Transcript Schwann cells

12
Fundamentals of the
Nervous System and
Nervous Tissue
PowerPoint® Lecture Presentations prepared by
Leslie Hendon
University of Alabama, Birmingham
© 2014 Pearson Education, Inc.
I. Nervous System Overview
A. Master control and communication system
1. Stimulus—changes detected inside or outside the body
2. Sensory receptors - monitor changes inside and outside the body
2. Sensory input—information gathered by receptors
3. Integration—Processes and interprets sensory input
4. Motor output—Dictates response; activates effector organs
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Stimulus
Sensory receptor
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
Spinal
cord
Nerves
Ganglia
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PNS
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
Somatic and visceral sensory
nerve fibers
Somatic sensory fiber
Motor (efferent) division
Motor nerve fibers
Somatic nervous
system
Skin
Somatic motor
(voluntary)
Conducts impulses
from the CNS to
skeletal muscles
Visceral sensory fiber
Stomach
Autonomic nervous
system (ANS)
Visceral motor
(involuntary)
Conducts impulses
from the CNS to
cardiac muscles,
smooth muscles,
and glands
Skeletal
muscle
Motor fiber of somatic nervous system
Sympathetic division
Mobilizes body systems
during activity
Paraysmpathetic
division
Conserves energy
Promotes housekeeping functions
during rest
Sympathetic motor fiber of ANS
Heart
Structure
Function
Sensory (afferent)
division of PNS
Motor (efferent)
division of PNS
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Parasympathetic motor fiber of ANS
Bladder
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 sense—position 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 motor—contraction 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. phagocytes—the 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
Microglial
cell
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
Ependymal
cells
Brain or
spinal cord
tissue
Ependymal cells line cerebrospinal
fluid-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
Myelin sheath
Oligodendrocytes
Myelin sheath gap
Oligodendrocytes have processes that form myelin
sheaths 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
Satellite
cells
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 sheath
formed from Schwann cells
Schwann
cell plasma
membrane
1 A Schwann cell
envelops an axon.
Schwann cell
cytoplasm
Axon
Schwann cell
nucleus
2 The Schwann cell
then rotates around
the axon, wrapping
its plasma membrane
loosely around it in
successive layers.
Myelin sheath
Myelin
sheath
Schwann cell
cytoplasm
3 The Schwann cell
cytplasm is forced
from between the
membranes. The tight
membrane wrappings
surrounding the axon
form the myelin
sheath.
Myelin sheath
Schwann cell
cytoplasm
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
(receptive
regions)
Cell body
(biosynthetic center
and receptive region)
Neuron
cell body
Dendrites
Nucleus with
nucleolus
Nucleus
Nuclei of
neuroglial
cells
Axon
(impulse-generating
and -conducting
region)
Nucleolus
Axon hillock
Myelin sheath gap
(node of Ranvier)
Terminal boutons
(secretory region)
Schwann
cell
Terminal
arborization
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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
(receptive
regions)
Cell body
(biosynthetic center
and receptive region)
Neuron
cell body
Dendrites
Nucleus with
nucleolus
Nucleus
Nuclei of
neuroglial
cells
Axon
(impulse-generating
and -conducting
region)
Nucleolus
Axon hillock
Myelin sheath gap
(node of Ranvier)
Terminal boutons
(secretory region)
Schwann
cell
Terminal
arborization
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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 >>> 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
(receptive
regions)
Cell body
(biosynthetic center
and receptive region)
Neuron
cell body
Dendrites
Nucleus with
nucleolus
Nucleus
Neurofibril
Nuclei of
neuroglial
cells
Axon
(impulse-generating
and -conducting
region)
Nucleolus
Axon hillock
Myelin sheath gap
(node of Ranvier)
Terminal boutons
(secretory region)
Schwann
cell
Terminal
arborization
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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
Presynaptic
neuron axon
Terminal boutons
at synapse
Dendrites
Postsynaptic
neuron
Postsynaptic
neuron axon
Two neurons connected
by synapses
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Presynaptic axon
Nerve
impulses
Terminal bouton
Vesicle releasing
neurotransmitter
Mitochondrion
Synaptic
vesicles
Synaptic
cleft
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 > one long dendrite > cell body > axon
e.g. olfactory cell; retinal cell
C. Unipolar neuron
► long dendrite > (passes by cell body) > 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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Stimulus
Sensory receptor
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
Endoneurium
Perineurium
Blood
vessels
Fascicle
Epineurium
Myelinated
axons
Myelin
sheath
Fascicle
Epineurium
Schwann
cell
nucleus
Axon
Myelin
Myelin
sheath
gap
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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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PNS
Dorsal root of the
spinal nerve
CNS
Gray matter
- Collection of nerve cell bodies
Sensory (afferent)
fiber
Spinal
nerve
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 and
vertebra, 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
Touch
receptor
Spinocerebellar pathway
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Dorsal column-medial lemniscal pathway
Spinothalamic pathway
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
Skin
1 Receptor
Cell
body
Synapse
Axon
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 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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1 Sensory (stretch) receptor
2 Sensory (afferent) neuron
3
4 Motor (efferent) neuron
5 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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1 Sensory receptor
2 Sensory (afferent) neuron
3 Interneuron
4 Motor (efferent) neuron
5 Effector organ
Polysynaptic withdrawal reflex
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IX. Neuronal Circuits
A. Diverging circuit—one presynaptic neuron synapses with several
other neurons (divergence)
B. Converging circuit—many neurons synapse on a single
postsynaptic neuron (convergence)
C. Reverberating circuit—circuit that receives feedback via a
collateral axon from a neuron in the circuit
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Input
Input 1
Input 2
Many outputs
Diverging circuit to
multiple pathways
Input 3
Output
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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Monosynaptic pathway
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Sensory pathway up to brain
Sensory pathway through brain
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Motor response from brain to
spinal cord to effector
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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Endoneurium
Schwann cells
Regeneration of an axon
in a peripheral nerve.
Droplets of
myelin
1 The axon
becomes
fragmented at
the injury site.
Fragmented
axon
Site of nerve damage
Schwann cell
Macrophage
Aligning Schwann cells form
regeneration tube
2 Macrophages
clean out the dead
axon distal to the
injury.
3 Axon sprouts,
or filaments, grow
through a
regeneration tube
formed by
Schwann cells.
Fine axon sprouts
or filaments
Schwann cell
Single enlarging
axon filament
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Site of new myelin
sheath formation
4 The axon
regenerates, and a
new myelin sheath
forms.