of the cord. But within the brainstem the gray matter diverges, passing through the center of the cerebrum and terminating in the gray matter of the cortex (see below). The cerebellum has its own gray and white matter distribution which is like that of the cortex. Other parts such as the basal nuclei have both gray and white matter. Gray matter in the brain functions much like that in the cord: it is the site of connections between neurons and contains the cell bodies of motor and interneurons. It is composed of unmyelinated neurons. White matter in the brain, like that in the spinal cord, is composed of myelinated fibers in tracts which carry information from one place to another. (See Figure 12.10) The ventricles: (See Figure 12.6) Ventricles are cavities within the brain derived from the original lumen of the neural tube. They exist in each cerebral hemisphere (the lateral ventricles), between the hemispheres (the third ventricle), and beside the cerebellum (the fourth ventricle). Canals connect these ventricles together. The interventricular foramen connects the lateral with the third ventricle, the cerebral aqueduct connects the third and fourth ventricles. The fourth ventricle connects to the central canal of the spinal cord, and other canals, called apertures, lead to the subarachnoid space of the meninges. |
| Circulation of the cerebrospinal fluid: (See Figure
12.22)
The cerebrospinal fluid (CSF) is produced by filtration from blood capillaries located in tissue known as the choroid plexus which lines the ventricles. Filtration forces water, electrolytes, and nutrients out of the blood. These substances are then processed by the ependymal cells which release them into the cerebrospinal fluid. At the same time waste products are removed from the CSF and released into the blood. The cerebrospinal fluid circulates between the ventricles and into the spinal canal. It also enters the subarachnoid and subdural spaces through apertures near the fourth ventricle (see Figure 12.24). It therefore also bathes the brain and spinal cord providing buoyancy. Eventually it must be reabsorbed into the bloodstream and this occurs at pockets of arachnoid tissue which invaginate into a large vein called the superior sagittal sinus. These pockets are called the arachnoid granulations or arachnoid villi and allow the fluid to move into the veins by osmosis and a pressure gradient produced by lower pressure in the vein. The absorption must occur at the same rate as production of CSF in order to prevent an imbalance. If absorption is insufficient it causes a condition known as hydrocephaly, "water on the brain". This condition usually shows up in early childhood and can damage the brain and lead to abnormal development. It is usually corrected surgically with a shunt, a small tube which drains fluid from the meninges and short circuits it into a nearby vein. |
| The Cerebral Hemispheres: (See Figures 12.8 and
12.10)
The cerebrum is divided into two hemispheres separated by the longitudinal fissure. The terms fissure and sulcus describe grooves in the surface of the cerebrum. Most of the time fissure refers to a larger groove than a sulcus, although they are somewhat interchangeable. In between grooves is found a raised area called a gyrus. Certain of these structures are consistent landmarks for the brain. The outer layer of the cerebrum is composed of gray matter and is called the cerebral cortex. The cerebral cortex is the area of conscious thought and perception. For this reason, and because it forms a cap over the rest of the brain the cerebral cortex has been called the "thinking cap". The cortex can be described as made up of regions called lobes. Each lobe bears the name of the bone lying above it. The central sulcus separates the parietal from the frontal lobe. The lateral fissure divides the parietal from temporal lobe. A short parieto-occipital fissure indicates the upper delineation of the parietal from occipital lobe. The largest fissure is the longitudinal fissure which divides the two hemispheres from one another. The cerebral hemispheres are connected by fiber tracts which permit the hemispheres to communicate with one another. The major connection is the corpus callosum, a second is the fornix. The hemispheres normally divide up the tasks with one hemisphere, usually the left, being dominant. This is the principle of lateralization and dominance. In most people (90 to 95%) the left hemisphere is dominant and responsible for logic, mathematics, and language. The right hemisphere is the center for emotions and artistic endeavors. However, when one hemisphere is damaged the other may be capable of performing the lost functions. |
| The Projection Areas (Functional Areas) of the Cerebrum: (See Figure
12.8 and test diagram)
Projection areas are regions of the cortex where specific motor or sensory activity is localized. The pre-central gyrus (the raised area in front of the central sulcus) is the primary motor area. It is the center for voluntary control of the skeletal muscles. Each area of the body, for that matter each muscle, is controlled by a specific part of the pre-central gyrus. (See Figure 12.11) The orientation is generally reversed in position compared to the body location, i.e. the muscles of the leg and foot are generally projected to the top of the gyrus, those in the head and neck to the bottom of the gyrus. The area represented reflects the level of activity and control over the muscles, not their size. For example more area is devoted to the muscles of facial expression, speech, and hand movement as to all the rest of the muscles. The pre-central gyrus is the origin of the corticospinal tracts. The post-central gyrus is the comparable area for sensory perception, called the somatosensory area. It receives the conscious sensations from the musculocutaneous regions of the body: pain, temperature, touch, and pressure. These sensations are brought by the spinothalamic tract and the fasciculus gracilis and cuneatus. As before, these sensations terminate in specific positions on the gyrus which are inversely related to body position and directly related to the degree of sensation and its importance, not the size of the area. Other projection areas of importance: (See Figure 12.8 for locations) The pre-motor area - in front of the pre-central gyrus, a motor association area partially responsible for learned reflexes. Frontal eye field - synchronizes eye movements. Broca's area - the motor speech area for control of the muscles of speech. Prefrontal cortex - Important in planning complex movements and in general planning and elaboration of thoughts. This area normally exerts control over other areas such as those responsible for emotions and stress response, and is thus thought to be the center for self-control, reasoning, and such. It is responsible in part for personality and some aspects of memory. Wernicke's area - Language comprehension and elaboration; a general interpretive area. Primary auditory area - this area receives and perceives hearing. Auditory association areas - areas involved with association of hearing with other functions such as speech and memory, necessary to speak and to understand speech. Primary visual area - this area perceives visual stimuli and constructs a three-dimensional image using stimuli from both eyes. Visual association area - this area interprets the image and relates it to images in memory for recognition. |
| Other areas of the cerebrum:
Deep to the cortex are both fiber tracts and gray matter areas considered part of the cerebrum. The corona radiata consists of fibers which bring impulses to and from the cerebral cortex from the thalamus. The internal capsule consists of fibers which connect the cortex with the basal nuclei and corticospinal tracts. (See Figure 12.10). The basal nuclei (Figure 12.11) (sometimes misnamed the basal ganglia [Q: why is this incorrect?]) consist of a group of structures with both gray and white matter which in various ways modify motor functions coming from the cerebral cortex. Its structures include the caudate nucleus, putamen, and globus pallidus, and it connects to the substantia nigra and amygdaloid nucleus. The substantia nigra is actually part of the midbrain. The basal nuclei function in association with the corticospinal system to control complex patterns of motor activity. When there is serious damage the basal nuclei the motor cortex can no longer provide the patterns for many skilled and repetitive actions. Included are: writing letters of the alphabet, using scissors, hammering nails, shooting a basketball, passing a football, shoveling dirt, controlled eye movements and many others. Several circuits connect the basal nuclei with the motor association areas, sensory association areas, and the motor cortex in loops which provide both positive and negative feedback. The basal nuclei utilize a wide range of neurotransmitters: GABA (gamma amino butyric acid) and dopamine are inhibitory neurotransmitters. Ach, norepinephrine, serotonin, and enkephalin are found in connecting pathways, both excitatory and inhibitory, to other areas, and there are multiple glutamate pathways which provide most of the excitatory signals within the basal nuclei. (See Table 11.3 for a summary of neurotransmitters) Disorders:Parkinson's Disease - caused by destruction of dopamine secreting cells in the substantia nigra which send impulses to the caudate nucleus and putamen. The result is that without these inhibitory impulses, tremor and rigidity occur. Also akinesia occurs, or lack of the ability to perform willful movements. Treatments: 1) use of L-dopa which is converted into dopamine in the brain (eventually the brain resists this); 2) transplanted fetal dopamine cells and genetically engineered dopamine cells; 3) the use of the MAO inhibitor Deprenyl. MAO (mono amine oxidase) is the chemical which breaks down dopamine); growth factors which stimulate recovery or block deterioration of the dopamine cells. The activation of existing stem cells in the new frontier of research for possibly curing Parkinson's and other neurological disorders. [Parkinson's and Dopamine] [Parkinson's Article] [Interpreting Alzheimer's Abnormalities] [Alzheimer's Article] [The Prefrontal Lobes and Schizophrenia] [Schizophrenia Article] Huntington's Disease - Begins with flicking movements a joints which progress to severe distortional movements of the entire body leading eventually to severe dementia. It is caused by a genetic mutation of an enzyme-producing gene. This results in loss of GABA secreting neurons in the basal nuclei, and in loss of ACH neurons in many parts of the brain. Alzheimer's Disease - caused by the development of neurofibrillar tangles and beta-amyloid plaques in the brain. The tangles occur when a support protein called "tau", which is important in support of the transport proteins for the cell, breaks down, causing the proteins to intertwine and loose their functional transport structure. Without transport proteins neurons cannot function or survive. The plaques accumulate in large masses in certain people. In response to these the immune cells produce chemicals called cytokines which attack the plaques. Unfortunately these chemicals damage and kill the neurons they are supposed to protect. Treatments focus on various aspects of the problem. On one front factors which cause the breakdown of tau may be able to be blocked, thus preventing the tangles. On another front antibodies against the beta amyloid plaque have been developed which show promise in eliminating it in mice.
|
| Parts of the brain below the cerebrum:
(See Figure 12.12 [human brain sectioned] [Sagittal Section Diagram] [Test Diagram) corpus callosum - connects the hemispheres. The fornix - also connects hemispheres and part of the limbic system. septum pellucidum - a membrane which covers the opening into the lateral ventricle. The Dienchephalon (See Figure 12.13) The thalamus - its halves connected by the intermediate mass, the thalamus receives all conscious sensations and acts as a relay center. Sensations from the spinothalamic tract and the fasciculus gracilis and fasciculus cuneatus synapse in the thalamus before continuing to the cortex. Afferent impulses are routed by the thalamus to their proper destinations. The thalamus also lies at the top of the reticular formation and is part of the alert mechanism of the reticular activating system. The thalamus also helps to filter out unwanted stimuli. The hypothalamus is a small yet very important part of the brain below the thalamus (hypo=below). It is part of the control mechanism for many of the endocrine glands. Through its connection with the pituitary gland through the infundibulum the hypothalamus directs the pituitary's secretions, which in turn direct many other endocrine glands. The hypothalamus also coordinates many autonomic and visceral functions such as control of blood glucose, heart rate and respiration in response to stresses, control of thermoregulation, the perception of hunger, thirst, control of electrolyte and water balance, and the sleep-wake cycle. The pineal gland is considered part of the epithalamus (epi=upon the thalamus) and receives stimuli from the hypothalamus. The pineal gland secretes melatonin during the dark periods. This establishes our biological clock and regulates our circadian rhythm (day-night cycle) which affects many behaviors such as sleeping, eating, sexual desire, etc. Individuals who receive insufficient light, such as those living in the far north during the winter, may experience Seasonal Affective Disorder. This disorder affects their mood and mental state as well as their physiology. Lights which stimulate the retina in a pattern similar to that of normal sunlight has been shown to alleviate the problem. The Brainstem (See Figures 12.15 and 12.16) The midbrain is comprised of the corporal quadrigemina and cerebral peduncles. The corpora quadrigemina (four bodies, twins) are the superior colliculi, which are the center for visual reflexes (blinking, accommodation of the lens), and the inferior colliculi where auditory reflexes are centered (contraction of the stapedius muscle). The pons - the name means bridge and in part the pons is a bridge to the higher brain. However its main functions are as part of the regulation of the rate and depth of respiration. It also contains the CNS connection for cranial nerves V, VI, VII, and VIII. The medulla - center for the vital functions of heart rate, respiration, and blood pressure. The medulla also has CNS connections for VIII, as well as IX, X, XI, and XII. The pyramids are also part of the medulla. The medulla stimulates the muscles of respiration and controls the breathing rhythm. It regulates the heart rate and volume and controls blood pressure and overall blood distribution. The cerebellum (See Figure 12.17) - this part coordinates the skeletal muscles. It receives unconscious proprioception as well as input from all the higher motor centers. From this input the cerebellum monitors muscle contractions and planned muscle contractions and maintains a constantly adapting system to coordinate them. |
| Functional (Conceptual) Areas of the brain: these areas are considered by some to work together to
perform certain functions, but are not related structurally.
The cingulate gyrus - lies above the corpus callosum, connects the system to the cerebral cortex. The hypothalamus - contains the pleasure, reward, or satiety center. Also mediates hormonal responses to stress. Mamillary body - contains memory pathways, especially for olfaction. Hippocampus - responsible for short and long term memory and learning. Amygdaloid nucleus - the center for many emotional response pathways. The thalamus - the sensory center for input which stimulates emotions The fornix.- provides connecting pathways for the limbic system from one hemisphere to the other. Functions of the Limbic System - this system is sometimes called the "emotional brain". It is the site of: 1) emotional states and behavior. 2) the bridge between the conscious and subconscious brain. This is how subconscious feelings surface as voluntary behavior. 3) short term memory and information storage, especially short term recognition of facts, objects, people, etc. |
| The Reticular Formation and Reticular Activating System: (See Figure
12.119).
The Reticular Formation is a brainstem pathway which receives sensory input of many types including vision, auditory, and somatic senses. It directs these stimuli to the thalamus as part of the Reticular Activating System which is an alert system for make the cortex. This allows unwanted and unimportant stimuli to be filtered out, while making us aware of important and critical stimuli. |
| Next: The Cranial Nerves |
Revised: April 23, 2003
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