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NERVOUS SYSTEM NERVOUS TISSUE Nervous System - Overview Control System; Regulator of Homeostasis Electrical Impulses & Neurotransmitters Rapid & Transient Effects Comparison of Control Systems Endocrine System Hormones Targets: cells with receptors Changes metabolic activity (enzymes, genes) Slower onset, more persistent effects Widespread effects Slow recovery Nervous System Neurotransmitters Targets: neurons, muscle cells, gland cells Stimulates contraction or glandular secretion Rapid onset, brief duration Local effects Rapid recovery 18-3 Nervous System - Functions Sensory - Monitors Internal & External Environments Integrative - Analyzes Sensory Information - Stores - Makes Decisions Regarding Appropriate Responses Motor – Controls muscles & glands; responds to sensory information Structural Subdivisions Central Nervous System (CNS) - Brain, Spinal Cord - Dorsal Body Cavity - Integration & Command Center Peripheral Nervous System (PNS) - All Neural Tissue outside CNS Nerves (Cranial & Spinal) Carries Info. Between CNS & Rest of Body Sensory & Motor Peripheral Nervous System (PNS) Functional Subdivisions Sensory/Afferent (“carrying to/toward”) - Conveys sensory information from receptors to CNS - Somatic afferent fibers (from skin, skeletal muscles, joints) - Visceral afferent fibers (from organs in ventral body cavity) Motor/Efferent (“carrying away”) - Conveys motor commands from CNS to muscles & glands Motor Divisions of PNS Somatic Nervous System (SNS) - Voluntary Control of Skeletal Muscles Autonomic Nervous System (ANS) - Involuntary Control of Smooth Muscle, Cardiac Muscle & Glands - Sympathetic Division - Parasympathetic Division Divisions of the ANS Two Antagonistic Branches that Serve same Organs: - Parasympathetic * “Housekeeping” * Conserves Energy * Craniosacral Origin * Effects include: increased gastrointestinal activity, increased healing rate, decreased heart & resp. rates Divisions of the ANS continued - Sympathetic * “Fight or Flight” * Expends Energy * Thoracolumbar Origin * Effects Include: Increased heart & resp. rates, constriction of blood vessels, increased blood pressure, pupil dilation, bronchiole dilation, inhibition of G.I. activity Fig. 8.39 Cells of Nervous System Neurons - Basic Unit of Nervous System - Most Specialized Cell in Body - Conduct Impulses Neuroglia (“Nerve Glue”) - Support, Framework (fill spaces) & Phagocytes - Most Numerous - Can Divide & Multiply Neurons - Structure Cell Body (Soma) - Nucleus - Nissl Bodies (Rough ER + polyribosomes) - Neurofibrils (Cytoskeletal) Processes - Dendrites - Axon Neuron Processes Dendrites - Carry Impulse Towards Cell Body (Afferent) - Numerous, Short, Branched - Receive Impulse from other Neurons or Receptors Neuron Processes Axon - Carries Impulse Away from Cell Body (Efferent) - Carries Towards: * Neuron * Muscle Fiber * Gland - Long, Singular Neuron Processes - Axon continued - Many Mitochondria, Neurofibrils Axon Hillock (Joins Cell Body & Axon) Collaterals (Axon Branches) Axon/Synaptic Terminals *Numerous, Fine Processes at end of Axon & Axon Collaterals *Some with Synaptic Knobs Neurons – Axon continued - Myelination *Most Axons *Enclosed in Schwann Cells (Neurolemmocyte) Myelin Sheath – Inner, Fatty, Tight Layers of Membrane Neurolemma – Outer Schwann Cell Membrane & Cytoplasm Myelination Addition of insulating material Most Axons Provided by Schwann Cells in PNS Myelin Sheath – inner, fatty, tight layers of membrane Neurolemma – outer, Schwann Cell membrane & cytoplasm Neurons – Axon Myelination continued *Insulates & Increases Speed of Conduction *Nodes of Ranvier Occur Along Axon Between Schwann Cells No Myelin Fig. 8.6 Neurons – Functional Classification Sensory - Afferent - Connect Receptors & CNS Motor - Efferent - Carry Commands from CNS to Effectors Interneurons (Association) - CNS - Integrate Sensory & Motor - Most Numerous Sensory Receptors Receptors are sensitive only to specific stimuli Free nerve endings (pain, temp, itch, tickle) Encapsulated nerve endings - Meissner’s & Pacinian corpuscles, Stretch receptors Complex, clusters of cells (sound, light, taste, balance) Sensory Cells Neurons – Structural Classification Unipolar - One Process (Dendrites & Axon Fused) - Sensory Bipolar - Two Processes: One Dendrite, One Axon - Rare (Special Senses) Multipolar - Several Dendrites, One Axon - Common - Motor & Interneurons Location of Neuron Cell Bodies The cell bodies of most neurons are found in the CNS - Gray matter contains cell bodies and unmyelinated fibers - Nuclei are clusters of cell bodies within white matter of CNS Ganglia are groups of cell bodies in the PNS Neuron Terminology: PNS vs CNS Bundle of Fibers (Axons) Cluster of Neuron Cell Bodies PNS CNS Nerve Tract Ganglion (Ganglia) Nucleus (Nuclei) Neuroglia – Supporting Cells in CNS Astrocytes - Large, Star-shaped - Link Neurons & Blood Vessels; Help form Blood-brain Barrier Oligodendrocytes - Form Myelin Sheath in CNS Microglia - Derived from WBCs; Phagocytic Ependymal Cells - Epithelium; Line Ventricles & Central Canal - Produce & Help Circulate CSF Neuroglia – Supporting Cells in PNS Schwann Cells (Neurolemmocytes) - Form Myelin Sheath Satellite Cells - Protect, Cushion Ganglia Nerve Impulse Transmission Two mechanisms involved - Transmission along a neuron *An electrical process - Transmission between neurons *A chemical process *Occurs at synapse Neuron Physiology Transmission Requirements: - Resting Membrane Potential (cell membrane is polarized in neuron at rest) - Delivery of Threshold Stimulus *Neurotransmitter, light, sound, stretch, pressure, etc… - Ion Channels in Cell Membrane (allow ions to diffuse when open) Resting Membrane Potential Conduction Along Neuron Resting Membrane Potential (+/Na+ outside, -/K+ inside (polarized) Appropriate Threshold Stimulus Opens Na+ Channels Na+ Diffuses into Neuron, Results in Depolarization (+ inside/- outside) Depolarization wave spreads from dendrite to axon (Action Potential) + Na Opening of Channels Causes Depolarization Conduction Along Neuron continued Na+ Channels Close, K+ Channels Open & K+ Diffuses Out of Neuron Results In Repolarization (+ outside/- inside) Repolarization Required before another Action Potential Sodium-Potassium Pump moves Na+ out & K+ in (Requires Energy) + K Opening of Channels Causes Repolarization Action Potential: self-propagating reversal of charge across plasma membrane Conduction continued All-or-None Principle - Neurons respond to stimuli by generating an impulse (action potential), or don’t respond at all Refractory Period - Neurons must repolarize their cell membranes before they respond to subsequent stimuli - Occurs when K+ diffuses out of neuron Types of Conduction Continuous - Typical of Unmyelinated Neurons (Slower) - Steps as Previously Described (“transmission along a neuron”) Saltatory - Occurs along Myelinated Neurons - No Current where Myelin occurs - Action Potential Leaps from Node of Ranvier to Node - Faster! Fig. 8.12 Synaptic Transmission Arriving Action Potential Depolarizes Synaptic Knob Ca++ Enters Cytoplasm of Presynaptic Neuron Exocytosis of Synaptic Vesicles, Releasing Neurotransmitter Neurotransmitter Diffuses across Synaptic Cleft & Binds to Receptors on Postsynaptic Membrane Na+ Channels Open, Postsynaptic Membrane Depolarizes Neurotransmitters Excitatory – Cause Depolarization/Na+ ions channels open Inhibitory – Raise the Threshold - Hyperpolarize membrane/K+ or Cl- ion channels open Removed by specific enzymes, diffusion, active transport back into neuron