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Chapter 2

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Zachariah Campbell

Chapter 2 – Central Nervous System: Structure and Function Prenatal Development External Factors Prenatal development = all steps involved with the formation of various structures and functions of the body. Parental decisions: • Proper nutrition is critical – low birth weight, smaller head circumference may be caused by malnutrition. May later lead to obesity and problems in programming the appetite regulatory system correctly in the fetus • Abstaining from alcohol and other drugs, decreasing stress level, and avoiding physical/emotional abuse. Alcohol and drugs can cross the placental barrier around the fetus o Fetal alcohol syndrome: can cause serious cognitive difficulties o Increased stress levels in the mother can raise cortisol amount in the body. Cortisol: a steroid hormone that elevates blood sugar and metabolism to help the body adapt to prolonged stress. Too much can lead to depression of the immune system. Physical abuse: • Severe physical abuse to the mother can lead to brain impairment in the baby • Shaken baby syndrome: when the baby is shaken so hard that their brain impacts with the skull. Characteristic injuries subdural hemorrhages, retinal hemorrhages, fractures of ribs or long bones Emotional abuse: • Abuse to mother could lead to her neglecting herself and fail to care for physical needs. Can affect child and cause neglect of other children. Development of the Central Nervous System Germinal period: period of time from conception to when implementation in the uterine wall occurs – about 2 weeks between • Developing cells are called a zygote • By end of first week, it’s a blastocyst – consists of tissue that will become the embryo and the trophoblast (provides nutrition) • With implantation, the cells are now considered an embryo Embryonic stage: lasts until the end of the first trimester Embryo then begins to differentiate into 3 distinct layers: • Ectoderm: outermost layer. Later develops into the skin, sense organs, and nervous system • Mesoderm: middle layer. Becomes the muscles, blood, and excretory system • Endoderm: innermost layer. Becomes the digestive system, lungs, other internal organs. The life support system for the embryo also develops: • Amnion: sack of fluid where embryo floats for temperature regulation and protection • Umbilical cord: connects embryo to placenta • Placenta: group of tissues in which blood vessels from the embryo and the mother mix but do not join. o Particles (water, salt, oxygen) go from mother-to-child while waste and carbon dioxide go from child-to-mother Next is development is the beginning of the nervous system 1. Neural plate: made up of ectodermal tissue on the dorsal side of the embryo a. Becomes evident at 3 weeks and appears to be stimulated by signals from the mesoderm b. At this time, all the cells are totipotent – can be any cell in the body. As the neural plate develops, the cells lose this ability 2. The neural plate folds to forms the neural groove 3. The edges of the groove fuse to form the neural tube – about 24 days after conception. a. The inside of the neural tube becomes the spinal canal and the cerebral ventricles. b. The swellings at the end of the tube become the forebrain, midbrain, and hindbrain. At the end of 7 weeks, the embryo is referred to as the fetus and resembles a more human shape. The brain is an almost complete replication of the adult brain at approximately 100 days from conception, even though the structures are not completely developed. The neural crest is dorsal to the neural tube. It’s formed from cells that differentiate from the neural tube as it is being formed. • This develops into the neurons and glial cells Prenatal Neuronal Development Induction: begins when part of the ectoderm becomes the nervous system. This begins during the development of the neural plate. • The cells are still totipotent at this stage or stem cells: they are able to develop into different types of cells and have an almost unlimited capacity for self-renewal o As the stages progress, these cells become more specific = specialized cell function. Proliferation: used to describe a time of immense cellular division, which occurs when the neural tube is formed. • Also termed neurogenesis because is beginning of development of neurons occurs in the first 5 months of gestation. Cell migration: begins after the first neurons are developed and continues several weeks after neurogenesis is complete. As cells migrate to their place, they follow chemical pathways that lead them to their correct location. Cell aggregation: occurs once migration is complete. The cells move to other cells that have migrated to a similar are to form nervous system structures. Maturation: time of axonal, dendritic, and synaptic formation. • Some axonal growth occurs during migration • When a primitive neuron reaches a specific location, maturation will give the neuron all of its distinguishing features • 1 neuron can grow 1 axon. It takes 2 neurons for one synapse. o Synaptogenesis: the formation of new synapses. This will occur all through a person’s life but especially during early periods of development. Apoptosis: if a neuron does not receive nerve growth factor or does not bond correctly to receive this factor, the neuron will begin the programmed process of nerve cell death. • The space that is left by the dead cells is filled with the axons of the living neurons = rearrangement of synaptic connections. At the 12 week, the brain starts to assume more of a concrete model of the adult brain. • Most noticeable characteristics: ventricles; telencephalon; diencephalon; mesencephalon; mylencephalon. The Neuron A baby may have as many as 100 billion neurons at birth. Structure of the Neuron Neurons are the main cells in the body that specialize in communicating with one another or with muscles, glands, and other tissues. Soma: area where neurons assemble proteins, generate energy, and maintain metabolism. • Nucleus: contains DNA (deoxyribonucleic acid) o Each strand of DNA is made up of chromosomes: long strands of nucleotides that are paired together o When nucleotides form sequences, they make up the components of genes. o Each gene carries instructions that can be added to other genes in the DNA segment to synthesize a specific protein necessary to form organelles. • Cytoplasm: internal fluid that holds organelles in place within the cell • Mitochondria: site of energy production for cells o Adenosine triphosphate (ATP): the energy source for neurons and other cells; consists of adenosine bound to ribose and three phosphate group. Fats, sugars, proteins from food react with oxygen to produce ATP o Cycle in which ATP is formed is called the Krebs cycle • Endoplasmic reticulum (ER): network of tubules within a cell that transports synthesized lipids and membrane proteins to other locales o Smooth ER: does not contain ribosomes; important for the synthesis and production of lipids that are used in carbohydrate metabolism. Also involved in the detoxification from drugs and poisons o Rough ER: contains ribosomes (takes in materials and synthesizes proteins) • Golgi complex: system of membranes that package molecules into vesicles or can modify the molecules further • Lysosome: contains digestive enzymes and provides the neuron help in recycling and reusing materials • Microtubules: tubules that quickly break down materials within the neuron and maintaining cellular structural support • Messenger RNA: information coded in the DNA is transcribed on this o Transcription: synthesis of RNA from a DNA template. o Accomplished through RNA polymerase (enzyme) pretty much unzipping the DNA helix and reading one half of the DNA strand while concurrently building the same strand to match the strand that is being transcribed o The RNA strand breaks away from the DNA and is carried elsewhere o RNA is similar to DNA except that it contains uracil instead of thymine. Its backbone is phosphate and ribose instead of ribose and deoxyribose o Messenger RNA carries the genetic code from the neuron to the rough ER where it is transcribed into proteins • Lipid bilayer: membrane that covers the neuron and is made of two layers of fat; it allows selective permeability to certain substances o Made up of hydrophilic exterior and hydrophobic interior. This gives it polarity and the selective permeability o Channel proteins: certain molecules can pass through here, providing nutrients and ions into the cell o Signal proteins: transfer a signal to the inside of the neuron when certain molecules bind to them on the outside of the membrane Dendrite: structures that receive information and send it to the body of the neuron • Neuron many have many dendrites that branch out to other neurons • Dendritic branches are divided into segments, called orders, named based on their location in relation to the soma o First order is closest to the soma and then so on Axons: structure that sends information from the cell body to the synapse • Signals from the axon usually travel in one direction, starting from the soma • A neuron usually only has one axon. The axon can have many branches. • Axons can vary in length from millimeters to a meter Axon hillock: structure on the body of the axon that determines whether an impulse is strong enough to cause an action potential Action potential: massive momentary reversal of the membrane potential from -70 to +50 mV; synonymous with firing of the neuron • Of the sum of depolarization and hyperpolarization is sufficient to depolarize the membrane to the threshold of excitation, the action potential is generated • All-or-nothing principle: either the signal is strong enough to produce an action potential or it dissipates • Resting state: without any stimulation, the electrical charge within the neuron is + - -70 mV and the neuron is polarized. The concentration of Na and Cl are greater outside the neuron while the concentration of the K is more concentrated outside the neuron o Sodium-potassium pump: functions to maintain the cell potential; pumps out sodium ions and potassium ions in by active transport o Diffusion: tendency of molecules to move from areas of high concentration to areas of low concentration o Concentration gradient: attraction of a region of high levels of molecules to an area of low concentration • Absolute refractory period: brief period of time (1-2 milliseconds) when it is impossible to have another action potential take place • Relative refractory period: period when the neuron can respond to a series of impulses that have a greater depolarization charge. This is when the neuron has started to repolarize but has not returned to resting potential Myelin she
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