12 HUMAN ANATOMY fifth edition MARIEB MALLATT WILHELM
12 HUMAN ANATOMY fifth edition MARIEB | MALLATT | WILHELM Power. Point® Lecture Slides prepared by Leslie Hendon, University of Alabama, Birmingham 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. 14 a, b
Myelin Sheaths in the PNS Copyright © 2008 Pearson Education, Inc. , publishing as Benjamin Cummings Figure 12. 14 c, d
Myelin Sheaths in the PNS Copyright © 2008 Pearson Education, Inc. , publishing as Benjamin Cummings Figure 12. 15 a
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. 15 b
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. 16 a
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. 18 a, 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
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