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Chapter 2

PSYC32 (31) Week 1 Notes on Chapter 2.docx

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Zachariah Campbell

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PSYC31H3 Week 1 Notes on Chapter 2 Prenatal Development: External Factors: • Prenatal Development refers to all of the steps involved with the formation of the various structures and functions of the body and, for our purpose, specifically of the central nervous system. • Many fetal difficulties such as low birth weight and smaller head circumference may be caused by malnutrition. Improper nutrition may also lead to other physiological difficulties later in life such as obesity and problems in programming the appetite regulatory system correctly in the developing fetus. • Abstaining from alcohol and other drugs, decreasing stress level, and avoiding physical or emotional abuse are also desirable for the proper formation of the central nervous system. Alcohol and various other drugs which cross the placental barrier can directly affect the fetus. Fetal alcohol syndrome, which can cause serious cognitive difficulties, is directly related to alcohol consumption. • Studies have shown that increased stress levels raise the amount of cortisol in the body. Cortisol is a steroid hormone released by the adrenal cortex which elevates blood sugar and metabolism and helps the body adapt to prolonged stress. However, excess cortisol for too long a period of time may lead to the depression of the immune system and leave the individual vulnerable to various illnesses. Examples of illnesses that could affect a compromised immune system include pneumonia, bronchitis, or other serious systemic infections. • Severe physical abuse to the mother may lead to brain impairment in the baby especially is she is struck in the stomach. Shaken baby syndrome may result from physical abuse and can lead to traumatic brain injury. In cases of shaken baby syndrome, the infant is shaken so hard that his or her brain impacts the skull in a manner similar to the effects of a motor vehicle accident. • Emotional Abuse is further form of abuse in which an individual is verbally made to feel bad about him- or herself and abilities. Emotional abuse to the mother may lead her to neglect herself and fail to care for her physical needs. Lack of care of the mother while pregnant could greatly affect the child and also cause neglect of any other children. Development of the Central Nervous System: • When the sperm and egg unite, the process of cell division initiates very quickly. The period from the time of conception to approximately 2 weeks later when implantation in the uterine wall occurs is termed the germinal period. • The developing cells are called a zygote. • By the end of the 1 week, a mass of cells with a fluid center has formed which is th th called the blastocyst. The blastocyst enters the uterus and by the 11 -15 day following conception, the blastocyst implants on the uterine wall. • The blastocyst consists of the inner tissue which will become the embryo, and the trophoblast. • Once implantation occurs, it is now considered an embryo. • The embryonic stage lasts until the end of the first trimester. The growth of cells is fairly rapid. The embryo then begins to differentiate into three distinct layers.  The ectoderm is the outermost of three layers of the embryo which develops into the skin, sense organs, and nervous system.  The mesoderm is the middle layer and becomes the muscles, blood, and excretory system.  The endoderm is the innermost layer and becomes the digestive system, lungs, and other internal organs.  In addition to the cell layers, the life support system for the embryo develops simultaneously. • The amnion is a sack of fluid in which the embryo floats for temperature regulation and protection. • The umbilical cord connects the embryo to the placenta. The placenta is a group of tissues in which blood vessels from the embryo and mother mix but do not join. Very small particles cross from the mother’s blood to the embryo, such as water, salt, and oxygen, while carbon dioxide and waste from the baby return to the mother. • The first recognizable structure during the nervous system development is the neural plate made up of the ectodermal tissue on the dorsal surface of the developing embryo. The neural plate begins to become evident at approximately 3 weeks of age and appears to be induced by signals from the mesoderm layer. • At the time the neural plate becomes evident the cells are totipotent or able to become any cell in the body. As the neural plate develops, the cells begin to lose their ability to form into any type of tissue making them specific to central nervous system development. • As time progresses, the neural plate folds to form the neural groove. The lips of the neural grove fuse to form the neural tube by approximately the 24 day after conception. The inside of the neural tube becomes the spinal canal and the cerebral ventricles. • The swelling at the end of the neural tube develop into the forebrain, midbrain, and hindbrain. • The central nervous system changes greatly in the first 7 weeks of development. By the end of these 7 weeks, the embryo is referred to as the fetus which has begun to resemble a more human shape. Although it appears to be a very short period of time, the brain is almost a complete replica of the adult brain at approximately 100 days from the time of conception, although the structures are not completely developed. • The neural crest is dorsal to the neural tube. It is formed from cells that differentiate from the neural tube as it is being formed. Neural crest cells develop into the neurons and glial cells of the peripheral nervous system. Glial cells appear in many parts of the body as development continues. They colonize at specific locations such as the gastrointestinal tract where they help to continue formation of both the central nervous system and the peripheral nervous system. Prenatal Neuronal Development: • Most researchers divide the process into several stages: induction, proliferation (neurogenesis), migration and aggregation, differentiation, axonal growth and synaptic formation (maturation), programmed cell death (apoptosis), and synaptic rearrangement. • The process of induction begins when part of the ectoderm becomes the nervous system. This occurs during the development of the neuronal plate. Cells at this point are described as totipotent. Another name for these cells is multipotent because the embryonic neural crest tissue tends to give rise to many cells that are highly specific later in the adult’s life, a unique quality of vertebrates. • In modern neuropsychology these cells are most commonly termed stem cells, because they are able to develop into different types of cells and have almost unlimited capacity for self-renewal. • Proliferation is a term used to describe a time of immense cellular division, which occurs once the neural tube is formed. It is also termed neurogenesis because it is the beginning of the development of neurons and occurs for the first 5 months of gestation. • Cell migration begins after the first neurons are developed and continues several weeks after neurogenesis is complete. As these cells migrate to the appropriate place, they follow chemical pathways that help to lead them to the correct area or location. • Once cells have migrated, they begin to aggregate; that is, they move toward other cells that have migrated to a similar area to form nervous system structures. • After aggregation comes a time of axonal, dendritic, and synaptic formation also called maturation. Some axonal growth takes placed while the cell is migrating, with the early cell often looking as though it has a rudimentary tail. These structures develop along with other features of the neuron when they reach the location where they will take on a specific role. • When the primitive neuron reaches its specific location, the process of maturation will give the neuron all of its distinguishing features. The axon (the sending end of the neuron), if it has not already started to form, will do so upon arrival at the appropriate site. Also, the dendrites (the receiving end of the neuron) will also show signs of growth. Once axons have reached their intended sites, they develop synapses with the appropriate surrounding neurons. The formation of new synapses is termed synaptogenesis. This is a process that is carried out throughout life. • Bonding depends very specifically on the levels of nerve growth factor, which has been shown to promote the survival and growth of a neuron. Nerve growth factor has been found to be released by organs with which the neuron has begun to form a synapse. If a neuron does not receive nerve growth factor or does not bond correctly to receive this factor, the neuron will begin the preprogrammed process of nerve cell death (apoptosis) and degeneration after a certain time. After death, the space that is left on the postsynaptic membrane is filled by sprouting axons of living neurons. This leads to a massive rearrangement of synaptic connections. • At the 12 week, the brain starts to assume more of a concrete model of the adult brain. Some of the most noticeable characteristics that can be found include the ventricles, which have developed their characteristic butterfly shape. Even more evident are all of the major subdivisions of the brain that now appear: telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon. The Neuron: • The building blocks of the central nervous system are the neurons. Structure Of the Neuron: • Neurons are the main cell sin the body that specialize in communicating with one another or with muscles, glands, or other tissues. • The cell body or soma is the area where neurons assemble proteins, generate energy, and maintain metabolism. Various organelles reside in the soma, where they have their own specific tasks and functions, and are also covered in a specific membrane. • The nucleus of the neuron contains deoxyribonucleic acid (DNA), the genetic code of the human being. If neurons are the building blocks of the central nervous system, DNA would be considered the building blocks of life. • Each strand of DNA is composed of chromosomes, which are long strands of nucleotides that are paired together. When these nucleotides form sequences, they make up the components of genes. • When these nucleotides form sequences, they make up the components of genes. Each gene carries instructions that can be added to other genes in the DNA segment to synthesize a specific protein necessary to bind with other proteins and finally form organelles. From there, the neuron is given structure and function as defined by the DNA and organelles. • The remainder of the cell is made up of cytoplasm, which is the clear, semigelatinous internal fluid of the cell that helps to hold all of the other organelles in place and gives the cell a definite shape. • Mitochondria reside within the cell body and are the sites of energy production; sometimes they are referred to as the power plant inside of the cell. Within the mitochondria, fats, sugars, proteins from food react with oxygen to produce adenosine triphosphate (ATP). ATP is the fundamental source of energy for all cells including the neuron. • The cycle in which ATP is produced is termed the Krebs cycle after its discoverers Hans Krebs. • The endoplasmic reticulum (ER) on appearance resembles a system of spherically shaped membranous sacs. The ER is composed of two areas, the smooth ER and the rough ER.  The smooth ER, which does not contain ribosomes, is important for the synthesis and production of lipids that are used in carbohydrate metabolism. The smooth ER is also involved in the detoxification of the cell from drugs and poisons.  The rough ER receives its name from the rough or lumpy looking appearance that occurs because of the presence of ribosomes. • Ribosomes are the structures inside of the rough ER which take in materials and synthesize proteins. From this point, the proteins are carried either to different areas of the cell where they are needed or to the Golgi apparatus for further modification. • The Golgi complex is a system of membranes that package molecules into vesicles or can also modify the molecules further. These vesicles can then travel anywhere they are needed in a cell or can also be transported out of the cell if need be. • Lysosome: A cellular organelle that contains digestive enzymes and provides the neuron help in recycling and reusing materials. • The roles of the microtubules are a means for quick transport of materials within the neuron or cell as well as maintaining cellular structural support. Microtubules aid in both transportation and ensuring the cell or neuron maintains its integrity. • Information coded in the DNA is transcribed as messenger RNA. Transcription is the synthesis of RNA from a DNA template. This is accomplished through RNA polymerase (an enzyme) basically unzipping the DNA helix and reading one half of the DNA strand while concurrently building the same strand to match the strand being transcribed. When this process is done, the new RNA strand breaks away from the mother DNA molecule and is carried elsewhere for further use. • Neurons are covered by a cell membrane composed of a lipid bilayer, essentially two layers of fat. Protein molecules are embedded in this bilayer and from the basis for the cell membrane’s functions. This bilayer is made up of a hydrophilic exterior (attracting water) and hydrophobic interior (repelling water). This gives it polarity and allows the cell to be selectively permeable to different substances. Parts of the membrane contain channel proteins through which certain molecules may pass, allowing needed nutrients and ions into the cell. Other parts of the membrane contain signal proteins, which transfer a signal to the inside of the neuron when particular molecules bind to them on the outside of the membrane. • The point at which a neuron first receives a signal from another neuron is where the dendrites are located. Dendrites are relatively short, treelike structures, which receive information and bring it to the soma. A neuron may have many dendrites that branch out to other neurons and act as message recievers bringing in multitudes of chemical and electrical information. Dendritic branches are divided into segments, which are called orders. These orders are named based on their locations in relation to the soma. The locations that are closest to the soma are denoted as first order, those that branch from the first order are known as second order, and so on. • Axons send information from the soma to the presynaptic terminal. Signals traveling through the axon generally travel in one direction, starting at the soma. There is almost always only one axon per neuron. However, the axon may have many branches, which, in most cases, eave the axon some distance from the cell body. Axons can also be of various lengths depending on their placement and function, ranging from a few millimeters to a meter. • The axon hillock resides at the junction of the soma and the axon. It is at this point where an impulse or signal is determined to be strong enough to be sent down the axon and on to the next cell. If the sum of the depolarization and hyperpolarization reaching this section of the axon is sufficient to depolarize the membrane to a level called the threshold of excitation, then an action potential is generated. In a sense, the axon hillock is receiving mixed messages and has to sort out which message is the strongest. The all-or-nothing principles applies here: either the signal is strong enough to produce an action potential or it dissipates. • In the resting state, without any stimulation, the electrical charge within the neuron is -70 mV. At this point, the neuro+ is polarized. During t-e resting state, the concentration of the sodium ions (Na ) and chloride ions (Cl ) are greater outside the neuron than inside, while the potassium (K ) ions are more concentrated on the inside. • Sodium-Potassium Pump: Functions to maintain the cell potential; it pumps sodium ions out of the cell and potassium ions into the cell by active transport. The pumps are membrane-embedded protein mechanisms which used the majority of ATP produced by the neuron to propagate the ion flow. • The action potential is a massive momentary reversal of the membrane polarization from -70 to + 50 mV. The two processes which aid the generation of the action potential are diffusion and the concentration gradient.  Diffusion is the tendency of molecules to move from areas of high concentration to areas of low concentration.  The Concentration gradient is the attraction of a region of high levels of molecules to an area of low concentration. • After this has taken place, there is a brief period of time (1-2 milliseconds) in which it is impossible for another action potential to take place. This brief period is called the absolute refractory period. • There is also a relative refractory period when the neuron can respond to a series of impulses which have a greater depolarization charge. This is at a point when the neuron has started to repolarize, but has not yet reached its resting potential; therefore, it requires a very large stimulus or series of stimuli. • Myelin sheath: The fatty substance that covers the axon and speeds conduction.  Myelin is produced by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. Schwann cells are the only cells capable of guiding axonal regeneration. Therefore, in the human, axonal regrowth occurs only in the peripheral nervous system. The myelin sheath speeds conduction of impulses and provides insulation to the axon similar to an electrical wire.  Nodes of Ranvier: The gaps in the myelin sheath that speed axonal conduction.  The electrical movement of the action potential along the nodes of Ranvier is termed salutatory conduction. Jumping between nodes allows for much faster conduction than waiting for an action potential to build up along the axon. • Synapse: The junction across which a nerve impulse passes from an axon terminal to a neuron, muscle cell, or gland. • Synaptic Vesicles: Structures that store neurotransmitters; the vesicles release the neurotransmitters into the synaptic cleft when a nerve impulse reaches the synaptic cleft. • Neurotransmitters are proteins which have been packaged and stored by Golgi bodies in a series of vesicles infused by microtubules to gather at the terminal ends of the axon. The vesicles gather together to await an action potential. Often the neurotransmitters are located nex
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