Glial cells and Myelin Flashcards
Myelin?
An electrical insulator that increases the conduction velocity of electrical signals transmitted along axons in the nervous system. Formed by oligodendrocytes (central nervous system) and Schwann cells (peripheral nervous system).
Which neuroglia cell regulates chemical composition of tissue fluid by absorbing excess neurotransmitters and ions
Astrocytes
Which neuroglia cell forms the blood brain barrier
Astrocytes
White matter vs gray matter
- White matter - Regions of brain & spinal cord with dense collections of myelinated fibers – usually fiber tracts
- Gray matter - Mostly neuron cell bodies and nonmyelinated fibers
Schwann cells MOA
spirals repeatedly around a single nerve fiber • lays down as many as a hundred layers of its own membrane
• no cytoplasm between the membranes
• neurilemma – thick outermost coil of myelin sheath. contains nucleus and most of its cytoplasm
• external to neurilemma is basal lamina and a thin layer of
fibrous connective tissue – endoneurium
Oligodendrocytes MOA
reaches out to myelinate several nerve fibers in its immediate vicinity
anchored to multiple nerve fibers
• cannot migrate around any one of them like Schwann cells • must push newer layers of myelin under the older ones
• so myelination spirals inward toward nerve fiber
• nerve fibers in CNS have no neurilemma or endoneurium
Types of neuroglia cells
Astrocytes Ependymal cells Microglia Oligodendrocytes Satellite cells Schwann cells
Astrocytes functions
have extensions (perivascular feet) that contact blood capillaries that stimulate them to form the blood-brain barrier
convert blood glucose to lactate and supply it to neurons as energy source
nerve growth factors secreted by astrocytes promote neuron growth and
synapse formation
regulate chemical composition of tissue fluid by absorbing excess neurotransmitters and ions
• astrocytosis or sclerosis – when neuron is damaged, astrocytes form hardened scar tissue and fill space formerly occupied by the neuron
Oligodendrocyte function
form myelin sheaths in CNS
Microglia?
“macrophages”, wander in search of cellular debris to phagocytize
Ependymal cells
lines internal cavities of the brain, cuboidal epithelium with cilia on apical surface
• secretes and circulates cerebrospinal fluid (CSF)
Schwann cells
produces a myelin sheath in PNS
Satellite cells
surround the neurosomas in ganglia of the PNS
• provide electrical insulation around the soma
Myelination
production of the myelin sheath • begins the 14th week of fetal development
• proceeds rapidly during infancy
• completed in late adolescence
• dietary fat is important to nervous system development
Myelin sheath gaps / Nodes of Ranvier
Gaps between adjacent Schwann cells
• Sites where axon collaterals can emerge
Myelin composition
40% water
Dry mass: about 80% lipids, about 20% proteins
Typical lipid for myelin are cerebroside and sulfatide
Typical proteins for myelin are myelin basic protein (MBP) and proteolipid protein (PLP)
Other myelin specific proteins are: 2′:3′- cyclic nucleotide 3′- phosphodiesterase (CNP) and myelin-associated glycoprotein (MAG)
Cerebrosides?
(group of glycosphingolipids)
a ceramide with a single sugar residue at the 1-hydroxyl moiety
Sulfatide
sulfolipids, specifically a class of sulfoglycolipids, which are glycolipids that contain a sulfate group
CNS vs PNS myelin protein
CNS myelin is rich in myelin basic protein (MBP) and proteolipid protein (PLP).
PNS: myelin protein 0 and myelin protein 22
Myelin-associated glycoprotein (MAG)
- Single transmembrane domain glycoprotein
- Localized in periaxonal Schwann cell and oligodendrocyte membranes
- Involved in myelination during nerve regeneration in the PNS
- In the CNS MAG is one of three main myelin-associated inhibitors of axonal regeneration after injury
- Anti-MAG antibodies result in complement deposits in the myelin sheath and demyelination (neuropathies).
MS
Multiple sclerosis
• oligodendrocytes and myelin sheaths in the CNS deteriorate
• myelin replaced by hardened scar tissue
• nerve conduction disrupted (double vision, tremors, numbness, speech defects) • onset between 20 and 40 and fatal from 25 to 30 years after diagnosis
• causemaybeautoimmunetriggeredbyvirus
Proteolipid
Protein (PLP)
The major myelin protein in the central nervous system (CNS)
Important for the formation and maintenance of the multilamellar structure of myelin.
Determines membrane spacing
