Nervous Tissue Flashcards
What glial cells are present in the CNS?
WHat glial cells are present in the PNS?
CNS: astrocyte, oligodendrocytes, microglia, ependymal cells
PNS: Schwann cells, satellite cells
Which one has against connective tissue, PNS or CNS?
PNS only
Embryologically, what forms the PNS? WHat forms the CNS?
Neural crest forms the PNS (sensory neurons, motor neurons, glia, etc)
Neural tube forms the CNS (neurons and glia)
What cells are mitotically active in the nervous system?
neurons are NOT
glial cells maintain mitotic capbility
Where are pyramidal neurons located? WHere are purkinje fibers located?
hippocampus
Cerebellum
What is the term for all the neural tissue that contains all the synapses, processes, axons, dendrites and glial cells?
neuropil
WHat cells line the canal of the spinal cord?
ependymal cells
What stain will strongly strain RER?
the Nissl stain
it forms Nissle bodies = clumps of RER (there’s lots of RER in neurons because they need lots of proteins)
What filaments make up the cytoskeleton of the neuron cell body?
a class of intermediate filemtns called neurofilaments
microtubules ar epresent as well, but moreso in the axons
What are the two major pigments that can be picked up by neurons?
lipofuscin (from breakdown of lipids in lysosomes)
neuromellenin (in locus cerruleus and substantia nigra)
WHy does the axon hillock stain white under stains?
there’s no RER in the hillock. the RER is what’s being stained.
WHat do dendrites do for a neuron?
provides receptive areas for receipt of input
What is the example of naked nerve ending?
What are two examples of encapsulated nerve endings?
Merkel’s cells are associated with naked nerve endings in the skin (touch)
The encapsulated nerve endings are meisner’s corpuscles (touch) and pacinian corpuscles (pressure)
What are the places an axon can send information?
- another nerve cell in the CNS
- autonomic ganglia in the PNS
- effector organ in the PNS
What are some general characteristics of axons?
- usually single, but can have collateral braches
- myelinated or non myelinated
- variable length
- constant diameter
- have terminal branches
Which axon fibers tend to be non-myelinated?
the small ones = C fibers
If an axon terminal branches to form up to 1000 or more terminals, what is it called?
telodendria
Swellings at the end of axon terminals are called what?
terminal boutons
end-feet
knobs
telodendria
What are some characteristics of the plasma membrane of neurons?
it’s semipermeable and electrically active
integral membrane proteins serve as channels/carriers for ions or receptors for NT
they maintain the resting membrane potential and also propagate action potentials
What forms the synaptic vesicles for NT secretions?
the golgi apparatus
What really determines the characteristics of different nerve cell types?
the arrangement of the dendrites
What is the initial segment of a neuron?
It’s the region of the axon that extends from the cell body to the beginning ot myelination
It is the site of impulse initiation due to lower threshold of excitability
What is the content of an axon?
NO RER or ribosomes
synaptic vesicles in the terminals
microtubules running parallel to the long axis of the axons
What do terminal ends of the axon do? What are the different types?
THey form synapses - specialized communicating junctions - with other nerve cells or effector organs
- axo-dendritic
- axo-somatic
- axo-axonal
- Serial, like axo-axo-dendritic
- Dendrodendritic (very rare)
What are the NT released into?
the synaptic cleft between the axon terminals and their targets
What are 4 things you’d find at an axon terminal?
- NT vesicles
- presynaptic dense projections
- neurofilaments
- mitochondria
What is the importance of presynaptic dense projections in the terminals? (also called presynaptic grid)
it’s an electron dense material on the cytoplasmic side of the membrane.
it provides sites for the vesicles to approach and fuse with the presynaptic membrane
What are some characteristics of the post synaptic structure?
It has receptor integral membrane proteins to interact with the neurotransmitters
interaction with the NT may cause an electrical change in the postynaptic nerve
it’s electron dense material at the cytoplasmic side of post-synaptic membrane
it may be thicker or the same thickness as the presynaptic density
MUCH less compplex than presynaptic dense projections
What is a peptidergic neurons?
It’s a neuron that packages the neurotransmitters into vesicles within the cell body and then transports them to the axon terminals via fast axonal transport via kinesins. the vesicles are then exocytosed
How is a small-molecule secreting neuron different from a peptidergic neuron?
In small molecule secreting neurons, the golgi makes empty vesicles that are transported to the axon terminals by fast transport and that’s where they are constantly being packaged, recycled and repackaged
Describe the slow component of axonal transport?
It is ONLY in the orthograde direction
It transports non-packaged molecules and cytoskeletal components
0.5 to 3 mm per day
mechanism unclear
Describe the fast component of axonal transport.
It can be orthograde (400 mm/day) or retrograde (200-300 mm/day)
In the orthograde direction it transports synaptic vesicles, mitochondria and other membrane associated proteins
In the retrograde direction it transports worn out membranes of synaptic vesicles, old mitochondira, etc.
What is the importnat clinical issue with retrograde axonal transport?
Viruses can travel to cell bodies that way
ravies does this
For fast axonal transport, what motor cells are used for the orthograde direction? How about the retrograde motor?
othograde = kinesin
retrograde = dynein
What two glial cell types form macroglia?
astrocytes and oligodendrocytes
What are the two types of astrocytes?
fibrous (white matter) and protoplasmis (gray matter)
What do astrocyte processes terminate as? WHere
they terminate as “end feet”
they insinuiate themselves between nerve cells and processes and also end on blood vessels
- they form a layer outside the basement membrane of the endothelial cells - they help to form tight junctions between the endothelial cells to make the BBB
- THey end on the inner surface of the pia mater to form the external glial limiting membrane
- They end on the basal surface of ependymal cells to form the internal glial limiting membrane
What are the likely function of astrocytes?
- structural support
- uptake of excess potassium form extracellular space following prolonged neuronal activity
- phagocytosis and scar formation after injury (gliosis)
- Isolation of nerve terminals and fibers form their neighbors
- regulation of the entry of substances to the interneuronal space
How can you distinguish an oligodendrocytes from an astrocyte?
the nucleus will stain darker in oligodencrotyes
they’ll have fewer projections
What is the function of oligodencrocytes?
myelination of CNS axons
they have arms, each of which will myelinate a different internode
What is the funciton of microglia?
they are macrophages of the nervous system, derived from monocytes, and actively phagocytic
What do ependymal cells do?
they line the central canal of the spinal cord and the ventricles
in the ventricles they have cilia that beat and move CSF
They are possibly involved with the transport of substances form the CSF to the brain, and secretion into the ventricles
What kind of cells are ependymal cells?
simple cuboidal o columnar epithelium; ciliated
In the roof of the ventricles, the ependymal cells are modified to make what?
choroidal epithelium covering the choroid plexus
What do choroidal epithelial cells do?
they control the composition of CSF produced by the choroid plexus
they prevent movmeent of plasma proteins from the capillaries of the choroid plexus into the CSF by having tight junctions between adjacent choroidal epithelial cells
What are the three layers of connective tissue that bind together groups of axons in peripheral nerves?
- epineurium (the external, tough layer of dense collagen)
- Perineurium (envelopes bundles of fasicles of fibers and includes numerous epithelioid-like cells on the inner aspect)
- Endoneurium (delicate collagen around individual axons - contacts the basement of schwann cells which cover the axon)
Unmyelinated fibers of the PNS consist of an axon and what?
a sheath of Schwann, or neurilemma
Describe a Sheath of Schwann
- schwann cell embraces a number of small axons (C fibers)
- eventually eac axon gains its’ own trough in the periphery of the schwann cell
- the axons still remain outside the schwann cell membrane even though they are engulfed by it
- the area of membrane that encloses over the axon is called a mesaron
what is the collective term fo a group of unmyelinated fibers encompassed in a single Schwann cell?
a bundle of remak
Where do unmyelinated fibers occur in the pNS?
a. Axons of most post-ganglionic autonomic neurons
b. Axons of small sensory neurons in dorsal root ganglia
WHat do myelinated fibers of the PNS consist of?
axon
myelin
schwann cells
What is the major consistuent of myelin?
lipids 80%
protein 20%
What will myelin look like…
when fresh?
stained with osmium?
stained with H&E?
fresh: glistening white
Osmium: black empty rings - like black cheerios
H&E: network of residual proteins called neruokeratin (the lipid is removed in the fixation)
What are nodes of ranvier?
periodic interruptions in the myelin sheath
the space between adjacent schwann cells along the length of the axon
What are internodes?
Segments of myelin between adjacent Nodes of Ranvier
Sometimes there are areas where the myelin didn’t wrap quite right and the cytoplasm wasn’t squeezed out. THese form oblique clefts in the myelin under stainig. WHat are these called?
Schmidt-Lantermann clefts
What is the myelin jelly-roll theory?
- the myelin is formed by a spiral wrapping of the Schwann cell membrane around hte nerve fiber
- cytoplasm of the SC is squeezed out and compation results in the formation of intraperiod and major dense lines
ONE schwann cell will form ONE segment of internode on ONE axon with the nucleus in the cytoplasm of the final wrap
Largest axons will have the —— myelin and the —— internodes, and are the most —— conducting.
Largest axons have the thickest myelin and the longest internodes –
and are the most rapidly conducting
What forms the major dense lines in myelin? THe intraperiod lines?
the major dense lines are former cytoplasmic surfaces of schwan cell membranes that fuse together
the intraperiod liens are former outer surfaces of schwann cells that fuse and ecome thin
What are the internal and external mesaxons?
the regions where the SC meets itself as it encircles the axon initially (internal) and where it overlaps itself on the outermost turn (external)
What do the nodes of ranvier look like in the PNS?
it’s partially covered by loosely interdigitating cytoplasmic processes of the schwann cell
the basement membrane is outside the SC and inside the endoneurium
Do unmyelinated fibers in the CNS have a glial cell?
no
What glial cell myalinates fibers int he CNS? Describe how.
oligodendrocyte
one oligodencrocyte may myelinate more than one axon - 10 to 40
Oligodendrocyte cell body is located distant form the axon so there is little cytoplasm in the outer wrap and never a nucleus
How are nodes of ranvier in the CNS different from thos ein the PNS?
in the CNS the internodal distance is shorter and the node is exposed to the extracellular space because ther eis no oligodendrocyte process covering it (as in the PNS)
There isn’t a basement membrane outside the myelin - no connective tissue
Are there any schmidt lantermann clefts in the CNS?
No - differen cell, different type of wrapping
How do action potentials doncut down myelinated axons?
it’s faster because it uses slatatory conductoin - it jumps from node to ndoe
this correlates with the fact that the N of R have the highest concentration of sodium channels anywhere on the nerve cell
Where are sensory ganglia?
dorsal root ganglia of all spinal enrves and the sensory ganglia of cranial nerves 5, 7,8,9 and 10
Do sensory ganglia have synapses inside?
No - they hold pseudounipolar cells, so synapses aren’t necessary- one process goes to the periphery and the other goes to the CNS
What type of peripheral glia cell is located in sensory ganglia?
satellite cells
Describe satellite cells.
Very small cells which form a covering around each ganglionneuron; only the nucleus of these glial cells is demonstrated inlight microscopy
they might just be schwann cells - the satellite cell layer is ocnitnuous with the schwann cell sheath of the peripheral nerve and they share a continuous bsement membrane
What connective tissue cells can also be found in sensory gnaglia
fibroblasts!
they are continuous with the endoneurium of the peripheral nerve
The entire ganglion is surrounded by what?
a connective tissue capsule that is continuous with th perineurium and epineurium of the peripheral nerve
it extends into the ganglion to separate nerve ell bodies into groups
How do sympathetic ganglia and parasympathetic ganglia differ?
sympathetic ganglia are discrete structures with CT capsules
parasympathetic ganglia are smal encapsulated structures in the head, but are isolated cell groups in the thoracid, abdominal and pelvic viscera
What are the contents of autonomic ganglia?
synapses of the preganglionic fibers on the dendrives nand cell bodies of the post-ganglionic neuron
the cell bodie of the postganglionic neurons (multipolar)
satellite cells and schwann cells
connective tissue and fibroblasts
What is one way you can differentiate sensory ganglia from autonmoic ganglia uder a microscope?
sensory ganglia have central nuclei
autonomic ganglia have eccentric nuclei and are often binucleate
Which are the myelinated fibers: pre or postganglionic nerve fibers?
the pregnaglionis are myalinated B fibers
the postganglionic axons are small unmyelinated C fibers
How do the satellite cells of the autonomic ganglia differ form that of sensory ganglia?
they don’t form as complete a covering of each nerve fiber as they do through autonomic
Describe Wallerian degeneration?
it’s orthograde degeneration
the axon, axon terminals, and myelin disintegrate
sometimes the schwann cell sheath and CT layers remain intact, but not always
Describe retrograde degeneration.
It’s degeneration in the cell body and or portion of axon still attached to the cell body
- degeneration of axon and myelin sheath for a short distance toward the cell body
- chromatolysis (cell body and nucleus swell, nucleus becomes eccentric, RER disintegrates and move to periphery
- somtimes cell death without chromatolysis
Which injury is worse: a crushing injury or a slice injury?
a crushing injury
regeneration can occur in the PNS if the growing axon can find and follow the tube of the schwann cells and connective tissue that’s left
it won’t be present in a crushing injury
What happens first, reinnervation or remyelination?
reinnervation
Is reinnervation specific?
not really
sensory fibers will innervate any sensory receptor and motor fibers will innervate any muscle
so the regenerated product is never as good as the original - sensation is less critical and movement is less discrete
the regenerated axons generally don’t retain their oriinal diameter either, so conduction velocity is decreased
What is a traumatic neuroma?
a. Large heterogeneous mass of entangled nerve fibers, Schwann
cells, connective tissue cells, etc.
b. Sometimes develops as regenerating neurons are unable to overcome
the obstacles between the cut ends of the nerve
c. the afferent neurons involved in the neuroma still transmit messages
centrally which are often regarded as painful
Why does regeneration typically not occur in the CNS?
THe Oligodendrocytes don’t retain that guiding tube
scar tissue produced by astroglia is a significant barrier as well
WHat is a possible explanation for surprising CNS regeneration in some cases?
plasticity
- collateral sprouting and establishment of new connections
- unmasking of previously less significant connections that had redundant function
