Fundamentals of the Nervous System and Nervous Tissue

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• Fundamentals of the Nervous System andNervous
  Tissue

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I. Nervous System Overview

• A. Master control and communication system
• 1. Stimuluschanges detected inside or outside
  the body
• 2. Sensory receptors - monitor changes inside
  and outside the body
• 2. Sensory inputinformation gathered by
  receptors
• 3. IntegrationProcesses and interprets sensory
  input
• 4. Motor outputDictates response activates
  effector organs

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Sensory receptor
Stimulus
Sensory input
Integration
Motor output
Action
Effector
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II. Basic Organization of the Nervous System

• A. Central nervous system (CNS)
• 1. composed of the brain and spinal cord
• 2. is the integrating and command center
• B. Peripheral nervous system (PNS)
• 1. Consists of nerves extending from brain and
  spinal cord
• a. cranial nerves (off the brain)
• b. spinal nerves (off the spinal cord)
• 2. Peripheral nerves link all regions of the body
  to the CNS
• 3. Nucleus group of nerve cell bodies in the
  brain/cord
• 4. Ganglion group of nerve cell bodies outside
  brain/cord

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Brain
CNS
Spinalcord
Nerves
PNS
Ganglia
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• C. Sensory (afferent) division
• 1. somatic sensory information from skin,
  muscle and tendon
• 2. visceral sensory information from organs,
  glands, all else
• 3. information carried to CNS by spinal and
  cranial nerves
• D. Motor (efferent) division
• 1. somatic motor (voluntary) information to
  muscles
• 2. visceral motor (involuntary) information to
  organs, glands, etc.
• a. also called the autonomic nervous system
• i. sympathetic division
• ii. parasympathetic division

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Central nervous system (CNS)
Peripheral nervous system (PNS)
Cranial nerves and spinal nerves
Brain and spinal cord
Sensory (afferent) division
Motor (efferent) division
Somatic and visceral sensorynerve fibers
Motor nerve fibers
Somatic nervoussystem
Autonomic nervoussystem (ANS)
Somatic sensory fiber
Skin
Somatic motor(voluntary)
Visceral motor(involuntary)
Conducts impulsesfrom the CNS toskeletal muscles
Conducts impulsesfrom the CNS tocardiac
muscles,smooth muscles, and glands
Visceral sensory fiber
Stomach
Skeletalmuscle
Motor fiber of somatic nervous system
Sympathetic division
Paraysmpatheticdivision
Mobilizes body systemsduring activity
Conserves energy
Promotes house-keeping functionsduring rest
Sympathetic motor fiber of ANS
Heart
Structure
Function
Sensory (afferent)division of PNS
Bladder
Parasympathetic motor fiber of ANS
Motor (efferent)division of PNS
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• E. Somatic sensory - general (widespread) somatic
  senses
• 1. Receptors spread throughout outer tube of
  body
• a. Touch, Pain, Vibration, Pressure,
  Temperature
• 2. Proprioceptive senses - detect tendon/muscle
  stretch
• a. Body senseposition and movement of body in
  space
• 3. Special somatic sense - balance

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• F. Visceral sensory
• 1. General visceral senses
• a. stretch, pain, temperature, nausea, and hunger
• b. felt in digestive and urinary tracts, and
  reproductive organs
• 2. Special visceral senses hearing, vision,
  taste and smell

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III. Somatic vs. Visceral Motor

• A. Somatic motor
• 1. general somatic motorcontraction of skeletal
  muscles
• a. Under our voluntary control
• b. Often called voluntary nervous system
• B. Visceral motor
• 1. regulates the contraction of smooth and
  cardiac muscle
• 2. controls function of visceral organs and
  glands
• 3. also called the autonomic nervous system
  (involuntary)

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IV. Nervous Tissue

• A. Neurons - electrical signals to transmit
  information
• 1. basic structural unit of the nervous system
• 2. can send an action potential (nerve
  impulse) down its axon
• 3. Longevity - can live and function for a
  lifetime
• 4. amitotic - fetal neurons lose their ability
  to undergo mitosis neural stem cells are an
  exception
• 5. High metabolic rate - require abundant oxygen
  and glucose
• a. Neurons die after 5 minutes without oxygen

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• B. Neuroglial cells supporting cells of
  neurons
• 1.Most neuroglia have branching processes and a
  central cell body
• 2. Outnumber neurons 10 to 1
• 3. Make up half the mass of the brain
• 4. Can divide throughout life
• C. Astrocytes most abundant type of glial cell
• 1. Extract blood sugar from capillaries for
  energy
• 2. Take up and release ions to control
  environment around neurons
• 3. Involved in synapse formation in developing
  neural tissue
• 4. Produce molecules necessary for neuronal
  growth

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Capillary
Neuron
Astrocyte
Astrocytes are the most abundant CNS neuroglia.
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• D. Microglia smallest and least abundant glial
  cell
• 1. phagocytesthe macrophages of the CNS
• a. engulf invading microorganisms and dead
  neurons
• 2. derived from blood cells called monocytes
• 3. migrate to CNS during embryonic and fetal
  periods

Neuron
Microglialcell
Microglial cells are defensive cells in the CNS.
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• E. Ependymal cells help circulate cerebrospinal
  fluid (CSF)
• 1. line the brain ventricles and central canal
  of spinal cord
• 2. have cilia to help circulate the CSF

Fluid-filled cavity
Ependymalcells
Brain orspinal cordtissue
Ependymal cells line cerebrospinalfluid-filled
cavities.
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• F. Oligodendrocytes wrap around axons in the
  CNS
• 1. this results in the myelin sheath around the
  axons

Axons
Oligodendrocytes
Myelin sheath gap
Myelin sheath
Oligodendrocytes have processes that form
myelinsheaths around long axons in the CNS.
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• G. Schwann cells wrap around axons in the PNS
• 1. this results in the myelin sheath around the
  axons

Satellitecells
Cell body of neuron
Schwann cells(forming myelin sheath)
Axon
Satellite cells and Schwann cells (which form
myelin)surround neurons in the PNS.
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• H. Myelin sheath
• 1. segmented structures composed of the
  lipoprotein myelin
• 2. surround thicker axons
• 3. forms an insulating layer
• 4. prevent leakage of electrical current
• 5. increase the speed of impulse conduction
• 6. non-myelinated axons are slower
• 7. nodes of Ranvier gaps between the
  surrounding cells

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Myelinated axon in PNS
An axon wrapped with a fatty insulating
sheathformed from Schwann cells
Schwanncell plasmamembrane
A Schwann cellenvelops an axon.
Schwann cellcytoplasm
Axon
Schwann cellnucleus
The Schwann cellthen rotates aroundthe
axon, wrappingits plasma membraneloosely around
it insuccessive layers.
Myelin sheath
The Schwann cellcytplasm is forcedfrom
between themembranes. The tightmembrane
wrappingssurrounding the axonform the
myelinsheath.
Myelin sheath
Schwann cellcytoplasm
Myelin sheath
Schwann cellcytoplasm
Axon
Cross section of a myelinated axon (TEM 135,000?)
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V. The Structure of a Neuron (nerve cell)

• A. Nerve Cell Body
• B. Dendrites
• C. Axon (and axon hillock)
• ? Myelin Sheath (w/ Nodes of Ranvier)
• ? Axon Terminals (terminal boutons)
• D. Synapse

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Dendrites(receptiveregions)
Cell body(biosynthetic centerand receptive
region)
Neuroncell body
Dendrites
Nucleus withnucleolus
Nucleus
Nuclei ofneuroglialcells
Axon(impulse-generatingand -conductingregion)
Nucleolus
Myelin sheath gap(node of Ranvier)
Terminal boutons(secretory region)
Axon hillock
Schwanncell
Terminalarborization
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• A. Nerve cell body
• 1. site of nucleus, nucleolus and organelles
• 2. size is from 5 150 µm
• 3. most nerve cell bodies located in the CNS
• 4. has colorful organelles called Nissl bodies
• 5. group of cell bodies in the CNS nucleus
• 6. group of cell bodies in the PNS ganglion
• a. a nucleus or ganglion usually has a common
  function

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Dendrites(receptiveregions)
Cell body(biosynthetic centerand receptive
region)
Neuroncell body
Dendrites
Nucleus withnucleolus
Nucleus
Nuclei ofneuroglialcells
Axon(impulse-generatingand -conductingregion)
Nucleolus
Myelin sheath gap(node of Ranvier)
Terminal boutons(secretory region)
Axon hillock
Schwanncell
Terminalarborization
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• B. Dendrites
• 1. extend off the nerve cell body
• 2. can be 10 100 in number
• 3. receive electrical signals from other nerve
  cells
• C. Axons
• 1. one long extension of the plasma membrane
• 2. send signals from cell body to axon terminals
  (the synapse)
• 3. signal is sent in only one direction (cell
  body gtgtgt synapse)
• 4. axon hillock first part of axon attached to
  cell body
• 5. may or may not have a myelin sheath wrapped
  around it
• 6. end in terminal arboration (tree) with many
  terminal boutons

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Dendrites(receptiveregions)
Cell body(biosynthetic centerand receptive
region)
Neuroncell body
Dendrites
Nucleus withnucleolus
Nucleus
Neurofibril
Nuclei ofneuroglialcells
Axon(impulse-generatingand -conductingregion)
Nucleolus
Myelin sheath gap(node of Ranvier)
Terminal boutons(secretory region)
Axon hillock
Schwanncell
Terminalarborization
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• D. The Synapse
• 1. site where the presynaptic neuron sends
  signal to postsynaptic neuron
• 2. almost all synapses are chemical using a
  neurotransmitter
• 3. some synapses are electrical using gap
  junctions between cells
• 4. most are axondendritic small number are
  axosomatic (cell body)
• 5. space between pre- and postsynaptic cell is
  the synaptic cleft
• 6. terminal bouton have vesicles with
  neurotransmitter

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• D. The Synapse

Presynapticneuron axon
Terminal boutonsat synapse
Dendrites
Postsynapticneuron
Postsynapticneuron axon
Two neurons connectedby synapses
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Nerveimpulses
Presynaptic axon
Terminal bouton
Mitochondrion
Vesicle releasingneurotransmitter
Synapticvesicles
Synapticcleft
Postsynaptic dendrite
Enlarged view of the synapse
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VI. Classification of Neurons by Structure
A. Multipolar neuron ? cell body surrounded by
dendrites one very long axon e.g. Purkinje
cell of cerebellum pyramidal cell of
hippocampus B. Bipolar neuron ? many dendrites
gt one long dendrite gt cell body gt axon e.g.
olfactory cell retinal cell C. Unipolar
neuron ? long dendrite gt (passes by cell body) gt
long axon e.g. sensory cell of the dorsal root
ganglion
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VI. Classification of Neurons by Function

• A. Sensory neurons
• 1. send nerve impulses toward the CNS
• 2. almost all are unipolar
• 3. cell bodies are located in ganglia outside
  the CNS
• B. Motor neurons
• 1. send nerve impulses away from the CNS
• 2. most motor neurons are multipolar
• 3. cell bodies located in nuclei within the CNS
• 4. form synapses with the organs, glands,
  tissues they innervate

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• C. Interneurons
• 1. between sensory and motor between themselves
• 2. most numerous of all types
• 3. only located in the brain and spinal cord
  (CNS!)
• 4. mostly multipolar

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Sensory receptor
Stimulus
Sensory input
Integration
Motor output
Action
Effector
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VII. Structure of a Nerve
A. Nerve bundle of axons wrapped together by
connective tissue 1. like a bunch of wires
wrapped together in electrical cord 2. most
nerves contain myelinated axons 3. Schwann cells
form the myelin sheath around single cells B.
Layers of Connective Tissue 1. epineurium
around the entire nerve 2. perineurium around
a fascicle of axons 3. endoneurium around each
individual axon
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Axon
Myelin sheath
Myelinatedaxons
Endoneurium
Perineurium
Myelinsheath
Bloodvessels
Fascicle
Fascicle
Epineurium
Schwanncellnucleus
Epineurium
Axon
Myelin
Myelinsheathgap
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C. Grey matter anywhere there are nerve cell
bodies located 1. in various regions of the
brain 2. in the central butterfly region of
the spinal cord D. White matter indicates the
presence of myelinated axons 1. in various
regions of the brain 2. surrounding the central
grey butterfly of the spinal cord 3. tracts -
bundles of axons carrying common information in
the CNS NOTE bundle of axons in the PNS
nerve bundle of axons in the CNS tract
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Dorsal root of the spinal nerve
PNS
CNS
Sensory (afferent)fiber
Gray matter
- Collection of nerve cell bodies
Spinalnerve
White matter

• Bundles of axons carrying common
• information

Motor (efferent)fiber
Ventral root of the spinal nerve
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Grey matter
White matter
Cross section of spinal cord andvertebra,
cervical region
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White matter - ascending and descending TRACTS
of the spinal cord.
Ascending tracts
Descending tracts
Grey matter
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Grey matter
White matter
Grey matter
White matter
Grey matter
White matter
Touchreceptor
Spinothalamic pathway
Dorsal column-medial lemniscal pathway
Spinocerebellar pathway
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VIII. Reflex Arcs

• A. Reflex arcs simple chain of neurons that
  allow for reflexes
• 1. Mechanism for action of simple reflexes
• e.g. patellar tendon reflex biceps tendon
  reflex
• 2. can be either a somatic reflex or a visceral
  reflex
• 3. Consists of five components
• a. receptor detects the stimulus
• b. sensory neuron transmits the information
• c. integration center relay station
• d. motor neuron sends message to the effector
• e. effector muscle or organ that is activated

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Stimulus
Cellbody
Synapse
Axon
Skin
Interneuron
Receptor
Sensory neuron
Integration center
Motor neuron
Effector
Spinal cord(in cross section)
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• B. Monosynaptic reflex arc
• 1. simplest of all reflex pathways
• 2. one sensory neuron and one motor neuron
• 3. fastest type of reflex

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Sensory (stretch) receptor
Sensory (afferent) neuron
Motor (efferent) neuron
Effector organ
Monosynaptic stretch reflex
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• B. Polysynaptic reflex arc
• 1. more common type of reflex pathway
• 2. one or more interneurons between sensory and
  motor
• 3. common in withdrawal reflexes

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Sensory (afferent) neuron
Sensory receptor
Interneuron
Motor (efferent) neuron
Effector organ
Polysynaptic withdrawal reflex
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IX. Neuronal Circuits

• A. Diverging circuitone presynaptic neuron
  synapses with several other neurons (divergence)
• B. Converging circuitmany neurons synapse on a
  single postsynaptic neuron (convergence)
• C. Reverberating circuitcircuit that receives
  feedback via a collateral axon from a neuron in
  the circuit

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Input
Input 1
Input 2
Input 3
Output
Many outputs
Diverging circuit tomultiple pathways
Converging circuit
Input
Output
Reverberating circuit
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X. Types of Processing
A. Serial processing - Neurons pass a signal to a
specific destination along a single pathway from
one to another B. Parallel processing - Input is
delivered along many pathways a single
sensory stimulus results in multiple perceptions
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XI. Integration Between PNS and CNS

• A. Neuronal circuits form networks of
  interneurons
• Example painful stimulus
• ? Immediate response is spinal reflex
• ? Sensory information passed along to brain
• ? Pain is felt after reflexive withdrawal

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Sensory pathway up to brain
Monosynaptic pathway
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Sensory pathway through brain
Motor response from brain to spinal cord to
effector
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XII. Neuronal Regeneration

• A. Neural injuries may cause permanent
  dysfunction
• B. If axons alone are destroyed, cell bodies
  often survive, and the axons may regenerate
• 1. In PNS macrophages destroy axon distal to the
  injury
• a. Axon filaments grow peripherally from injured
  site
• b. Partial recovery is sometimes possible
• 2. In CNS macrophages destroy axon distal to the
  injury
• a. neuroglia cannot guide axon back to proper
  re-growth
• b. no effective recovery of neurons in natural
  patient
• c. stem cell therapy may change this in the
  future

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Regeneration of an axonin a peripheral nerve.
Endoneurium
Schwann cells
The axonbecomesfragmented atthe injury
site.
Droplets ofmyelin
Fragmentedaxon
Site of nerve damage
Macrophagesclean out the deadaxon distal
to theinjury.
Schwann cell
Macrophage
Axon sprouts,or filaments, growthrough a
regeneration tubeformed bySchwann cells.
Aligning Schwann cells formregeneration tube
Fine axon sproutsor filaments
The axonregenerates, and anew myelin
sheathforms.
Schwann cell
Site of new myelinsheath formation
Single enlargingaxon filament