Myelination…
Oligodendrocytes and Schwann Cells
Speed of Action Potential Conduction…
Ranges from 1 to >100 m/s
increased by:
a) larger size - giant axons – invertebrates (do not have axon
insulation in the form of myelin sheath)
b) increased body temperature faster diffusion
c) fatty insulation - myelin sheaths – vertebrates
Myelin…
Specialised membrane
sheath surrounding
(most) vertebrate
axons.
Insulates axons from
each other (electrical
wiring)
Speeds conduction of nervous impulse (saltatory conduction
between nodes).
Folding structure wrapped around axons at various points which is
generated by different types of glial cells that contact the axon and
produce the sheath.
Breaks between myelinated areas are called Nodes of Ranvier –
having these means that action potential is able to jump along the
axon rather than having to move gradually, speeding up the rate of
conduction.
Difference between PNS and CNS:
In CNS
with
oligodendrocytes, myelination
occurs on multiple different
axons.
Schwann cell of PNS only myelinates a single axon at a single place.
, Myelin Sheath is composed of many loops of a Glial Process…
Each oligodendrocyte has several processes, each of which
produces a myelin sheath on a different axon
Schwann cells each form only a single myelin sheath
Myelin sheath has a large surface area but forms a tightly
compacted structure wrapped around the axon, which is important
in its ability to insulate the axon.
Schwann cells & Oligodendrocytes…
These cells have different origins.
Schwann cells:
Derived from neural crest (small contribution from ventral neural
tube).
Migrate and differentiate under control of peripheral axons.
Oligodendrocytes:
One of the two major classes of CNS glia (cf. astrocytes – common
origin).
Derived from progenitors that reside in ventricular zone of neural
tube.
Astrocytes have lots of roles too - calcium homeostasis, signalling to
neurones and other glia, formation of blood-brain barrier, sink for
neurotransmitters and free ions, control of entry of inflammatory cells,
etc… but not dealt with here.
Embryonic development summary:
Mechanism of neural tube
and crest formation
Schwann cells will arise from
the neural crest,
oligodendrocytes from the
neural tube
The Oligodendrocyte precursor
cell…
Oligodendrocytes and Schwann Cells
Speed of Action Potential Conduction…
Ranges from 1 to >100 m/s
increased by:
a) larger size - giant axons – invertebrates (do not have axon
insulation in the form of myelin sheath)
b) increased body temperature faster diffusion
c) fatty insulation - myelin sheaths – vertebrates
Myelin…
Specialised membrane
sheath surrounding
(most) vertebrate
axons.
Insulates axons from
each other (electrical
wiring)
Speeds conduction of nervous impulse (saltatory conduction
between nodes).
Folding structure wrapped around axons at various points which is
generated by different types of glial cells that contact the axon and
produce the sheath.
Breaks between myelinated areas are called Nodes of Ranvier –
having these means that action potential is able to jump along the
axon rather than having to move gradually, speeding up the rate of
conduction.
Difference between PNS and CNS:
In CNS
with
oligodendrocytes, myelination
occurs on multiple different
axons.
Schwann cell of PNS only myelinates a single axon at a single place.
, Myelin Sheath is composed of many loops of a Glial Process…
Each oligodendrocyte has several processes, each of which
produces a myelin sheath on a different axon
Schwann cells each form only a single myelin sheath
Myelin sheath has a large surface area but forms a tightly
compacted structure wrapped around the axon, which is important
in its ability to insulate the axon.
Schwann cells & Oligodendrocytes…
These cells have different origins.
Schwann cells:
Derived from neural crest (small contribution from ventral neural
tube).
Migrate and differentiate under control of peripheral axons.
Oligodendrocytes:
One of the two major classes of CNS glia (cf. astrocytes – common
origin).
Derived from progenitors that reside in ventricular zone of neural
tube.
Astrocytes have lots of roles too - calcium homeostasis, signalling to
neurones and other glia, formation of blood-brain barrier, sink for
neurotransmitters and free ions, control of entry of inflammatory cells,
etc… but not dealt with here.
Embryonic development summary:
Mechanism of neural tube
and crest formation
Schwann cells will arise from
the neural crest,
oligodendrocytes from the
neural tube
The Oligodendrocyte precursor
cell…
