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7. Neuroscience 2 - The Neuron - Structure.doc

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University of Toronto St. George
Mac Burnham

NEUROSCIENCE 2 THE NEURON – STRUCTURE Reading: Carlson – Chapter 2 (p. 29-41) Carlson, Chapter 2, p. 42-53. OUTLINE -General Characteristics of the -Neuron -Special Characteristics of the Neuron -Names for Parts of the Neuron -Types of Neuronsof Synapse 1. GENERAL CHARACTERISTICS OF THE NEURON FIGURE 2.5 The neuron has the same basic characteristics of any eukaryotic single cell: nucleus, cytoplasm, cell membrane (not porous) Nucleus: • DNA • mRNA • nuclear membrane (porous) Cytoplasm – components • water (80%) with electrolytes (potassium, magnesium, phosphate, sulfate, bicarbonate plus trace levels of sodium, chloride, calcium) • proteins (10-20%) – structural (fibrillar) and enzymes (globular) • lipids (2-3%) – triglycerides plus cell membrane (below) • carbohydrates (1%) – glycogen Cytoplasm – organelles • mitochondria (oxidative metabolism) • rough endoplasmic reticulum with ribosomes (protein manufacture) • smooth endoplasmic reticulum (fats, etc.) • Golgi apparatus o Special form of smooth E.R. Some complex molecules, made up of simpler individual molecules, are assembled here. o Produces lysosomes: small sacs that contain enzymes that break down substances no longer needed by the cell • Cytoskeleton: thickest of the protein strands: microtubules. They produce hollow tube. • Cell membrane (not porous) - phospholipid bilayer, cholesterol, membrane proteins 2. SPECIAL CHARACTERISTICS OF THE NEURON (“CONNECTORS”) o receives and sends chemical signals o very long – “biological cables” (microtubules) o synapses with other neurons (also muscles, glands) o biological cable with chemical “interruptions” (conditional) Interneurons are neurons located entirely within the central nervous system (i.e. they are not motor or sensory neurons).  Local interneurons form circuits with nearby neurons and analyze small pieces of information  Relay interneurons connect circuits of local interneurons in one region of the brain with those in other regions Terminal buttons need some items that can be produced only in the soma. There therefore needs to be a system that can transport these items rapidly and efficiently through the axoplasm (the cytoplasm of the axon)  Axoplasmic transport enables this transport. It is an active process by which substances are propelled along microtubules that run the length of the axon. o Movement from the soma to the terminal buttons is called anterograde axoplasmic transport. This is accomplished by molecules of the protein kinesin (they resemble a pair of legs and feet attached to a vesicle). Energy is supplied by ATP molecules produced in the mitochondria.  ~500 mm per day o Dynein is another protein that carries substances from the terminal buttons to the soma, a process known as retrograde axoplasmic transport.  ~half as fast as anterograde transport Neurons have a high rate of metabolism but no means of storing nutrients – therefore need to be constantly supplied with nutrients and oxygen.  Glial cells surround neurons and hold them in place, controlling their supply of nutrients and some of the chemicals they need in order to exchange message with other neurons; they insulate neurons from one another so that neural messages do not get scrambled; and they even act as housekeepers, destroying and removing the carcasses of neurons that are killed by disease or injury. Three most important tyepes of glial cells: o Astrocyte: star-shaped and provide physical support to neurons and clean up debris within the brain. Also produce some chemicals neurons need to fulfill their function. Help control the chemical composition of the fluid surrounding neurons by keeping concentrations within critical levels. Also involved in providing nourishment to neurons  Partially wrapped around both blood vessels and neurons: allow transportation of nutrients from the blood as well as allowing disposal of waste products into blood. Take glucose from capillaries, break it into lactate, which neurons take up. Neurons transport this lactate into the mitochondria and use it for energy.  Astrocytes also store small amount of glycogen.  Also serve as matrix that holds neurons in place “nerve glue” Glial cells continued: o Oligodendrocytes are another type of glial cell. Their principal function is to provide support to axons and to produce the myelin sheath.  Myelin is 80% lipid and 20% protein and is discontinuous  Discontinuous spaces where myelin sheath is not present is called node of Ranvier. o Microglia are the smallest of glial cells. Like some types of astrocytes, they act as phagocytes, engulfing and breaking down dead and dying neurons.  Microglia also serve an immune function. They protect the brain from invading microorganisms and are primarily responsible for the inflammatory reaction in response to brain damage. Schwann cells: are cells in the peripheral nervous system that are wrapped around a myelinated axon, providing one segment of its myelin sheath.  Most axons in PNS are myelinated.  Schwann cells provide myelin for only one axon as opposed to in the CNS where the oligodendrocytes wrap around a number of axons. Blood-Brain Barrier: barrier between the blood and the cerebrospinal fluid (CSF) of the CNS. It is selectively permeable unlike blood vessels, which allow free exchange of most substances between blood plasma and the fluid outside of the capillaries.  The BBB is not uniform throughout the nervous system. It is relatively permeable in some places. E.g., the area postrema is part of the brai
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