NROB60_Chapter_2.docx

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Department
Human Biology
Course
HMB430H1
Professor
Malcolm Mac Kinnon
Semester
Fall

Description
Chapter 2: Neuron and Glia  Neurons and Glia are cells in the nervous system and are broad categories, within which are many types of cells that differ based on their structure, chemistry and function  Glia outnumber neurons by tenfold  Neurons are the most important cells for the unique functions of the brain. o They sense the changes in the environment, communicate these changes to other neurons, and command the body’s responses to these sensations.  Glia (glial cells) are thought to contribute to brain function mainly by insulating, supporting, and nourishing neighboring neurons The Neuron Doctrine  The Golgi Stain o The Golgi Stain shows that neurons have at least two distinguishable parts: a central region that contains the cell nucleus and numerous this tubes that radiate away from the central region o The swollen region containing the nucleus has several names that are used interchangeably; most notably the cell body and soma o The thing tubes that radiate away from the soma are called neuritis and are of two types: axons and dendrites o The cell body usually gives rise to a single axon.  The axon is of uniform diameter throughout its length, and if it branches, the branches generally extend at right angles  Axons can travel long distances (a meter or more); dendrites, however, rarely extend more than 2 mm in length.  Cajal’s Contribution o Argued that the neuritis of different neurons are not continuous with one another and must communicate by contact, not continuity o This became known as the neuron doctrine and the brother won a Nobel Prize The Prototypical Neuron  The Soma o Roughly spherical central part of the neuron o Is about 20 µm in diameter o Watery fluid inside the cell (cytosol) is salty, potassium rich and separated from the outside o The cell body of the neuron contains the same organelles that are found in all animal cells  The most important:  Nucleus  The rough ER  The smooth ER  The Golgi apparatus  Mitochondria o Everything within the cell membrane, excluding the nucleus is collectively known as the cytoplasm o The Nucleus  Spherical, centrally located and is contained within a double membrane called nuclear envelope  Contains chromosomes and DNA o The Rough Endoplasmic Reticulum  Not far from the nucleus  Enclosed stacks of membrane dotted with ribosomes  Contains an abudance of neurons  Major site of protein synthesis in neurons  Some are produced on free ribosomes  So what’s the difference?  If the protein is destined to reside within the cytosol of the neuron, then the protein’s mRNA moves towards the free ribosome  If the protein is destined to be inserted in a membrane or an organelle, then it is synthesized in the rough ER. o Smooth ER and the Golgi Apparatus  Is actually quite heterogeneous and performs different functions in different locations  Some types of smooth ER play no direct role in the processing of protein molecules but instead regulate the internal concentrations of substances  The Golgi apparatus is believed to be the sorting of certain proteins that are destined for delivery to different parts of the neuron (e.g. axon and dendrites) o The Mitochondrion  Very abundant in the soma  Site of cellular respiration and uses ATP as energy currency o The Neuronal Membrane  Serves as a barrier to enclose the cytoplasm inside the neuron and to exclude certain substances between both sides of the barrier o The Cytoskeleton  Gives the neuron its characteristic shape  The bones that make up the cytoskeleton are the microtubules, microfilaments, and neurofilaments  Microtubules  Big and run longitudinally down neuritis  Appears as a straight, thick-walled hollow pipe  Microfilaments  About the same thickness as a cell membrane  Found throughout the neuron and are particularly numerous in the cell membrane  Neurofilaments  Are intermediate in size  Only neurons they are called neurofilaments but exist in all cells  The Axon o Found only in neurons and is highly specialized for the transfer of information over distances in the nervous system o It begins with a region called the axon hillock o Two features distinguish the axon from the soma:  No rough ER extends into the axon, and there are few, if any, free ribosomes  The protein composition of the axon membrane is fundamentally different from that of the soma membrane o Since there are no ribosomes, no protein synthesis can occur in the axon and thus, all proteins present must originate in the soma.  The different proteins in the axonal membrane allow it to serve as a telegraph wire and send information over great distances o Axons often branch and these branches are called axon collaterals  Those branches that return to communicate with the same cell that gave rise to the axon or with the neighbouring cells are called recurrent collaterals. o The diameter of an axon is variable and this variation is important  The thicker the axon, the faster the information is sent and the thinner the axon, the slower the information is sent  The Axon Terminal o All axons have a beginning (the axon hillock), a middle (axon proper) and an end called the axon terminal or terminal bouton o It is the site where the axon comes in contact with other neurons and passes info onto them o This point of contact is referred to as the synapse o Sometimes axons have many branches at their ends and e
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