NROB60 Chapter 2.doc

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Human Biology
Janelle Le Boutillier

NROB60 – Chapter 2 Introduction: - Structure of different types of cells in the nervous system: o Neurons – most important cells for the unique functions of the brain  Sense changes in the environment, communicate these changes to other neurons and command the body’s responses to these sensations.  10% of brain cells are neurons o Glia – outnumbers neurons by tenfold  Contributes to brain function mainly by insulating, supporting and nourishing neighboring neurons. The Neuron Doctrine: - Most cells are th the range of 0.01-0.05 mm in diameter - Early in the 19 century, scientists discovered how to harden/fix tissues by immersing them in formaldehyde o They developed a special device called a microtome, to make very thin slices o Histology – the microscopic study of the structure of tissues - Stains were used to selectively color parts of the cells in brain tissue o Nissl showed that a basic class of basic dyes would stain the nuclei of all cells and clumps of material surrounding the nuclei of neurons  These clumps are called Nissl bodies  The stain is known as the Nissl stain, useful for 2 reasons • Distinguishes neurons and glia from one another • Enables histologists to study the arrangement, or cytoarchitecture, of neurons in different parts of the brain The Golgi Stain: - Golgi discovered that by soaking brain tissue in a silver chromate solution (now called Golgi stain), a small percentage of neurons became darkly colored in their entirety. o The Golgi stain shows that neurons have at least 2 distinguishable parts:  Central region – that contains the cell nucleus  Numerous thin tubes – that radiate away from the central region - The swollen region containing the cell nucleus has several names that are used interchangeably: o Cell body o soma o perikaryon - The thin tubes that radiate away from the soma are called neurites and are of 2 types: o Axons – is of uniform diameter throughout its length and if it branches, the branches extend at right angles.  Axons must act like wires that carry the output of the neurons o Dendrites – rarely extend more than 2 mm in length  They must act as the antennae of the neuron to receive incoming signals, or input Cajal’s Contribution: - Cajal used the Golgi stain to work out the circuitry of many regions of the brain. - Golgi and Cajal had opposite conclusions about neurons: o Golgi – viewed that the neuritis of different cells are fused together to form a continuous reticulum or network similar to the arteries and veins of the circulatory system. o Cajal – argued that the neuritis of different neurons are not continuous with one another and must communicate by contact not continuity. - Neuron doctrine – the idea that the neuron adhered to the cell theory - With the help of the later invention of the electron microscope, it was possible to show that the neurites of different neurons are not continuous with one another The Prototypical Neuron: - The inside of the neuron separated from the outside by the limiting skin, the neuronal membrane, lies like a circus tent on an intricate internal scaffolding The Soma: - The cell body of the typical neuron is about 20 um in diameter. o Cytosol – the watery fluid inside the cell, that is a salty, potassium-rich solution that is separated from the outside by the neuronal membrane o Organelles – A number of membrane-enclosed structures within the soma - The most important organelles that are found in all animal cells are: o Nucleus o Rough endoplasmic reticulum o Smooth endoplasmic reticulum o Golgi apparatus o Mitochondria - Cytoplasm - everything contained within the confines of the cell membrane, including the organelles but excluding the nucleus The Nucleus: - Is spherical, centrally located and about 5-10 um across - It is contained within a double membrane called the nuclear envelope o The nuclear envelope is perforated by pores that measure about 0.1 um across - Within the nucleus are chromosomes, which contain the genetic material, DNA (deoxyribonucleic acid) o The specific parts of the DNA that are used to assemble the cell are what distinguish a neuron from a liver cell.  These segments of DNA are called genes - Gene expression – the reading of the DNA o Proteins – the synthesis of molecules which are the final product of gene expression  Protein synthesis – the assembly of protein molecules, occurs in the cytoplasm - Messenger ribonucleic acid (mRNA) – a long molecule that carries the genetic message to the sites of protein synthesis in the cytoplasm o Consists of 4 different nucleic acids strung together in various sequences to form a chain o Transcription – process of assembling a piece of mRNA that contains the information of a gene  Transcript – the resulting mRNA - Promotor – is at one end of the gene, the region where the RNA-synthesizing enzyme, RNA polymerase, binds to initiate transcription o Transcription factors – proteins that regulate the binding of the polymerase to the promoter - Terminator – a sequence on the other end of DNA that the RNA polymerase recognizes as the end point for transcription - Introns – interspersed regions o Exons – coding sequences o RNA splicing – removal of introns and the remaining exons are fused together - Transcription of a single gene can ultimately give rise to several different mRNAs and protein products. - Amino acids – building blocks of a protein, there are 20 different kinds o Translation – assembling of proteins from amino acids under the direction of the mRNA Rough Endoplasmic Reticulum: - Ribosomes – enclosed stacks of membrane dotted with dense globular structures, measuring about 25 nm in diameter o Rough endoplasmic reticulum – the stacks of ribosomes  Abounds in neurons far more than in glia  Another name for Rough ER is Nissl bodies  Is a mjor site of protein synthesis in neurons o Ribosomes take raw material in the form of amino acids and manufacture proteins using the blueprint provided by the mRNA o Free ribosomes – freely floating ribosomes that are not attached to the rough ER  Polyribosomes – several free ribosomes appear to be attached by a single strand of mRNA Smooth Endoplasmic Reticulum and the Golgi Apparatus: - Smooth endoplasmic reticulum –stacks of membranous organelles that look a lot like rough ER without the ribosomes o Continuous with rough ER and is believed to be a site where the proteins that jut out from the membrane are carefully folded, giving them their 3-dimensional structure o Other types of smooth ER just regulate the internal concentrations of substances - Golgi apparatus – stack of membrane-enclosed disks in the soma that lies farthest from the nucleus o A site of extensive post-translational chemical processing of proteins o One important function is the sorting of certain proteins that are destined for delivery to different parts of the neuron, such as axon and dendrites. The Mitochondrion: - Sausage-shaped structures measuring about 1 um in length. o Cristae- multiple folds within the enclosure of their outer membrane o Matrix – an inner space between the cristae - Are the site of cellular respiration o When a mitochondrion inhales, it pulls inside pyruvic acid and oxygen  Pyruvic acid enters into a series of biochemical reactions called the Krebs cycle  Krebs cycle provide energy within the cristae called the electron-transport chain • Resulting in the addition of phosphate to adenosine diphosphate (ADP), yielding adenosine triphosphate (ATP) o When the mitochondrion exhales, 17 ATP molecules are released for every molecule of pyruvic acid  ATP is the energy currency of the cell The Neuronal Membrane: - Serves as a barrier to enclose the cytoplasm inside the neuron and to exclude certain substances that float in the fluid that bathes the neuron - The membrane is about 5 nm thick and is studded with proteins o Some of the membrane-associated proteins pump substances from the inside to the outside o Other form pores that regulate which substances can gain access to the inside of the neuron - An important characteristic of neurons is that the protein composition of the membrane varies depending on whether it is in the soma, dendrites or the axon The Cytoskeleton: - Internal scaffolding that gives the neuron its characteristic shape. o Bones of the cytoskeleton are:  Microtubules  Microfilaments  Neurofilaments Microtubules: - 20 nm in diameter, they are big and run longitudinally down neurites o Appears as a straight, thick-walled hollow pipe o Tubulin – protein that makes up the smaller strands of the wall of the pipe  A single tubulin molecule is small and globular o Polymerization - the process of joining small protein to form a long strand  Polymer – the resulting strand o Microtubule-associated proteins (MAPs) – one class of proteins that partic
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