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Classification of Neurons Nature of the Nervous System Camillo Golgi The Synctium continuous network no gaps ~ Nature of the Nervous System Santiago Ramon y Cajal The Neuron Doctrine discrete cells communication across gaps synapses ~ Nature of the Nervous System Golgi won Proved himself wrong Golgi Stain silver chromate Supported Neuron Doctrine Golgi & Cajal shared Nobel Prize (1906) ~ Classification of Neurons Structural – based on the number of cytoplasmic processes Multipolar neurons Bipolar neurons Unipolar neurons Functional – based on the direction of impulse transmission Sensory neurons Motor neurons Interneurons (association) Histology of Nervous Tissue 2 types of cells Neurons – Structural & functional part of nervous system – Specialized functions Neuroglia (glial cells) – Support & protection of nervous system NEURONS Basic functional unit of N.S. Specialized cell All cells have same basic properties information processing Transmits Integrates Stores Regulation of behavior ~ Neurons Function Conduct electrical impulses Structure Cell body – Nucleus with nucleolus – Cytoplasm (perikaryon) Cytoplasmic processes – Dendrites – Axon Review of Neuron Structure Nervous Tissue: Neurons Neurons = nerve cells Cells specialized to transmit messages Major regions of neurons Cell body – nucleus and metabolic center of the cell Processes – fibers that extend from the cell body (dendrites and axons) Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.8 Neuron Cell Body Location Most are found in the central nervous system Gray matter – cell bodies and unmylenated fibers Nuclei – clusters of cell bodies within the white matter of the central nervous system Ganglia – collections of cell bodies outside the central nervous system Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.13 Neuron Anatomy Cell body Nucleus Large nucleolus Figure 7.4a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.9b Neuron Anatomy Extensions outside the cell body Dendrites – conduct impulses toward the cell body Axons – conduct impulses away from the cell body (only 1!) Figure 7.4a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.10 Axon Structure Long, specialized Collaterals = branches Telodendria = termination of axons & collaterals Cytoplasm = axoplasm Plasma membrane = axolemma Anatomy of a Neuron Anatomy of a Neuron Axons and Nerve Impulses Axons end in axonal terminals Axonal terminals contain vesicles with neurotransmitters Axonal terminals are separated from the next neuron by a gap Synaptic cleft – gap between adjacent neurons Synapse – junction between nerves Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.11 Nerve Fiber Coverings Schwann cells – produce myelin sheaths in jelly-roll like fashion Nodes of Ranvier – gaps in myelin sheath along the axon Figure 7.5 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.12 Application In Multiple Scleroses the myelin sheath is destroyed. The myelin sheath hardens to a tissue called the scleroses. This is considered an autoimmune disease. Why does MS appear to affect the muscles? Neuron Classification Figure 7.6 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.15 Types of Neurons Functional classification Sensory or afferent: Action potentials toward CNS Motor or efferent: Action potentials away from CNS Interneurons or association neurons (CNS) Principle neurons / projection neurons (CNS) Structural classification Multipolar, bipolar, unipolar Shorthand Neuron Classification Categories overlap ~ Functional Classification of Neurons Interneurons (association neurons) Found in neural pathways in the central nervous system Connect sensory and motor neurons Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Functional Classification of Neurons Sensory (afferent) neurons Carry impulses from the sensory receptors Cutaneous sense organs Proprioceptors – detect stretch or tension Motor (efferent) neurons Carry impulses from the central nervous system Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide By function (connections) Sensory Motor Interneuron By morphology (# of neurites) Multipolar Bipolar Unipolar Structural Classification of Neurons Multipolar neurons – many extensions from the cell body Figure 7.8a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Structural Classification of Neurons Bipolar neurons – one axon and one dendrite Figure 7.8b Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Structural Classification of Neurons Unipolar neurons – have a short single process leaving the cell body Figure 7.8c Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Neuroglia Neuroglia of CNS Astrocytes – – – – Form the blood-brain barrier Form a structural framework for the CNS Repair damaged neural tissue Control the interstitial environment of the CNS Oligodendrocytes – Form myelin sheaths CNS Microglia – Phagocytose foreign microbes, etc. Ependymal – Line ventricles of the brain, secrete cerebrospinal fluid Neuroglia of PNS Schwann cells – Form myelin sheaths of PNS Classification of Glial Cells Neuroglia of CNS Nerve Fibers of the PNS An axon and its sheaths Myelinated axon – Axon is surrounded by a myelin sheath Unmyelinated axon – Axon has no myelin sheath Myelin White matter of nerves, brain, spinal cord Composed primarily of phospholipids Production Developing Schwann cells wind around axon Function Increases speed of impulse conduction Insulation and maintenance of axon Schwann Cells and Peripheral Axons Myelin Neurilemma Peripheral cytoplasmic layer of the Schwann cell enclosing the myelin sheath Nodes of Ranvier Unmyelinated gaps between segments of myelin Impulses “jump” from node to node A Myelinated Axon Nerve Fibers of the CNS Umyelinated Myelinated Production of myelin is from oligodendrocytes Nodes of Ranvier are less numerous Nerve Fibers of the CNS Structural Classification of Neurons Functional Classification of Neurons By Dendrite Structure Pyramidal vs. Stellate Dendritic Spines Spiny or Aspinous ~ Other Classification Schemes Axon length Golgi Type I - long Golgi Type II - short (local signalling) Chemistry neurotransmitter released ~ How Neurons Function (Physiology) Irritability – ability to respond to stimuli Conductivity – ability to transmit an impulse The plasma membrane at rest is polarized Fewer positive ions are inside the cell than outside the cell Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.17 Starting a Nerve Impulse Depolarization – a stimulus depolarizes the neuron’s membrane A deploarized membrane allows sodium (Na+) to flow inside the membrane The exchange of ions initiates an action potential in the neuron Figure 7.9a–c Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.18 The Action Potential If the action potential (nerve impulse) starts, it is propagated over the entire axon Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane The sodium-potassium pump restores the original configuration This action requires ATP Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.19 Nerve Impulse Propagation The impulse continues to move toward the cell body Impulses travel faster when fibers have a myelin sheath Figure 7.9c–e Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.20 Stimuli Receives information Integrates information~ Axon carries information away from soma Electrical signal Axon terminal releases chemical message~ Continuation of the Nerve Impulse between Neurons Impulses are able to cross the synapse to another nerve Neurotransmitter is released from a nerve’s axon terminal The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter An action potential is started in the dendrite Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.21 Neuronal Communication: Within a neuron, electrical signals called action potentials travel along the axon. Communication between neurons is mediated via synaptic transmission (e.g. by means of neurotransmitters such as glutamate, GABA, acetylcholine). Transfer of information from one part of body to another. How Neurons Communicate at Synapses Figure 7.10 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.22 Glia Neural Support Cells Nervous Tissue: Support Cells (Neuroglia or Glia) Astrocytes Abundant, star-shaped cells Brace neurons Form barrier between capillaries and neurons Control the chemical environment of the brain (CNS) Figure 7.3a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.5 Nervous Tissue: Support Cells Microglia (CNS) Spider-like phagocytes Dispose of debris Ependymal cells (CNS) Line cavities of the brain and spinal cord Circulate cerebrospinal fluid Figure 7.3b, c Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.6 Nervous Tissue: Support Cells Oligodendrocytes (CNS) Produce myelin sheath around nerve fibers in the central nervous system Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.3d Slide 7.7a Neuroglia vs. Neurons Neuroglia divide. Neurons do not. Most brain tumors are “gliomas.” Most brain tumors involve the neuroglia cells, not the neurons. Consider the role of cell division in cancer! Support Cells of the PNS Satellite cells Protect neuron cell bodies Schwann cells Form myelin sheath in the peripheral nervous system Figure 7.3e Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.7b Support Cells / Glia CNS PNS Astrocytes Provide physical support Regulating chemical content of extracellular fluid localizes neurotransmitters K+ concentration Blood-brain Barrier ~ Blood-Brain Barrier Typical Capillary Brain Capillary BBB: Function Maintains stable brain environment large fluctuations in periphery Barrier to poisons Retains neurotransmtters & other chemicals Regulates nutrient supplies glucose levels active transport ~ Myelin Wrap around axon Saltatory Conduction faster transmission CNS: oligodendroglia or oligodendrocytes PNS: Schwann cells ~ Myelinated and Unmyelinated Axons Myelinated axons Myelin protects and insulates axons from one another Not continuous Nodes of Ranvier Unmyelinated axons Saltatory Conduction Other non-neuronal cells Microglia phagocytosis immune-like function Ependymal cells line walls of ventricles role in cell migration during development ~ Chemical Synapses Neuronal Pathways and Circuits Organization of neurons in CNS varies Convergent pathways Divergent pathways Oscillating circuits