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Chapter 2

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Janelle Leboutillier

Chapter 2 NROB60 Chapter 2 Neurons and Glia Introduction  In neuroscience there is no need to separate mind from brain o Once we fully understand the individual and concerted actions of brain cells we will understand the origins of our mental abilities  Neurons and glia are two diff. types of cells  Neurons: information processing cell of the nervous system; also called nerve cells o Most neurons use AP to send signals, others communicate using synaptic transmission o Sense changes in the environment , communicate these changes to other neurons and command the body’s response to these sensations  Glia/glial cells: a support cell in the nervous system o 4 categories: astrocytes, oligodendroglia, schwann cells and microglia o Thought to contribute to brain function mainly by insulating, supporting, and nourishing neighboring neurons o Perform bulk of information processing in the brain o Help hold in all the neurons together The Neuron Doctrine  Most challenging part of studying brain cell structure is the size since they are about 0.01- 0.05 mm in diameter th  Neuroscience only developed after compound microscopes in the late 17 century  Brain tissue needs to be hardened (fixed) in formaldehyde and then sliced into thin slices by a microtome before it could be studied  Histology: microscopic study of the structure of tissues  Thin slices are usually all the same color under a microscope so there is no difference in pigmentation to identify structures o Stains were introduced that could selectively color some parts of the tissue  Franz Nissl – introduced a stain that is still used today o Introduced a class of basic dyes that would stain the somata of neurons; nuclei of all cells and also stain clumps of material surrounding the nuclei of neurons (Nissl bodies) o ^Nissl stain  2 Advantages of Nissl Stain: o Distinguishes neurons and glia from one another o Enables histologists to study the arrangement (cytoarchitecture) o Cytoarchitecture: The arrangement of neuronal cell bodies in various parts of the brain  The study of this help progress the idea that each region of the brain performs a different function The Golgi Stain  Golgi stain: a method of staining brain tissue that shows neurons and all of their neuritis; o Silver chromate solution discovered by Camillo Golgi  A small percentage of neurons became darkly colored in their entirety o Revealed the neuronal cell body, the region of the neuron around the nucleus that shown with the Nissl stain, is actually only a small fraction of the total structure of the neuron  Golgi stain shows that neurons have at least two distinguishable parts: a central region that contains the cell nucleus, and numerous thin-tubes that radiate away from the central region  Cell body: The central region of the neuron containing the nucleus o swollen region containing the cell nucleus o (AKA soma, perikaryon)  Neurites: thin tubes that radiate away from the soma Chapter 2 NROB60 o Axons  Neurite specialized to conduct nerve impulses or action potentials, normally away from soma  Carry the output of neurons o Dendrites  A neurite specialized to receive synaptic inputs from other neurons  Rarely extend more than 2mm In length  Extend from cell body and generally taper to a fine point  Cell body usually gives rise to a single axon; uniform in diameter along length o Branches are generally extended at right angels  Because dendrites come in contact with many axons, they must act as antennae of neuron to receive input Cajal’s Contribution  Santiago Ramon y Cajal – skilled histologists that along with Golgi worked out many circuitry of many regions of brain  Golgi – found neuritis of different cells are fused together to form a continuous reticulum, or network, similar to the arteries and veins of the circulatory system  Cajal – argued forcefully that the neuritis of different are not continuous with one another and must communicate by contact, not continuity  Neuron doctrine: The concept that the neuron is the elementary functional unit of the brain and that neurons communicate with each other by contact, not continuity  Later discovered that neuritis of different neurons are not continuous with one another The Prototypical Neuron  Inside of the neuron is separated from the outside by the limiting skin, the neuronal membrane, which lies like a circus tent on an intricate internal scaffolding giving each part of the cell its special 3D appearance The Soma  Round, spherical central part of the neuron  Cytosol: salty, potassium-rich solution that is separated from the outside by the neuronal membrane  Organelles: membrane-enclosed structures within soma  Cell body of neuron contains same organelles that are found in all animal cells o Most important ones are nucleus, rough ER, smooth ER, Golgi apparatus and mitochondria  Cytoplasm: Cellular material contained by the cell membrane, including the organelles but excluding the nucleus The Nucleus  Is spherical, centrally located and about 5-10um across  Contained within a double membrane called nuclear envelope (perforated by pores that measures about 0.1um across)  Chromosomes: contain genetic material that are found within the nucleus o Each one contains an uninterrupted double-stranded braid of DNA, 2nm wide  DNA (deoxyribonucleic acid): genetic material o Was passed on to us from each of our parents and contains blueprints for our body  Genes: a unit of heredity; a sequence of DNA that encodes a single polypeptide or protein  Gene expression: The process of transcribing the information from a gene into messenger RNA; a gene is a segment of DNA carrying instructions for a single protein  Proteins: A polymer of amino acids strung together by peptide bonds o Exists in a wide variety of shapes and sizes, perform many different functions and bestow upon neurons virtually all of their unique characteristics Chapter 2 NROB60  Product of gene expression is proteins synthesis  Protein Synthesis: The assembly of protein molecules in the cell’s cytoplasm according to genetic instructions  Messenger ribonucleic acid/mRNA: a molecule constructed from four nuclei acids that carries the genetic instructions for the assembly of a protein from the nucleus to the cytoplasm  Transcription: The process of synthesizing a mRNA molecule according to genetic instructions encoded in DNA o Resulting mRNA is called a transcript  Promoter: A region of DNA that binds RNA polymerase to initiate gene transcription o RNA polymerase  Transcription factors: a protein that regulates the binding of RNA polymerase to a gene promoter  Terminator: RNA polymerase recognizes this as the end point  Introns: additional stretches of DNA within the gene itself that cannot be used to code for protein  Exons: coding sequences  RNA splicing: introns are remove and the remaining exons are fused together  In some cases, specific exons are also removed with the introns leaving an “alternatively spliced” mRNA that actually encodes a different protein  mRNA transcripts emerge from the nucleus via pores in the nuclear envelope and travel to the sites of protein synthesis elsewhere in the neuron o At these sites, a protein molecule is assembled by linking together many small molecules into a chain  Amino acids: A chemical building block of protein molecules, containing a central carbon atom, an amino group, a carboxyl group, and a variable R group  Translation: The process of synthesizing a protein molecule according to genetic instruction carried by a mRNA molecule Rough Endoplasmic Reticulum  Ribosomes: a cellular organelle that assembles new proteins from amino acids according to mRNA instructions  Rough Endoplasmic Reticulum/Rough ER: a membrane enclosed cellular organelle with ribosomes attached to its outer surface o Site of synthesis for proteins destined to be inserted into membrane or to be enclosed by membrane o Abounds in neurons, far more in glia o AKA Nissl bodies  Major site of protein synthesis in neurons  RNA transcripts bind to the ribosomes and the ribosome translate the instructions contained in the mRNA to assemble a protein molecule o Thus, ribosome take raw material in the form of amino acids and manufacture proteins using the blueprint provided by the mRNA  Some ribosomes are not attached to rough ER; freely floating (free ribosomes)  Polyribosomes: A collection of several ribosomes floating freely in the cytoplasm o Single strand of mRNA and the associated ribosomes are working on it to make multiple copies of the same protein  Free ribosomes make proteins that are destined to reside within the cytosol of the neuron  Rough ER makes proteins that are inserted into the membrane of the cell or an organelle Smooth Endoplasmic Reticulum and the Golgi Apparatus  Remainder of they cytosol of the soma is crowded with stacks of membranous organelles that look a lot like rough ER without the ribosomes o Smooth Endoplasmic Reticulum/Smooth ER: a membrane-enclosed cellular organelle that is heterogeneous and performs different functions in different locations Chapter 2 NROB60  Smooth ER is quite heterogeneous and performs different functions in different locations  Smooth ER is continuous with Rough ER and is believed to be a site where proteins that jut out from membrane are folded into 3D structure  Other types of smooth ER play no direct role in processing of protein molecules but instead regulate the internal concentrations of substances (ex. Calcium) o Usually in muscles cells  called sarcoplasmic reticulum  Golgi apparatus: stack of membrane-enclosed disks in the soma that lies farthest from the nucleus o Site of post – translational chemical processing of proteins o Sorting of certain proteins that are destined for delivery to different parts of the neurons (ex. Axons and dendrites) Mitochondria  Mitochondrion: sausage-shaped structures  Within enclosure of their outer membrane are multiple folds of inner membrane (cristae)  Between crista is an inner space called matrix  Mitochondria are site of cellular respiration o When mitochondria inhales, it pulls inside pyruvic acid and oxygen (both floating in the cytosol) o Within inner compartment of mitochondria, pyruvic acid enters into a complex series of biochemical reactions (Krebs cycle) o Products of Krebs cycle provide energy that results in ADP (reactions in cristae lead to ATP)  ATP – o Energy currency of the cell The Neuronal Membrane  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  Some membrane-associated proteins pump substances from inside to the outside  Others form pores that regulate which substances enter and leave  Protein composition of membrane varies if its in soma, dendrites or axons  Function of neurons is understood by understanding structure and function of the membrane and its associated proteins The Cytoskeleton  Cytoskeleton  Gives neuron its characteristic shape  Bones of cytoskeleton – microtubules, microfilaments and neurofilaments  Elements of the cytoskeleton are not stationary…neurons mover around all the time Microtubules  Microtubules  Bit and run long
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