Glia and Myelination

Question 1

Glia

Answer 1

Glue cells
Supporting of the CNS and PNS
Outnumber neurons by 3:1 to 10:1 but occupy only about 50% of CNS volume
No synapses and no action potentials
Retain mitotic ability into adult life; allows for CNS lesions by glia
Most CNS cancers are glial

Question 2

Astrocytes

Answer 2

Largest glial cells, most numerous
Star-shaped cell bodies, many long processes
Vascular end-feet or foot processes which contact

Question 3

Fibrtous astrocytes

Answer 3

Long thin processes, found in white matter

Question 4

Protoplasmic astrocytes

Answer 4

Shorter thicker processes and are found in gray matter

Question 5

Structural support and repair

Answer 5

GFAP: Glial fibrillary acidic proteins intermediate filaments
Proliferation of astrocytes after neuronal damage leads to gliosis

Question 6

K+ spatial buffering

Answer 6

K+ ions released by spiking neurons are taken up by astrocytes
Not electrically excitable

Question 7

Neurotransmitter and metabolite removal

Answer 7

Astrocytic processes around synapses and actively remove many neurotransmitters and metabolites
Aid in rapid termination of the synaptic signal
GLAST (glutamine astrocyte-specific transporter) for glutamate to glutamine conversion

Question 8

Neurotransmitter reeceptors

Answer 8

Trigger calcium waves

Question 9

Blood brain barrier

Answer 9

Contributed to by astrocytes

Question 10

Astrocyte communication

Answer 10

Gap junctions

Question 11

Astrocytes during development

Answer 11

Glial guides, act as stem cells in adult brain to generate neurons and glia

Question 12

Oligodendrocytes

Answer 12

Smaller than astrocytes, fewer processes round nuclei

Electron-dense cytoplasm

Question 13

Types of oligodendrocytes

Answer 13

Perineural oligodendrocytes
Interfasicular oligodendrocytes
Oligodendrocytes in white matter

Question 14

Function

Answer 14

Myelination of axons (CNS)
One oligodendrocyte may myelinate many axons
Inhibitor of axon outgrowth and regeneration

Question 15

Methods of neuronal growth inhibition (CNS)

Answer 15

Myelin-associated glycoprotein (MAG): periaxonal oligodendroglial membrane, axon-glial signaling, important in maintaining axon-myelin complexes
Neurite inhibitor of 35 kDa (NI-35): Chondrotin sulfate proteoglycan expressed by oligodendrocytes
Nogo genes and proteins (NI-220/250)

Question 16

Schwann cells

Answer 16

Function: PNS myelination (one segment of one axon, single PNS axon can be 50-500 cells)
Can surround multiple axons if axons are unmyelinated
Provide growth-promoting factors: laminin,NgCAML/L1 (cell adhesion), Schwann may secrete nerve growth factor

Question 17

Microglia

Answer 17

Smallest, oval cell bodies, many short processes
Few in number normally, hyperplasia and hypertrophy when neurons undergo degeneration
Recruited during infection, injury, and seizure
Phagocytosis

Question 18

Ependymal cells

Answer 18

Cuboidal or columnar epithelial layer lining inside of neural tube
Can be ciliated or have microvilli
Desmosomal junctions in ventricles
Modified cells form choroid plexus with tight junctions

Question 19

Function of ependymal cells

Answer 19

Provide some barrier between brain and CSF
Ciliary motion aids in CSF circulation
Choroid plexus produce CSF

Question 20

Myelin structure

Answer 20

Inner mesaxon - Both CNS and PNS
Other mesaxon - Present in PNS
Major dense lines (apposed cytoplasmic faces)
Minor dense lines or intraperiod (apposed extracellular faces)
Axons larger than 1 micron in diamter are myelinated
High lipid content
Thickness increases with axon diameter

Question 21

Myelin basic protein

Answer 21

Major structural proteins of myelin in CNS and PNS
On the cytoplasmic face of myelin membrane
Basis for CNS autoimmune disease: experimental allergic encephalomyelitis

Question 22

Myelin-associated glycoprotein (MAG)

Answer 22

Both CNS and PNS myelin

Question 23

Myelin-oligodendrocyte glycoprotein (MOG)

Answer 23

CNS only
On the surface of myelin sheath and oligodendrocytes
Implicated as a target antigen in autoimmune aspects of CNS demyelinating diseases

Question 24

Nodes of Ranvier

Answer 24

Axolemma - Exposed to extracellular fluid
Saltatory (“jumping”) conduction from node to node
High concentration of voltage-gated sodium channels
Internode typically 1-2 mm, longer with the size of axon
Axon branching (collaterals) always occurs at nodes
Nodes in PNS are covered by Schwann cell cytoplasm
CNS, the nodes are bare

Question 25

Clefts

Answer 25

PNS: Schmid-Lanterman incisures CNS: Longitudinal incisures Split in the amjor dense lines filled with myelin-forming cell cytoplasm Function: cytoplasmic nutrients to inner leaflets

Question 26

Myelin function

Answer 26

Insulation, decrease ionic flux across the axolemma, conserves cellular ATP

Question 27

Multiple sclerosis

Answer 27

Chronic demyelinating disease of the CNS Autoimmune disease with genetic and environmental risk Several myelin antigens may be involved Scarring throughout CNS Paralysis from abnormal conduction Disruption of blood brain barrier and acute inflammation Marked periods of remission and relapses over many years Affects both sensory and motor axons

Question 28

Guillain-Barre Syndrome

Answer 28

Acute inflammatory demyelinating diseases of the PNS Attacks myelin of the peripheral nerves that innervate muscle and skin Autoimmune disease against one's own myelin proteins Demyelination leads to slower and abnormal conduction of action potentials Difficulty with both sensory perception and motor coordination