Fundamentals of the Nervous System and Nervous Tissue
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Transcript of Fundamentals of the Nervous System and Nervous Tissue
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HUMAN ANATOMYfourth edition
MARIEB | MALLATT | WILHELM
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
PowerPoint® Lecture Slides prepared by Leslie Hendon,
University of Alabama, Birmingham
12
Fundamentals of theNervous System and
Nervous TissuePART 1
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Nervous System
• Master control and communication system
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Nervous System: Functions
• Three overlapping functions• Sensory receptors monitor changes inside and
outside the body• Change – a stimulus
• Gathered information – sensory input
• CNS Processes and interprets sensory input• Makes decisions – integration
• Dictates a response by activating effector organs• Response – motor output
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Basic Divisions of the Nervous System: CNS
• Central nervous system (CNS)• Brain and spinal cord
• Integrating and command center
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Basic Divisions of the Nervous System: PNS
• Peripheral nervous system (PNS)• Outside the CNS• Nerves extending
from brain and spinal cord• Cranial nerves• Spinal nerves
• Link all regions of the body to the CNS
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Sensory Input and Motor Output
• Sensory signals picked up by sensory receptors• Carried by afferent nerve fibers of PNS to the CNS
• Motor signals are carried away from the CNS • Carried by efferent nerve fibers of PNS to effectors
• Innervate muscles and glands
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Sensory Input and Motor Output
• Divided according to region they serve• Somatic body region
• Visceral body region
• Results in four main subdivisions• Somatic sensory
• Visceral sensory
• Somatic motor
• Visceral motor
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Somatic Sensory
• Somatic sensory • General somatic senses – receptors are widely
spread • Touch, pain, vibration, pressure, and temperature
• Proprioceptive senses – detect stretch in tendons and muscle
• Body sense – position and movement of body in space
• Special somatic senses • Hearing, balance, vision, and smell
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Visceral Sensory
• Visceral sensory• General visceral senses – stretch, pain,
temperature, nausea, and hunger• Widely felt in digestive and urinary tracts,
reproductive organs
• Special visceral senses – taste
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Somatic Motor
• Somatic motor• General somatic motor – signals contraction of
skeletal muscles• Under voluntary control
• Often called “voluntary nervous system”
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Visceral Motor
• Visceral motor• Regulates the contraction of smooth and cardiac
muscle and gland secretion
• Makes up autonomic nervous system
• Controls function of visceral organs
• Often called “involuntary nervous system”
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Peripheral Nervous System Summary
Figure 12.3
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Types of Sensory and Motor Information
Figure 12.3
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Types of Sensory and Motor Information
Figure 12.3
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Nervous Tissue
• Cells are densely packed and intertwined • Two main cell types
• Neurons – transmit electrical signals
• Support cells (neuroglial cells) – nonexcitable
• Surround and wrap neurons
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The Neuron
• The human body contains billions of neurons• Basic structural unit of the nervous system
• Specialized cells conduct electrical impulses along the plasma membrane
• Graded potentials
• Action potentials
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The Neuron: Special Characteristics
• Longevity – can live and function for a lifetime
• Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception
• High metabolic rate – require abundant oxygen and glucose
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Neuron Structure
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The Cell Body or Soma (also called Perikaryon)
• Size varies from 5–140µm
• Contains nucleus, organelles plus other structures• Chromatophilic bodies (Nissl bodies)
• Clusters of rough ER and free ribosomes
• Stain darkly and renew membranes of the cell
• Neurofibrils – bundles of intermediate filaments
• Form a network between chromatophilic bodies
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Nissl Body Staining
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The Cell Body
• Most neuronal cell bodies• Located within the CNS (clustered in nuclei)
• Protected by bones of the skull and vertebral column
• Ganglia – clusters of cell bodies in PNS
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Cell Body Structure
Figure 12.4
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Neuron Processes: Dendrites
• Dendrites • Extensively branching from
the cell body
• Transmit electrical signals (graded potentials) toward the cell body
• Chromatophilic bodies – only extend into the basal part of dendrites
• Function as receptive sites
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Dendritic Spines
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Dendritic Spines
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Neuron Processes: Axons
• Axons (nerve fibers)• Neuron has only one, but it can
branch
• Impulse generator and conductor
• Transmits action potentials away from the cell body
• Chromatophilic bodies absent
• No protein synthesis in axon
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Neuron Processes: Axons
• Axons• Neurofilaments, actin
microfilaments, and microtubules• Provide strength along
length of axon
• Aid in the transport of substances to and from the cell body
• Axonal transport
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Neuron Processes
Neuron Structure
• Axons• Branches along length are
infrequent• Axon collaterals
• Multiple branches at end of axon• Terminal branches (telodendria)
• End in knobs called axon terminals (also called end bulbs or boutons)
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Neuron Processes: Action Potentials
• Nerve impulse (action potential)• Generated at the initial segment of the
axon
• Conducted along the axon
• Releases neurotransmitters at axon terminals
• Neurotransmitters – excite or inhibit neurons
• Neuron receives and sends signals
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Synapses
• Site at which neurons communicate
• Signals pass across synapse in one direction
• Presynaptic neuron• Conducts signal toward a synapse
• Postsynaptic neuron• Transmits electrical activity away from a synapse
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Two Neurons Communicating at a Synapse
Figure 12.6
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Types of Synapses
• Axodendritic• Between axon terminals of one neuron and
dendrites of another• Most common type of synapse
• Axosomatic • Between axons and neuronal cell bodies
• Axoaxonic, dendrodendritic, and dendrosomatic• Less common types of synapses• Function not as well understood
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Types of Synapses
Figure 12.7
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Synapses
• Axodendritic synapses – representative type
• Synaptic vesicles on presynaptic side• Membrane-bound sacs containing neurotransmitters
• Mitochondria abundant in axon terminals
• Synaptic cleft separates the plasma membrane of the two neurons
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Structure of a Synapses
Figure 12.8a, b
PLAYPLAY Synapse
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Synapse
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Signals Carried by Neurons: Resting Membrane Potential
• Plasma membranes of neurons conduct electrical signals
• Resting neuron – membrane is polarized
• Inner, cytoplasmic side is negatively charged
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Changes in Membrane Potential
• Signals occur as changes in membrane potential
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Directional Signals
• Stimulation of the neuron depolarization
• Inhibition of the neuron hyperpolarization
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Action Potentials
Figure 12.9a, b
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Action Potentials on Axons
• Strong depolarizing stimulus applied to the axon hillock triggers• Action potential
• Membrane becomes positive internally
• Action potential travels the length of the axon
• Membrane repolarizes itself
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Action Potentials on Axons
Figure 12.9c–e
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Graded Potentials on Dendrites and the Cell Body
• Natural stimuli applied to dendrites and the cell body• Receptive zone of the neuron
• Membrane stimulation causes local depolarization• A graded potential – inner surface becomes less
negative• Depolarization spreads from receptive zone to the
axon hillock• Acts as the trigger that initiates an action potential
in the axon
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Synaptic Potentials
• Excitatory synapses• Neurotransmitters alter the permeability of the
postsynaptic membrane
• Leads to an inflow of positive ions • Depolarizes the postsynaptic membrane
• Drives the postsynaptic neuron toward impulse generation
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Synaptic Potentials
• Inhibitory synapses• The external surface of the postsynaptic membrane
becomes more positive• Reduces the ability of the postsynaptic neuron to
generate an action potential
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Classification of Neurons
• Structural Classification
• Functional Classification
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Structural Classification of Neurons
Classification based on number of processes• Multipolar
• Bipolar
• Unipolar (pseudounipolar)
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Multipolar Neurons
Figure 12.10a–c
Possess more than two processes
Numerous dendrites and one axon
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Bipolar Neurons
Figure 12.10a–c
Possess two processes Rare neurons – found in some special sensory organs
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Unipolar (Pseudounipolar) Neurons
Figure 12.10a–c
Possess one single processStart as bipolar neurons during development
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Functional Classification of Neurons
Classification based on direction of action potential propagation• Afferents – from CNS to periphery
• Efferents – from periphery to CNS
• Interneurons – within CNS
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Afferent neurons
• Afferent (sensory) neurons – transmit impulses toward the CNS• Virtually all are pseudounipolar neurons (some true
bipolar)
• Cell bodies in ganglia outside the CNS• Short, single process divides into
• The central process – runs centrally into the CNS
• The peripheral process – extends peripherally to the receptors
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Afferent Neurons
Sensory receptorsAxon terminals
Periphery CNS
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Efferent Neurons
• Efferent (motor) neurons • Carry impulses away from the CNS to effector
organs
• Most efferent neurons are multipolar
• Cell bodies are within the CNS
• Form junctions with effector cells
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Interneurons
• Interneurons (association neurons) – most are multipolar • Lie between afferent and efferent neurons
• Confined to the CNS
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Neurons Classified by Function
Figure 12.11
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Variety of Interneurons
• Purkinje cell, stellate cell, granule cell, and basket cell• Located in the cerebellum
• Pyramidal cell – located in the cerebral cortex
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Variety of Interneurons
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Glial Cells (Supporting Cells)
• Six types of glial cells• Four in the CNS
• Two in the PNS
• Provide supportive functions for neurons
• Cover nonsynaptic regions of the neurons
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Supporting Cells (Neuroglial Cells) in the CNS
• Neuroglia – usually only refers to supporting cells in the CNS, but can be used for PNS• Glial cells have branching processes and a central
cell body
• Outnumber neurons 10 to 1
• Make up half the mass of the brain
• Can divide throughout life
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Types of Glial Cells in the CNS
• Astrocytes
• Microglia
• Ependymal Cells
• Oligodendrocytes
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Astrocytes
• Astrocytes – most abundant glial cell type• Take up and release ions to control the environment
around neurons
• Recapture and recycle neurotransmitters
• Involved with synapse formation in developing neural tissue
• Produce molecules necessary for neural growth (BDTF)
• Propagate calcium signals that may be involved in memory
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Astrocytes
Figure 12.12a
Necessary for development and maintenance of theblood brain barrier
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• Microglia – smallest and least abundant
• Phagocytes – the macrophages of the CNS
• Engulf invading microorganisms and dead neurons
• Derived from blood cells called monocytes
Microglia
Figure 12.12b
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Ependymal Cells
• Ependymal cells• Line the central cavity of the spinal cord and brain
• Bear cilia – help circulate the cerebrospinal fluid
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Oligodendrocytes
• Oligodendrocytes – have few branches• Wrap their cell processes around axons in CNS
• Produce myelin sheaths
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Supporting Cells in the PNS
• Satellite cells – surround neuron cell bodies within ganglia
• Schwann cells (neurolemmocytes) – surround axons in the PNS• Form myelin sheath around axons of the PNS
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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
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Myelin Sheaths in the PNS
• Formed by Schwann cells
• 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
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Myelin Sheaths in the PNS
• Nodes of Ranvier – gaps along axon
• Allow current exchange across axon membrane
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Myelin Sheaths in the PNS
• Thick axons are myelinated• Fast conduction velocity
• Thin axons are unmyelinated• Slow conduction velocity
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Myelin Sheaths in the PNS
Figure 12.14a
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Myelin Sheaths in the PNS – myelinated axon
Figure 12.15b
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Myelin Sheaths in the PNS – unmyelinated axons
Figure 12.15b
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Myelin Sheaths in the CNS
• Oligodendrocytes form the myelin sheaths in the CNS• Have multiple processes
• Coil around several different axons
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Oligodendrocytes
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Nerves
• Nerves – cordlike organs in the PNS
• Consists of numerous axons wrapped in connective tissue
• Axon is surrounded by Schwann cells
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Nerves
• Endoneurium – layer of delicate connective tissue surrounding the axon
• Nerve fascicles – groups of axons bound into bundles
• Perineurium – connective tissue wrapping surrounding a nerve fascicle
• Epineurium – whole nerve is surrounded by tough fibrous sheath
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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
• Interneurons – located centrally • Provide communication between sensory and
motor neurons and between levels of the CNS
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Example of Neuronal Organization: Reflexes
• Reflex arcs – simple neural pathways• Responsible for reflexes
• Rapid, autonomic motor responses
• Can be visceral or somatic
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Five Essential Components to the Reflex Arc
• Receptor – detects the stimulus
• Afferent (sensory neuron) – transmits impulses to the CNS
• Integration center – consists of one or more synapses in the CNS
• Efferent (motor neuron) – conducts impulses from integration center to an effector
• Effector – muscle or gland cell• Responds to efferent impulses
• Contraction or secretion
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Example of the Five Components to the Reflex Arc
Figure 12.17
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Reflex Classification
• Monosynaptic or polysynaptic
• Spinal or cranial
• Somatic or autonomic
• Innate or learned
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Types of Reflexes: Number of Classes
• Monosynaptic reflex – simplest of all reflexes• Just one synapse
• The fastest of all reflexes
• Example – knee-jerk reflex
• Polysynaptic reflex – more common type of reflex• Most have a single interneuron between the
sensory and motor neuron
• Example – withdrawal reflexes
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Monosynaptic Reflex
Figure 12.18a, b
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Polysynaptic Reflex
Figure 12.18a, b
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Spinal vs Cranial Reflexes
• Spinal = spinal cord integration center• Ex. Knee-jerk reflex
• Cranial = brain as integration center• Ex. Pupillary light reflex
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Somatic vs Autonomic Reflexes
• Somatic = motor neurons to skeletal muscles• Ex. Knee-jerk reflex
• Autonomic = autonomic neurons to smooth muscle and glands• Ex. Pupillary light reflex
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Innate vs Learned Reflexes
• Innate = born-with• Knee-jerk reflex, pupillary reflex
• Learned = develops based on experiences• Pavlov’s dogs salivation in response to bell
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Neuronal Circuits
• Diverging circuit
• Converging circuit
• Reverberating circuit
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Diverging Circuit
• Diverging circuit – one presynaptic neuron synapses with several other neurons (divergence)
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Converging Circuit
• Converging circuit – many neurons synapse on a single postsynaptic neuron (convergence)
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Reverberating Circuit
• Reverberating circuit – circuit that receives feedback via a collateral axon from a neuron in the circuit
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Neural Processing
• Serial processing
• Parallel processing
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Serial Processing
• Serial processing – neurons pass a signal to a specific destination along a single pathway from one to another
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Parallel Processing
• Parallel processing – input is delivered along many pathways; a single sensory stimulus results in multiple perceptions
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Gray versus White Matter in the Central Nervous System
• Gray matter
• Cell bodies
• Dendrites
• Synapses
•White matter•Axons (myelin)
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Gray Matter in the Spinal Cord
• Gray matter in the spinal cord
• H-shaped (butterfly) region – surrounds central cavity
• Dorsal half contains cell bodies of interneurons
• Ventral half contains cell bodies of motor neurons
• Cell bodies are clustered in the gray matter
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White Matter in the Spinal Cord
• White matter in the spinal cord
• Located externally to the gray matter
• Contains no neuronal cell bodies, but millions of axons
• Myelin sheath – white color
• Consists of axons running between different parts of the CNS
• Tracts – bundles of axons traveling to similar destinations
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Gray Matter in Brain
• Cortex and nuclei
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White Matter in Brain
• Pathways, tracts and commissures
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Disorders of the Nervous System
• Multiple sclerosis – common cause of neural disability• Varies widely in intensity among those affected
• Cause is incompletely understood
• An autoimmune disease • Immune system attacks the myelin around axons in
the CNS
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Multiple Sclerosis Videos
• Symptoms of multiple sclerosis