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PowerPoint® Lecture Slides prepared by Leslie Hendon, University of Alabama, Birmingham 12 HUMAN ANATOMY fifth edition MARIEB | MALLATT | WILHELM PART 3 Fundamentals of the Nervous System and Nervous Tissue Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Myelin Sheaths Segmented structures composed of the lipoprotein myelin Surround thicker axons Form an insulating layer Prevent leakage of electrical current Increase the speed of impulse conduction Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Myelin Sheaths in the PNS Formed by Schwann cells (neurolemmacytes) Develop during fetal period and in the first year of postnatal life Schwann cells wrap in concentric layers around the axon Cover the axon in a tightly packed coil of membranes Neurilemma Material external to myelin layers Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Myelin Sheaths in the PNS Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.14a, b Myelin Sheaths in the PNS Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.14c, d Myelin Sheaths in the PNS Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.15a Myelin Sheaths in the PNS Nodes of Ranvier – gaps along axon Thick axons are myelinated Thin axons are unmyelinated Conduct impulses more slowly Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Myelin Sheaths in the CNS Oligodendrocytes form the myelin sheaths in the CNS Have multiple processes Coil around several different axons Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.15b Gray and White Matter in the CNS Gray matter Is gray-colored and surrounds hollow central cavities of the CNS Forms H-shaped region in the spinal cord Dorsal half contains cell bodies of interneurons Ventral half contains cell bodies of motor neurons Primarily composed of neuronal cell bodies, dendrites, unmyelinated axons Surrounds white matter of CNS in cerebral cortex and cerebellum Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Gray and White Matter in the CNS White matter Lies external to the gray matter of the CNS Composed of myelinated axons Consists of axons passing between specific regions of the CNS Tracts are bundles of axons traveling to similar destinations Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Nerves Nerves – cablelike organs in the PNS Consists of numerous axons wrapped in connective tissue Axon is surrounded by Schwann cells You see many nerves in lab Nerves of Brachial Plexus Radial, axillary, median, musculocutaneous, ulnar Nerves of lumbosacral plexus Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Nerves Endoneurium – layer of delicate connective tissue surrounding the axon Perineurium – connective tissue wrapping surrounding a nerve fascicle Nerve fascicles – groups of axons bound into bundles Epineurium – whole nerve is surrounded by tough fibrous sheath Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Structure of a Nerve Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.16a Integration Between the PNS and CNS The CNS and PNS are functionally interrelated Nerves of the PNS Information pathways to and from body periphery Afferent PNS fibers respond to sensory stimuli Efferent PNS fibers transmit motor stimuli from CNS to muscles and glands Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Integration Between the PNS and CNS Nerves of the CNS Composed on interneurons that Process and receive sensory information Direct information to specific CNS regions Initiate appropriate motor responses Transport information from one area of the CNS to another Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Reflex Arcs Reflex arcs – simple chains of neurons Explain reflex behaviors Determine structural plan of the nervous system Responsible for reflexes Rapid, autonomic motor responses Can be visceral or somatic Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Five Essential Components to the Reflex Arc Receptor – site where stimulus acts Sensory neuron – transmits afferent impulses to the CNS Integration center – consists of one or more synapses in the CNS Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Five Essential Components to the Reflex Arc Motor neuron – conducts efferent impulses from integration center to an effector Effector – muscle or gland cell Responds to efferent impulses Contracting or secreting Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Five Essential Components to the Reflex Arc Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.17 Types of Reflexes Monosynaptic reflex Simplest of all reflexes Just one synapse The fastest of all reflexes Knee-jerk reflex Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Types of Reflexes Polysynaptic reflex More common type of reflex Most have a single interneuron between the sensory and motor neuron Withdrawal reflexes Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Types of Reflexes Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.18a, b Simplified Design of the Nervous System Three-neuron reflex arcs Basis of the structural plan of the nervous system Similar reflexes are associated with the brain Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Simplified Design of the Nervous System Sensory neurons – located dorsally Cell bodies outside the CNS in sensory ganglia Central processes enter dorsal aspect of the spinal cord Motor neurons – located ventrally Axons exit the ventral aspect of the spinal cord Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Simplified Design of the Nervous System Interneurons – located centrally Synapse with sensory neurons Interneurons are neurons confined to CNS Long chains of interneurons between sensory and motor neurons Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Simplified Design of the Nervous System Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.19 Neuronal Circuits Diverging circuit – one presynaptic neuron synapses with several other neurons (divergence) Converging circuit – many neurons synapse on a single postsynaptic neuron (convergence) Reverberating circuit – circuit that receives feedback via a collateral axon from a neuron in the circuit Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Neuronal Circuits Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.20 Input Processing – not in notes Serial processing Neurons pass a signal to a specific destination along a single pathway from one to another Parallel processing Input is delivered along many pathways; a single sensory stimulus results in multiple perceptions Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Neural Processing Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.21 Disorders of the Nervous System Multiple sclerosis Common cause of neural disability An autoimmune disease Immune system attacks the myelin around axons in the CNS Varies widely in intensity among those affected More women than men are affected When men are affected disease develops quicker and is more devastating Cause is incompletely understood Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Nervous Tissue Throughout Life Nervous system develops from the dorsal ectoderm Invaginates to form the neural tube and neural crest Neural tube walls begin as neuroepithelial cells These cells divide and become neuroblasts Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Nervous Tissue Throughout Life Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.22 Neuronal Regeneration Neural injuries may cause permanent dysfunction If axons alone are destroyed, cells bodies often survive and the axons may regenerate PNS – macrophages invade and destroy axon distal to the injury Axon filaments grow peripherally from injured site Partial recovery is sometimes possible Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Neuronal Regeneration CNS – neuroglia never form bands to guide regrowing axons and may hinder axon growth with growth-inhibiting chemicals No effective regeneration after injury to the spinal cord and brain Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Regeneration of the Peripheral Nerve Fiber Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.23