Brain & nervous system
Brain is the part of the nervous system that is enclosed in the skull. All vertebrates have well-developed brains; most invertebrates do not have true brains. Instead, they have groups of nerve cells called nerve nets, nerve cords, or ganglia. The human brain is an extension of the spinal cord, and with it makes up the central nervous system. It contains billions of neurons, or nerve cells, each with more than 10, 000 synapses, or connections to other neurons. No two brain cells are alike. The brain cannot regenerate new brain cells but it can bypass dead or damaged cells to form new synapses between existing cells.
The brain receives information from all parts of the body and sends out instructions to the body’s various organs and systems. The information and instructions travel through the brain in the form of nerve impulses, electrical signals that elicit chemical changes. The impulses travel along the neurons and move from one neuron to the next across the synapses by means of chemicals called neurotransmitters. It is through nerve impulses that the brain controls such activities as voluntary and involuntary movement (Zoeller, 2003).
The brain is connected with the sense organs, muscles of the head, and internal organs of the body by 12 pairs of cranial nerves. Some of the cranial nerves, called motor nerves, carry impulses from the brain to various parts of the body. Others, called sensory nerves, carry impulses from the body back to the brain. Most pairs of cranial nerves contain one motor and one sensory nerve; a few pairs contain sensory nerves only. The brain consists of two types of tissue: (1) nerve cells, or gray matter; and (2) sheathed nerve fibers, or white matter. The sheath is composed of myelin, a fatty protein that protects and insulates the fibers.
A large number of blood vessels carry nourishment to the brain. The brain extracts certain substances from the blood and metabolizes them (that is, produces chemical changes in them) to produce energy. Glucose, its main source of energy, is metabolized by a chemical reaction with oxygen carried in the blood. The human brain is more complex and has more functions than the brain of any other animal. It is the seat of consciousness and the coordinator of the nervous system. Thought, memory, imagination, and other mental processes are functions of the brain.
Specific areas in the brain are responsible for language and emotions. The brain is the seat of sensations. All voluntary and some reflex muscular movements are initiated and regulated by the brain (Colzie, 2006). In addition, various parts of the brain control such automatic functions as heartbeat, temperature regulation, digestion, and breathing. The brain of the average human male weighs about 3 pounds (1. 4 kg); the human female, 2. 7 pounds (1. 2 kg). At birth, a baby’s brain weighs only 11 to 13 ounces (310 to 370 g), but it grows rapidly during the first years of life.
By the age of seven, a child’s brain has reached nearly its full weight and volume, after which its growth is slow. The brain of a human male is fully grown by the 20th year, that of a female somewhat earlier. After the age of 20, the brain loses about one gram (0. 04 ounce) of weight per year (Spear, 1995). This study discusses the brain development and how it functions. II. Discussion Brain tissue is very soft and easily injured. It is well protected, however, by the skull and by three membranes of connective tissue, collectively called the meninges, between the skull and brain.
The outermost membrane is thick and tough, and fits closely to the inner surface of the skull. This membrane is called the dura mater, which is Latin for “hard mother. ” The innermost membrane is the pia mater, Latin for “tender mother. ” This thin membrane contains a network of blood vessels. These blood vessels supply nourishment to the brain, and carry blood from its interior back to the heart. The pia mater conforms exactly to the outer surface of the brain itself (Cynader, 1994). Between the dura mater and the pia mater is the arachnoid, or “spider-like,” membrane.
It is a soft, delicate, transparent tissue. The subarachnoid space, between the arachnoid membrane and the dura mater, is filled with cerebrospinal fluid, a clear, colorless liquid composed of protein, glucose, urea, and salts. It moistens the tissues of the brain and protects them from injury. The brain is also protected by the blood-brain barrier, a network of tightly meshed capillaries (tiny blood vessels) that selectively filter out harmful chemicals and waste products while permitting other substances, such as nutrients, to pass directly into the brain (Zoeller, 2003).
This barrier prevents harmful compounds in the blood from being absorbed by brain tissue. A. Parts of the Brain The brain itself consists of three main parts: a large forward part called the forebrain; a narrow middle portion called the midbrain; and a rear part, called the hindbrain. It contains four cavities (hollow spaces called ventricles). The Forebrain, which is made up mainly a mass of neurons called the cerebrum, occupies most of the skull cavity and accounts for 90 percent of the weight of the entire brain. The surface of the cerebrum is a layer of gray matter called the cerebral cortex.
It has many folds, or convolutions, which greatly increase its area. The longitudinal fissure, a deep cleft running from front to back, partially divides the cerebrum into right and left hemispheres. A central band of nerve fibers called the corpus collosum connects the two hemispheres. It contains bundles of nerve fibers called nerve tracts that carry information between the two hemispheres. The corticospinal tract carries impulses from the cerebral cortex to the spinal cord. Its fibers cross each other at the region where medulla oblongata (an area in the hindbrain) meets the spinal cord (Sousa, 2006).
Thus, the left interprets the sensations of the right side of the body and vice versa. Each hemisphere performs unique tasks. The left hemisphere is responsible for logical thought, writing, and mathematical skills. The centers of language are also located here. Broca’s area, situated in the frontal lobe (the forward section of the hemisphere), is responsible for the production of language. Wernicke’s area, situated in the temporal lobe, a section above the ears, is responsible for the comprehension of language. The two areas are connected by a bundle of fibers called the arcuata fasciculus.
Damage to these fibers will cause speech impairment. The right hemisphere is responsible for intuition, musical and artistic ability, and analysis of visual patterns. Although each hemisphere is responsible for different functions, one can take over for the other in the event of localized brain damage (Puckett, 1999). The cerebral cortex contains two specialized areas: the somatic sensory cortex and the motor cortex. They are separated by the central fissure, a deep cleft perpendicular to the longitudinal fissure and extending across the roof of the brain.
The somatic sensory cortex receives sensory signals from the skin, bones, joints, and muscles. The motor cortex controls the voluntary movement of muscles. Almost every part of the human body has a specific region controlling it in both the somatic sensory cortex and the motor cortex. Body parts that perform intricate movements, such as lips, hands, and legs, are controlled by large parts of the cortex. Body parts that perform gross movements, such as the shoulders and trunk, are controlled by smaller areas. Adjacent regions in the brain control adjacent body parts (Spear, 1995).
The cortex of each cerebral hemisphere is divided into four sections, called lobes: 1. The Frontal Lobe, the forward, upper part of the cerebrum, includes the areas concerned with intelligence, judgment, emotional reaction, and the movement of skeletal muscles. 2. The Parietal Lobe, in the upper, back area of the cerebrum, receives and interprets the sensations of pressure, temperature, and position. 3. The Temporal Lobe, above ears, is concerned with hearing, memory, and understanding of speech. 4. The Occipital Lobe, in the back part of the cerebrum, is concerned with vision and the interpretation of objects that are seen.
Each hemisphere contains a mass of nuclei called the thalamus (plural: thalami). It consists of gray matter that integrates a wide range of sensations from the visual and motor cortexes. It also plays a role in emotions. Above each thalamus are two basal ganglia, clusters of neurons that help regulate body movements (Cynader, 1994). Beneath the thalami is the hypothalamus, a mass of nerve cells and fibers that controls the reaction of the body of stress and to strong emotion. It also regulates the body’s water balance, temperature, appetite, sleepiness, and heart rate.
Below and in front of the hypothalamus is the pituitary gland, which is partially controlled by the hypothalamus (Colzie, 2006). The pineal gland, or epiphysis, is a coneshaped organ located beneath the corpus callosum. It is connected by nerves to the eyes and is extremely sensitive to light. In response to darkness, it secretes melatonin, a hormone that is believed to induce sleep (Sousa, 2006). The olfactory bulbs, which govern the sense of smell, are located on the undersurface of the hemispheres. Nerves run from the nose through these bulbs to the cerebrum (Sousa, 2006).
The Midbrain contains tracts (bundles) of nerve fibers that connect with other parts of the brain and with the spinal cord. The midbrain also has centers for auditory and visual reflexes, such as the contracting of the pupils (Sousa, 2006). The Hindbrain consists of three parts: (1) the cerebellum, behind and beneath the cerebrum; (2) the pons, beneath the midbrain and opposite the cerebellum; and (3) the medulla oblongata, attached at its base to the spinal cord (Sousa, 2006). The cerebellum, like the cerebrum, has a convoluted surface. The chief function of the cerebellum is to coordinate and regulate movements of the skeletal muscles.
(The movements, however, are initiated and controlled by the cerebrum). When the cerebellum is damaged, ordinary movements directed by the cerebrum cannot be carried out. Limb movements become slow and jerky, and speech may become slurred. The pons is a smooth-surfaced bulge between the midbrain and the medulla oblongata. It contains tracts that connect the two sides of the cerebellum, and tracts that connect other parts of the brain with each other and with the spinal cord. Many of the cranial nerves pass through here. The pons controls the motor and sensory nerves to the eyes, jaw, face, and muscles.
Together with the cerebellum, it regulates posture and balance (Puckett, 1999). The medulla oblongata is smooth, without convolutions. It contains three important nerve centers: one affects the rate of heartbeat; one controls breathing; and one produces the constriction of blood vessels to control the volume of blood supply to the tissues. It is also the site where the nerves from the left hemisphere cross over to control the right side of the body and vice versa. Reflex centers of vomiting and swallowing also lie in the medulla. The midbrain, pons, and medulla, oblongata together form a structure called the brain stem.
Deep within the brains tem, extending from the medulla to the midbrain is a network of nerve cells and fibers called the reticular formation (Cynader, 1994). The reticular formation regulates the amount and speed of electrical activity in the cerebral cortex. Many sensory nerves feed into it. It is believed to be the seat of consciousness. Ventricles. There are four ventricles within the brain. These cavities are connected to each other and to the hollow core of the spinal cord. The largest cavities are the two lateral ventricles, located in each hemisphere of the cerebrum.
Beneath the lateral ventricles is the third ventricle and under it is the fourth. Cerebrospinal fluid is formed and stored in the ventricles (Cynader, 1994). Within the lateral ventricles is the limbic system, a group of structures that controls emotions and behavior, stores memories, and is involved in learning. It contains two masses if gray matter: the amygdale and the hippocampus. B. Chemistry of the Brain Since the early 1970’s, researchers have discovered that the brain contains more than 50 neurotransmitters, chemical substances that facilitate the transmission of nerve impulses between neurons.
They interact with specific receptor sites in the brain to elicit chemical changes. Some circulate throughout the body (Spear, 1995). All neurotransmitters have chemical precursors. These are substances, composed of glucose and amino acids, which are produced elsewhere in the body and are carried in the bloodstream. The precursors are able to cross the blood-brain barrier into the brain, where they are eventually converted into neurotransmitters (Spear, 1995). The quantities of neurotransmitters in the brain are affected by the consumption of certain foods and also by strenuous exercise.
Endorphins make up a family of neurotransmitters that act as natural painkillers. They moderate the amount of pain an individual feels. They are composed of chains of amino acids called peptides. Narcotic analgesic drugs, such as heroin or morphine, effectively reduce pain by occupying the same receptor sites and producing the same interactions as endorphins. These drugs are often prescribed for severe pain or when there is a delay or malfunction in the release of the natural painkillers (Zoeller, 2003). Acetylcholine is a neurotransmitter that functions in storing memories, regulating moods, and controlling body movements.
Consumption of such foods as eggs, soybeans, and liver increases its production. All of these contain lecithin, which is converted into choline in the liver. Choline is a chemical precursor that is converted into acetylcholine in the brain. Serotonin, a neurotransmitter found only in the hypothalamus and midbrain, relieves depression, reduces sensitivity to pain, and induces sleep. Its chemical precursor is tryptophan, which is found in the protein in meat, fowl, and fish. Norepinephrine is another neurotransmitter that helps relieved depression (Zoeller, 2003).
Its precursor is tyrosine, which is also found in protein. Strenuous exercise increases the production of endorphins and norepinephrine. It is this increased production that causes “runner’s high—an increased tolerance to pain and a state of euphoria experienced by many long-distance runners. III. Conclusion In conclusion, as I study this subject I have discovered one thing and that is— brain is like your computer’s body. It receives information from the internal organs such as the heart, intestines and from the sense organs—the eyes, the ears, the tongue, the skin and the nose.
It makes me think that the brain is the control center of the body. It receives all messages from different parts of the body, interpret them, and tell the parts what to do. Moreover, as what I have understood from the research I conducted, it gives me an idea that the brain is not completely developed even in full-term newborn infants. A great deal of brain development takes place in the first few months of postnatal life; and, in fact, brain growth continues at least until adolescence, and perhaps in adulthood. Some nerve fibers in the brain develop myelin sheaths.
Many of these nerve fibers have not become myelinated by the time of birth. The process of myelination continues for years, especially in the reticular formation and parts of the forebrain. Reference: 1. Colzie, Lakesha (2006). The First Three Years and Beyond: Brain Development and Social Policy. Childhood Education, Vol. 82. 2. Cynader, Max S. (1994). Mechanisms of Brain Development and Their Role in Health and Well-Being . Daedalus, Vol. 123. 3. Meltzoff, Andrew N. (2002). The Imitative Mind: Development, Evolution, and Brain Bases. Cambridge University Press. Cambridge, England. 4.
Puckett, Margaret (1999). Examining the Emergence of Brain Development Research. Childhood Education, Vol. 76. 5. Sousa, David A. (2006). How the Arts Develop the Young Brain: Neuroscience Research Is Revealing the Impressive Impact of Arts Instruction on Students’ Cognitive, Social and Emotional Development. School Administrator, Vol. 63. 6. Spear, Norman E. (1995). Neurobehavioral Plasticity: Learning, Development, and Response to Brain Insults. Lawrence Erlbaum Associates. Hillsdale, NJ. 7. Zoeller, R. Thomas (2003). Thyroid Toxicology and Brain Development: Should We Think Differently? Envir