20 - histology of the nervous system

7/31/2019 20 - Histology of the Nervous System

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THE NEURON

Sensory neurons

Motor neurons

Interneurons

Cell body:o Nucleus and other normal organelles

o Abundent RER (high protein synthesis) RER forms stacks called Nissl

bodieso Axon hillock (just before the axon) is usually free of cytoplasmic

organelles.o Newly synthesised proteins are transported along the axon by axonal

transport

Axon: transmit stimuli to other neurons / effector cellso

All neurons have only a single axono Axon originates from axon hillock

o Region between beginning of axon and the myelin sheath is the initial

segment

Dendrites: receive stimulio A neuron can have several dendrites

o Transmit impulses from periphery cell body

o Unipolar neurons: only an axon, no dendrites

o Bipolar neurons: one axon and one dendrite

o Multipolar neurons: one axon and two or more dendrites.

o

Dendrites are usually unmyelinatedo Extensively branched dendritic trees

o Dendrites have many of the same organelles as the cell body, except

they dont have Golgi apparatus.

Motorneurons and interneurons = multipolar

Sensory neurons = unipolaro Cell body in DRG

o One axonal branch extends to the periphery, the other into the CNS

Synapses:o Synapses can be:

Axodendritic

Axosomatic (between axons and the cell body)

Axoaxonico Golgi stain reveals synaptic boutons

o Synapse consists ofpresynaptic knob with numerous synapticvesicles

o Synapses can be classified as:

Gray I

Thick

Excitatory Round vesicles


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Gray II

Cylindrical vesicles

AXONAL TRANSPORT: Fast & slow axonal transport provide distal parts of the axon with newly

synthesised proteins & organelles.

In adult, large distance between neuronal cell body & motor end-plate

Proteins & organelles are synthesised in the cell body.

Tie a ligature around the axon, and the axon will become engorged on the cellbody side (due to blocking of anterograde transport)

3 ways that the axons transports substances:

1. Fast anterograde axonal transport: cell body nerve terminal2. Slow anterograde axonal transport

3. Fast retrograde axonal transport: nerve terminal cell body

All 3 types of axonal transport take place along microtubules

Microtubules form a network throughout cytosol, forming tracks overwhich vesicles, organelles and other cell components can be moved.

Drugs like colchicine distrupt microtubule formation inhibit axonal transport.

This movement is assisted by motor proteins.

Motor proteins use energy derived from repeated ATP hydrolysis to travelalong the microtubule in a single direction. The motor proteins also attach tosome other cell component, and thus carry it along the microtubule.

Kinesins are motor proteins that move outwards along microtubules from the

cell body to the axon terminal anterograde transport Dyneins are motor proteins that move inwards from axon terminal to cell

body reterograde transport

Both kinesins and dyneins consist of2 globular heads containing ATPasewhich binds to the microtubule, and a tail which binds to the cellcomponent to be transported.

Fast anterograde transport:

Newly synthesised organelles in cell body are exported to axon and dendriteso Include synaptic vesicules

o Precursors of peptide neurotransmitters.

Slow anterograde transport:

Export soluble cytoplasmic & cytoskeletal proteins

Fast retrograde transport:

Allows recycling of material used by nerve terminal in synaptic transmission.

Allows transport of nerve growth factor from terminal cell body

Reterograde transport is exploited by some viruses (HSV / polio / rabies) to gainentry to the CNS.

SUPPORTING CELLS OF THE NERVOUS SYSTEM:

PNS SCHWANN CELLS:


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In PNS the schwann cells form the myelin sheath and support unmyelinatedcells.

Develop from neural crest cells

Myelinated PNS axons

Form a lipid-rich myelin sheath around myelinated neurons insulating theaxon from surrounding extracellular fluid.

Myelin sheath is formed from compacted layers of Schwann cell membrane,wrapped concentrically around the axon.

The cytoplasm of the Schwann cell is extruded from the membrane layers

The node of ranvier represents the junction between 2 adjacent schwanncells.

The internodal segment is the length of myelin between 2 nodes.

Axon hillock and the terminal portion of the axon are not myelinated.

Unmyelinated PNS axons

Unmyelinated axons are still enveloped in Schwann cell cytoplasm

Axons fit into grooves on the cell surface.

Large Schwann cells may have 20+ grooves

Several axons may lie in each groove

Unmyelinated axons are in direct communication with the extracellular fluid viaa longditudinal cleft in the swan cell the mesaxon.

SATELLITE CELLS:

Satellite cells are small cuboidal cells which surround neuronal cell bodies inganglion

Same function as scwhann cell (but doesnt make myelin)o Insulation

o Maintain appropriate neuronal environment


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CNS NEUROGLIA

Neuroglia are CNS cells, which can be divided into 4 catagories:o Oligodendrocytes form myelin

o Astrocytes

o Microgliao Ependymal cells

When looking at micrographs of CNS, it is only the neuroglia cells which cannormally be visualised (not the neurones).

Oligodendroglia / oligodendrocytes:

o Account for 75% of all glial cells in white matter

o Produce and maintain myelin sheaths around axons.

o Each oligodendrocyte gives off several tongue-like processes which then

wraps itself around an axon.

o Myelin sheath formed by concentric layers of oligodendrocyte membrane

o A single oligodendrocyte may give a myelin sheath to many different

axons.

o (In PNS the myelin sheaths are formed by schwann cells).

Astroglia / astrocytes:o Cells with long processes which form a scaffold through the CNS

o The ends of the processes are expanded into end feet make firm

attachments to blood vessels.


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o The ends of the astocytic processes seal together creating additional

barrier between blood and ECF of brain and spinal cord (i.e. strengthensblood-brain barrier).

o Prevents changing composition of blood from affecting neuronal

function.

o Also form connections with neurons, and modulates the behaviour of the

axons.

Microglia:o Phagocytes

o Originate from monocytic precursors

o Scattered throughout grey and white matter

o Rapidly migrate to site of infection in CNS

Ependemal cells:o Ciliated cells

o Forms simple ciliated cuboidal-columnar epithelium ependyma

o Tightly bound by tight junctions at apical border

o Lines the vesicles of the brain & central canal of spinal cord.

o Modified ependemal cells in the choroid plexus are involved inproduction of CSF


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STRUCTURE OF PERIPHERAL NERVE TRUNKS

Axons are delicate structures transverse considerable distances to reach theirtarget organs

Outside the CNS they run in peripheral nerve trunks.

Run alongside major BVs

Within nerve trunk they are surrounded by several layers of CT:o Outermost layer: epineurium

Anchors the nerve trunk to adjacent tissueo Middle layer: perineurium

Surrounds fascicles bundles of axonso Inner layer: endoneurium

Surrounds individual nerve fibres

In the PNS individual axons are a covered with schwann cells

There is extensive collagen and fibroblasts in the PNS, but none in the CNS.

METHODS OF STAINING NEURONS:o Golgi stain:

Silver compound

Only stains 1% of neurons, but stains them along their entire length

o Lucifer yellow

o Nissl stain:

Blue stain

Stains the Nissl bodies (RER) and ribosomes.

DISEASES WHICH SPREAD VIA NEURONS:

o Polio virus spreads by axonal transport from gut neurons ventral hornmotorneurons

o Causes atrophy of motoneurons in spinal cord

o poliomyelitis

o Rabies virus travels to the CNS by retrograde transport in both sensory andmotor neurons.

o

Herpes zoster virus lies dormant in DRGo If a person becomes immunocomprimised, it can travel by anterograde

transport down sensory neuronso Vesicular rash at clearly marcated dermatomes.

REGENERATION:o Oligodendrocytes in the CNS prevent regerentation in the CNS

o However, neurons in the PNS to have the potential for regeneration, due to

properties of Schwann cells.

20 - histology of the nervous system

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