neurons and neuroglia

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By

Mohammad H. Farjoo

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Contents

Neuron propertiesNeuron morphologyNeuron membraneCell Body (Soma)Neuronal

cytoskeletonDendriteAxonAxonal transport

NeurogliaMyelinNode of Ranvier

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Neuron properties

• Irritable membrane

• Connection via NT

• NT may be specific

• Synapses– Divergent– convergent

• Reflex loops–

Monosynaptic

– polysynaptic

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Neuron morphology

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Neuron morphology

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Neuron morphology

• Unipolar• Pseudo unipolar• Bipolar• Multipolar

– Motor neurons– Pyramidal cells– Purkinje cells

• Type of NT• Type of receptor• Activation or

inhibition• Axon diameter• myelination

Based on extensionsBased on other criteria

There are 1,000 to 10,000Neuron type !!

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Neuron morphology

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• Extrinsic– Hydrophilic– Enzymatic– Modulated by

lipids• Intrinsic

– Hydrophobic– Amphipathic– Uptake NT &….

• 40% of dry weight

• Amphipathic• Made of:

– Polar head– Glycerol– Fatty acid.

Neuron membraneProtein Lipid

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Neuron membrane

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Neuron membrane

• Kinetics of transport– Similar to enzymes– Obey Michaelis-Menton equation– Different kinetics in different areas of a

neuron.

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Cell Body (Soma)

• Nucleus

• RER & SER

• Golgi apparatus

• Mitochondrion

• Peroxisome

• Lysosome &

Endosome

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Nucleus

1. Nuclear envelope2. Perinuclear cisterna3. Contiguous with ER & Golgi4. Pore 5. Connection of cytoplasm &

nucleuplasm.

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RER & SER

1. Ribosomes attach via Chaperonin & Foldase to RER

2. Chromatolysis sometimes occurs 3. Completely inserted proteins =

secretory protein4. Incompletely inserted proteins =

Receptor , Ion channel1. SER is extension of RER into the Axon

& Dendrite2. Essential membrane lipids are

synthesized in SER3. Protein synthesis is completed in SER.

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Golgi apparatus

1. Proteins in different steps of biosynthesis

2. Modifies proteins : 1. Glycosylation , Sulfation ,

Phosphorylation Proteolysis , Fatty acid addition

2. Protein destination determines modification type

3. Processed proteins are sent by “Secretory granules”.

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Mitochondrion

1. Power house of cell2. Specific DNA , RNA and Ribosomes3. Self duplicating ( like bacteria)4. Internal membrane similar to bacteria5. Occurs even in narrowest dendritic

branches 6. Some metabolize Glutamate & GABA 7. Some in Axon terminals contain MAO.

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Peroxisomes

1. Half of their proteins is Catalase

2. prevent accumulation of

oxidating agent (H2O2) .

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Lysosome & Endosome

1. Lysosomes Contain digesting

enzymes

2. Are made by budding off the golgi

3. Are degraded to “Lipofuscin”

4. Involved in reverse axonal

transport1. Endosomes are made by budding off the

SER

2. Contain stuff for membrane

maintenance.

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Neuronal cytoskeleton

• Microtubule

• Neurofilament

• Actin Filament

1. Structural unit is Tubulin 2. Parallel in A & D but

interlaced in Soma3. + end for polymerization & -

end for depolymerization4. 90% of + end toward Axon

but randomly oriented in Dendrites

5. MAPs (Microtubule associated protein)

6. High MW MAP = “MAP2” in dendrites & soma

7. Low MW MAP = “tau” in axon

1. Made of 3 protein subunits = Neurofilament triplet protein

2. In axons neurofilaments > microtubules

3. In dendrites microtubules > neurofilaments

1. Contains actin2. Not longitudinal but make a

sub- membranous mesh work3. Cross linked by “Actin binding

protein” => Have a jelly consistency

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Dendrite

• Acceptor of signal (Not always)• Single branch or immensely arborized• Spine

– three form: thin , stubby , mushroom– appear & increase in number from third

division on– Are associated with learning– Abnormal in mental retarded persons.

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Dendrite

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Axon

• Axon hillock • Initial segment = Trigger zone

– without organel– Summation of signals

• Synaptic varicosity• Axon collateral

– On renshaw cell.• Recurrent collateral

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Differences of dendrite & Axon

1. Tapered2. Many

branches3. + & - end of

MTs toward the tip

4. Contains “MAP2”

5. Microtubules >

neurofilaments

6 Has Ribosome

1. Uniform diameter

2. Few branches3. + end of MTS

toward the tip4. Contains “tau”

5. Neurofilaments > microtubules

6. Ribosome free

Dendrite Axon

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Differences of dendrite & Axon

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Axonal transport

Compared to Soma , How much do you think Axon

dimensions are ?

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1. Transport rate is measured by labeled amino acids

2. Is doubled by every 10 degrees increase in temperature

3. Not dependent to soma4. Absolutely dependent to oxygen5. Calcium deficiency affects the amount

not the rate6. Blocked by colchicine.

Axonal transport

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Anterograde

Retrograde

Axonal transport

Fast

Slow

1. 200 - 400 mm/day

2. Moves NTs , their precursors , membrane proteins

3. crowded vesicles move faster

1. 0.2 to 8 mm/day (faster in longer Axons )

2. % 80 of stuff moves by this type

3. Responsible for receptor insertion , organelle movement , providing proteins

1. 24 mm/day2. Transfers degenerated material &3. captured molecules from synaptic 4. cleft5. Transfers much less than

anterograde counterpart6. Informs cell of remote events7. Has scavenger role (HRP)8. Misused by viruses

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Axonal transport

Cytoskeleton

(Microtubules)

Motor proteins Kinesin Dynein Dynamin

Use ATP

Uses GTP

Propelling mechanisms (Fast transport)

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1. Myosin like2. Cross bridge of

vesicles & MTs3. Unidirectional motor ,

toward + end4. Causes anterograde

movement

1. Motor of cilia & flagella

2. Cross bridge of vesicles & MTs

3. Unidirectional motor , toward – end

4. Causes retrograde movement

Axonal transportKinesin

Dynein

1. Cross bridge of MTs

2. Involved in vesicle recycling.

Dynamin

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Axonaltransport

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Axonal transportSortingmechanism

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Neuroglia

1. Numerically,10 times of neurons2. Smaller than neurons; hence, ½ of brain

volume 3. Separate & insulate neurons4. Transfer nutrients between blood &

neurons5. Uptake many NTs6. Have receptor for some NTs7. Have different types.

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Astrocyte

Oligodendrocyte

Neuroglia

Synthesize Myelin

Microglia

Ependymal cells

1. %10 of NGs 2. More abundant in gray

matter3. In case of injury become

macrophage (Hala Ke Chee?)

1. Take up extra K+ (& Cl -) generated by firing neurons to protect them

2. Has protoplasmic & Fibrous type

3. protoplasmic in gray matter & Fibrous in white matter

4. Fibrous type contains Glial - fibrilary acidic protein or “GFAP”

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GFAP in Astrocytes

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Myelin

• Structure & components

• Mechanism of formation

• Function

• Pathology

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1. Made by Oligodendrocyte in CNS2. Made by schwan cell in PNS3. Not all the schwan cells synthesize

myelin4. schwan cells also provide support to

injured axons5. Composed of %70 lipid & %30

protein.

Myelin:Structure & components

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Found in brain andabundant in Myelin

In cytoplasmicmembrane of axon terminal

Lipids

Simple

Complex

Phospholipid

Glycolipid

FASpingosinCarbohydrate

CeramideCerebroside

Ganglioside Cerebroside

Sialic acid

Myelin:Structure & components (Lipids)

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Protein Zero (P0)

Proteolipid protein (PLP)

Myelin:Structure & components (Proteins)

Myelin basic protein (MBP)

• In oligodendrocyte• %50 of proteins in

CNS • %1 of proteins in PNS• Has role in

myelination

• A group of 7 proteins

• %30 in CNS• %5 - %15 in PNS• Has role in

myelination

• only in PNS• Constitutes %50 of

proteins

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Myelin:Mechanism of formation (PNS)

Myelin-associated glycoprotein (MAG) from MBP groupmajor dense lines & Intra period

lines

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Myelin:Mechanism of formation (PNS)

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Myelin:Mechanism of formation (CNS)

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Myelin:Mechanism of formation (CNS)

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Myelin:formation

Schmidt-Lanterman cleft

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Myelin: Mechanism of formation

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Myelin function

1. Insulation of neurons2. Increase in speed (up to 10 times)3. Decrease in volume (Down to 100

times)4. An important advance in

vertebrates.

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Myelin pathology

1. Mutant MBP gene is called “Shi” or “Shiverer”

2. Myelin in CNS is either absent or abnormal

3. Myelin in PNS is normal4. Intraperiod lines are tightly attached5. Major dense lines are full of cytoplasm6. MBP is absent .

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Node of Ranvier

1. Interdigitating fingers of schwan cells

2. Full of Na+ channel

3. High capacitance4. Low resistance5. Site of branching

.

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