Download Nerves

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Specialization
o Fxn: specialized to use electrical energy to conduct information in a precise way from
cell to cell & place to place
o Nervous TISSUE Composition:
 Receptors: transduce physical energy from environment to biologically useful
electrochemical energy
 Neurons: receive, analyze, conduct & transmit the coded information (signal)
 Use axons, specialized processes, to selectively communicate with
other neurons/target cells
 Supporting Cells: variety of specialized non-neuronal cells referred to as glia
o NEURON Composition: long-lived cells that contain many organelles
 Cell Body (soma):
 contains nucleus
 “receptive” surface of neuron
 contains genetic material & most of neuron protein synthesis capacity
 Dendrite: extension (conserves volume; maximizes surface area) of cell body
specializing in receiving input from neurons
 Axon: conducts information
 Synaptic Varicosities: transmitting region of neuron (swellings)
 Neurons do not possess an external basal lamina
 Nervous tissue is highly vascularized, esp where there are many neuron bodies
 Spatial arrangement classification (number of dendrites):
 Pseudounipolar - dendrite + axon emerging from same process (“T”
axon---dorsal root ganglion)
 Bipolar - single axon + single dendrite on opposite ends
 Multipolar - more than two dendrites (most common)
Neuron (argyrophilic; well vascularized)
o Soma Organelles:
 Nucleus – usually centrally placed
 Nucleolus – centrally placed & prominent
 Relatively little heterochromatin (transcriptionally inactive)/ a lot of
euchromatin, b/c neuron uses great deal of DNA to transcribe message
for the large variety of proteins produced by it
 Perikaryon – portion of cell body that surrounds the nucleus
 Nissl body (granule) – located in perikaryon; collection of polysomes + rER
 Conspicuous in CNS (lack of extracellular space & CT = CNS)
 v. sm. in PNS: homogenous basophilic (b/c of free ribosomes)
cytoplasmic stain in class prep’s
 Golgi apparatus – very prominent due to:
 Lg. amt. of CHO added to many membrane proteins
 Membrane packaging of many proteins
 Lg. number of lysosomes produced by neuron
 Mitochondria – supply ATP for:
 Carrying out many biochemical processes
 Many lysosomes active in autophagy
 Found throughout perikaryon & neuron processes
 Lysosomes – Autophagy (throughout perikaryon & neuron processes)
 Filamentous organelles
 Microtubules (μT’s): Cytoskeletal support & materials mvmt
 Neurofilaments/Intermediate: cytoskeletal support (argyrophilia)
 Microfilaments – pervasive (omnipresent), hard to see, & most
prominent in developing/regenerating axons
 sER (a lot of membrane synthesis) – thru-out neuron; membrane reserve;
sequester Ca2+ phospholipid synth.
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Inclusions:
 Lipofuscin granules – accumulate during aging
 Melanin – found in some neurons that synthesize monoamines
Staining Properties:
 EM of soma has same general appearance as in LM
 CNS: in areas surrounding soma (neurophil) is not extracellular space
or CT; it is densely packed w/ neuronal processes, glial cell processes
& blood vessels
 PNS: ganglia neurophil contains substantial amt. of CT in addition to
neuron processes & glial cells
Dendrites – specializations for receiving synaptic contacts from other neurons
 Conduct excitatory/inhibitory postsynaptic potentials (graded) twrd the cell body
 Extension of perikaryon; major receptive surface of neuron
 Organelles: μT’s, neurofilaments, sER, mitochondria, lysosomes & ribosomes
Axon Hillock – “sum” electrical postsynaptic potentials & produce the axn potential
 Conical giving rise to a single narrow process (initial segment); ends where 1st
myelin wrapping begins
 Produces action potential, an all-or-none (non-graded) electrical signal
Axon – conducts action potentials rapidly over long distances
 Organelles: sER, mt, lysosomes, actin, μT’s ( + ends toward axon terminal);
few if any ribosomes beyond initial segment
 Microtubules, μT’s: axonal transport
 Motor proteins: kinesin (anterograde) & dynein (retrograde)
 Myelin: spirally-wound wrapping of glial cell PM that surrounds axon
(excludes cytoplasm; v. e- dense)
 Tight wrapping via adhesion molecules (intergral membrane proteins)
 Node of Ranvier – where myelin segments meet; ea. segment acts as an
insulator causing the axn potential to regenerate at the nodes (salutatory
conduction)
 Myelination speeds conduction velocity; # of turns of myelin & axn
potential velocity are directly proportional to axon diameter
 Stain: Osmium tetroxide – myelin dark brown/black & axon unstained
 Schwann cells form PNS myelin & are surrounded by an ext. lamina
(aids in nerve regeneration)
 Oligodendrocytes form CNS myelin & lack ext. lamina (do not
“wrap”; they “organize”—nerves sit in grooves)
 Clinical Correlations:
o Demylenation diseases: Multiple Sclerosis, autoimmunity to
myelin cell proteins
o PNS axon regerneration: ext. lamina important in guiding
regenerating PNS axons after nerve damage
Axon Terminal (varicosity): specialized for neural transmission
 Loses myelination at its target & repeatedly branches (arborizes) into
varicosities that contain many synaptic vesicles (membrane bound vesicles filled
with chemical NT’s)
 Synapses – membrane specialization; sites of one way communications
 Presynaptic membrane – synaptic vesicles accumulate
 Synaptic cleft – extracellular space b/w neuron & target cell
 Postsynaptic membrane – target cell; contains receptors & ion channels
 Synaptic Vessicle docking & fusion
 Normally: synaptic vesicle (w/ V-snare) tethered by synapsin to actin
cytoskeleton; axonal variscosity has low [Ca2+]
 Action Potential:
o Varicosity depolarized & influx of Ca2+
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Cascade phosphorylates synapsin; synaptic vesicle is feed
Complimentary V-snare & t-snare (target) bind one another
(“lock-n-key”)
o Synaptic vesicle fuses with presynaptic membrane
exocytosing NT’s into cleft
o NT diffuses across cleft binding ligand-gated ion channels of
postsynaptic membrane  postsynaptic axn potential
o Transmitter pumps & transmitter-degrading enzymes rapidly
clear cleft of transmitter.
Neuromuscular Junction in Skeletal Muscle (or motor end plate):
 Specialized synaptic contact
 Axon arborization is embedded in a depression, synaptic trough
 Active zone = presynaptic vesicles crowding presynaptic membrane
 Jxn’l folds = postsynaptic troughs; lips contain transmitter receptors
 External lamina of muscle cell thru cleft & into jxn’l folds
 Release of transmitter (acetylcholine, ACh) produces axn potential in
muscle cell PM that quickly propagates throughout the muscle cell
 Clinical Correlation: Myasthenia gravis – Ab’s against ACh receptors,
preventing ACh binding to receptors  muscular weakness
Organization
o Central Nervous System (CNS); Brain & spinal cord
 Contains neurons, glial cells & blood vessels, but very little CT
 Tissue:
 White Matter – mostly axons & few neuronal cell bodies
o Tracts – collections of axons that have a common orgin &
destination (e.g. spinothalamic – from spinal cord to thalamus)
 Gray Matter – mostly neuron cell bodies & few axons
o highly vascularized
o Nuclei – collections of neuron cell bodies (not to be confused
with the nucleus of the cell
 Blood-brain barrier – occluding zonules b/w CNS endothelial cells
 Connective Tissue Coverings (meninges):
 Dura mater – a thick, tough dense irregular collagenous tissue
 Arachnoid – a thinner, collagenous tissue composed of many trabeculae
creating a labyrinthine space, the subarachnoid space
 Pia mater – A delicate collagenous tissue following contours of brain
 Morphology:
 CNS is hollow tube
 Ventricles – spaces lined with ependymal cells (cuboidal ciliated)
 Cerebrospinal fluid (CSF) – fills ventricles
 Choroid plexus – specialized secretory organ of ventricles that produce
CSF  drains into the subarachnoid space surrounding the CNS
 Cells:
 Oligodendrocytes – myelinating cells
 Atrocytes – glial-type supporting cell
 Microglia – glial-type phagocytic cell (monocyte origin)
 Ependymal Cells – line the ventricles (cuboidal ciliated cells)
o Peripheral Nervous System (PNS):
 Consists of neuron cell bodies, axon bundles, & supporting cells
 Vascularized; Considerable vascular CT (unlike CNS)
 Cells
 Ganglia – collection of neuron cell bodies in PNS
 Satellite cells (form of Schwann cell) – encapsulate some ganglion cells
(do not confuse with muscle satellite cells)
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Nerves – collection of axons in PNS (nearly all organs)
Peripheral Nerve Collagenous Tissue coverings: outer  inner
 Epineurium – outermost covering; continuous w/ dura mater
 Perineurium
o Wraps individual nerve fascicles
o Provide barrier & contractile properties (contain actin &
myosin)
 Endoneurium – LCT matrix; formed by Type I + III fibers around axon
 Schwann Cells – ensheath individual axons
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Identification: Neuron cell bodies I.D. is aided by cell size & location
o Somas—basophilic & larger than nonneuronal cell bodies
o Soma nucleoli are large
o Neuron cell bodies often b/w layers of smooth mm & G.I. tract
o Longitudinal or oblique section stain lightly w/ eosin (wavy
arrangement of axons (fig 37)
o Traverse section: look for light staining and a frothy
“vesiculated” appearance (due to dissolved myelin by the
tissue embedding procedure)
 Divsions:
 Sensory
o Enter dorsal horn of the spinal cord via its dorsal root
o Travels with the motor fibers peripherally
 Motor
o Somatic – innervates skeletal muscles
 Motor neurons in spinal cord exit by the ventral roots
 Join with sensory axons peripherally
o Visceral (autonomic) – innervates smooth muscle or glands
 Motor neurons in spinal cord exit by the ventral roots
 Join with sensory axons peripherally (fig 38)
 Trichrome stain: nerves relatively unstained against
darkly stained CT background (fig 39)
SUMMARY:
Definition
Cell bodies
Axon Bundles
CT
Tissue Coverings
(outside to inside)
Supporting Cells
(non-neuronal)
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myelinating
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other support
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phagocyte
Ext. Basal Lamina
CNS
Brain & Spinal Cord
Nuclei (gray matter)
Tract, fasciculus, lemniscus (white matter)
Very little
Dura
Arachnoid
Pia
PNS
All neuron cell bodies & processes
outside brain & spinal cord
Ganglia
Nerve
Substantial
Epineurium
Perineurium
Endoneurium
Oligodendrocytes
Astrocytes
Microglia
None
Schwann
Satellite
Monocyte/Schwann
Some supporting cells have E.B.L.