Histology (11/6) Flashcards
Embryology of Nervous System
Neural crest
PNS: sensory neurons of spinal & cranial ganglia; motor neurons of autonomic ganglia; glia of the PNS; other “non-neuronal” cell types
Neural tube
Neurons & glia of CNS
Mitotic potential of NS cells
Neurons are non-mitotic
Glial cells maintain mitotic capability
White Matter
Aggregations of axons, includes nerves in PNS and tracts, columns, fasciculi of axons in CNS, white d/t myelination
Gray Matter
AGgregations of nerve cell bodies + neuropil
Ganglia in PNS, nuclei etc in CNS
Gray d/t little myelin
Neuropil
complex network of nerve cell processes, synapses, and glial cells occupying intervals between nerve cell bodies in gray
Constituents of Nervous Tissue: CNS
Nerve cells
Glia: Astrocytes, oligodendrocytes, Microglia, ependymal cells
Blood vessels
Constituents of Nervous Tissue: PNS
Nerve cells
Glia: Schwann cells, Satellite cells
Blood vessels
connective tisse
General Cytology/Components of Nerve Cell
Plasma membrane, cell body, Dendrites, (Sensory receptors), axon, end terminal
RER in nerve cells
Nissl bodies
Extensive - present throughout whole cell body + dendrites, absent from axon hillock + axon
Golgi in Neurons
Just in cell body Package vesicles (full or empty) to send down axon
Neurofilaments
Type of intermediate filament found only in neurons - structural role - form neurofibrils - found in body and processes
Microtubules in neurons
Involved in transport of molecules/particles down axon (maybe growth and development)
Microfilaments in neurons
Specialized distribution (growth cones, some junctions)
Pigments in Neurons
Lipofuscin - insoluble remnants of lysosomes (product of digestion) - golden brown
Neuromelanin - naturally occuring in some areas, dark brown/black
Mitochondria in Neurons
Large numbers throughout cells, especially abundant in axons
Unencapsulated nerve endings (receptors)
Touch, pain (maybe hot/cold)
i.e. merkel cell (Touch)
Corpuscles of Ruffini + Krause’s end-bulbs
mechanoreceptors, glomerular (encapsulated)
Meissner’s Corpuscle
Touch
Fibrous capsule (encapsulated)
Found in hairless skin
Pacinian Corpuscle
Deep pressure, vibration
Lamellar corpuscle
Looks like onion
ABC grades of axons
A - large, myelinated
B - smaller, myelinated
C - smallest, not myelinated
Presynaptic dense projections
electron dense material on cytoplasmic side of membrane at axon end terminal
Vesicles approach and fuse with presynaptic membrane (guide into right place)
Axonal Transport : Slow
Only orthograde
Carries soluble macromolecules, small molecules not in vesicles, fibrillar components of axoplasm
Axonal Transport: Fast
Orthograde faster than retrograde (both much faster than slow), microtubules
Orthograde: vesicles, organelles, etc. - KINESIN –> plus end (faster)
Retrograde: Worn out membranes, etc. - DYNEIN –> minus end
Consequences of Retrograde transport
clinically: how viruses (i.e. rabies) get far into nervous system
developmentally: might be how axons are told to grow
experimentally: track process of marker molecules back to find cell bodies
Neuroglia of CNS
Astrocytes, Microglia, Oligodendrocytes, Ependymal cells
Outnumber neurons
Macroglia
Astrocytes + Oligodendrocytes
Types of Astrocytes
Fibrous or Protoplasmic (Golgi staining vs. H&E) - either due to functional states or location
Golgi stain appearance of astrocytes
Cell body usually obscured by mass of processes, found near blood vessels/surfaces of brain
H&E/Nissl Stain Astrocytes
Nucleus - oval, large
Functions of Astrocytes
Terminal end feet –> pervasive between neurons and processes –>
End on blood vessels (maybe induce blood brain barrier)
End on inner surface of pia mater - external glial limiting membrane
End on basal surface of ependymal cells to form internal glial limiting membrane
Structural support, uptake potassium after activity, phagocytosis, Isolation from other neurons, regulation of environment
Appearance of Oligodendrocytes
Fewer processes than astrocytes
Nucleus (Nissl stain) - round, smaller, dense staining
Functions of oligodendrocytes
Myelination of CNS axons, maybe nutritive/maintenance
Microglia
5% or less of all glial cells, derived from monocytes - actively phagocytic
Ependymal cells
Line central canal of spinal cord, ventricles of brain
Simple cuboidal, columnar epithelial - ciliated
Beat CSF to brain, secretion into ventricles
Modified to be choroidal epithelium - control composition of CSF in ventricles of brain
Epineurium
External layer, tough, dense collagen
Perineurium
Envelopes bundles or fascicles of fibers
Intermediate in character between epi and endoneurium
inner aspect has epithelioid like cells
Endoneurium
around individual axons
delicate collagen
contacts basement membrane of schwann cells which cover axon
Unmyelinated fibers of PNS consist of ____
Axon + Sheath of Shwann
Development of Sheath of Schwann
Schwann cells from neural crest - travel along axons
Each SC embraces a small number of axons
Trough, remain outside even though engulfed
Function of Sheath of Schwann in unmyelinated axons
unknown
Mesaxon
region where lips of schwann cell cytoplasm meet (gap)
Bundle of Remak
Collective term for a group of unmyelinated fibers encompassed by single schwann cell
c fibers
term for unmyelinated axons
axons of most post-ganglionic autonomic neurons
axons of small sensory neurons in Dorsal root ganglia
Not usually seen d/t not stained
Myelinated fibers of PNS
Axon, myelin, sheath of schwann
Largest axons have thickest myelin, fastest conducting
Myelin content
High lipid content (protein)
Appearance of myelin
Fresh: white d/t lipids
Osmium stained: Black empty rings
H&E: see the residual protein (neurokeratin)
EM: Black
Nodes of Ranvier
Periodic breaks in myelin sheath
space between adjacent schwann cells
Highest conc. Na channels
schmidt lantermann clefts
oblique clefts in myelin, faults in smooth wrapping myelin lamellae, light areas = trapped portions of schwann cell cytoplasm
Myelination via Jelly-roll theory
Myelin formed by spiral wrapping of SC membrane around nerve fiber
Cytoplasm of SC squeezed out –> compaction –> intraperior and major dense lines
Nucleus of SC in cytoplasm of final wrap
Intraperiod Line
former outer surfaces of schwann cell – fuse to become very thin
Major dense line
Former cytoplasmic surfaces of schwann cell
Internal and external mesaxons
regions where schwann cell meets itself as it encircles axon initially (internal) and where it overlaps on last turn (external)
Myelinated Fibers of CNS
Oligodendrocytes responsible for myelination, may myelinated more than one axon = little cytoplasm, never a nucleus near axon (remain distant, tongue)
Nodes of Ranvier in PNS
Cytoplasm of Schwann cell covers, basement membrane out SC inside endoneurium
Collateral branches, en passage synapses occur
Nodes of Ranvier in CNS
Shorter internodal distance Node exposed to extracellular space No basement membrane outside myeline No SL clefts No Connective tissue
Sensory Ganglia (PNS)
Light and dark cells No synapses in ganglia Satellite cells (capsule cells) Fibroblasts C.t. capsule Nuclei in middle of cells
Dorsal root ganglia and cranial nerve ganglia
Satellite Cells
Very small cells which form covering around each ganglion neuron, may be schwann cells, may play role in metabolism of ganglion neurons
peripheral glia
Connective tissue capsule (PNS ganglia)
surround entire ganglia, continuous with perineurium/epineurium, extend into ganglion to separate nerve cell bodies into groups
Autonomic ganglia
Synapses
Pre - Type B (myelinated)
Post - Type C (small, not myelinated)
Satellite cells and schwann cells
Wallerian or Orthograde Degeneration
Portion of axon separated from cell body changes
Axon/terminals/myelin disintegrate
Schwann cell + CT remain in PNS (none in CNS)
Phagocytosis of debris
Retrograde Degeneration
Changes in cell body/portion of axon still attached
Degeneration for short time toward cell body
Chromatolysis (cell body/nucleus swell, moves to side, RER disintegrate)
Death of cell with or without chromatolysis MAYBE
Or maybe no change at all
Regeneration
Successful only in PNS
Multiple sprouts emerge from central stump of axon –> one must get through surviving tube of schwann cells
Remyelination
Specificity of Reinnervation
Will innvervate any sensory receptor/muscle - sensation/motions less critical
Traumatic neuroma
Large heterogeneous mass of entangled nerve fibers, schwann cells, ct cells, etc
Regen. neurons unable to overcome obstacle
Afferents may still send messages –> painful
CNS regeneration
Abortive
No guiding tube (oligodendrocytes)
Scar tissue by astroglia barrier
Plasticity
REcovery after CNS injury
Collateral sprouting - new connections OR unmasking of other connections
