.png)
NEURON AND ITS CLASSIFICATION |FINDYOURSELF
Classification of neurons:
Neurons are
classified on the basis of:
STRUCTURE:
•
Unipolar
•
Bipolar
•
Multipolar
FUNCTION:
•
Motor
•
Sensory
•
Interneurons
Classification
of nerves
–
Unipolar
–
Bi-polar
–
Multi-polar
Myelination
•
Myelination
is the presence of myelin around the neuron. Myelin is not part
of the structure of the neuron but consists of a thick layer mostly made up of lipids,
present at regular intervals along the length of the axon.
•
Such
fibers are called myelinated fibers.
•
The
water-soluble ions carrying the current across the membrane cannot permeate
this coat, it act as an insulator, just like the white coating of
the electric wires and prevents the leakage of ions from the neuron through its
membrane.
How
does the process of myelination occur?
Myelination
is carried out by myelin-forming cells that wrap themselves around the axons in
jelly-roll fashion. These myelin-forming cells are Schwann cells in
the PNS (peripheral nervous system) and the Oligodendrocytes in
the CNS (brain & the spinal cord)
Myelination
•
Outside CNS
1.
Schwann cells
2.
Neurons
CAN regenerate
3.
Neuron
can recover after injury
•
Inside CNS
1. Oligodendrocytes
2. Neurons CANNOT regenerate
3.
Neurons
DIE after injury
Outside
the CNS: myelinated
fibers
•
Myelination
is not part of the neuron but is done by the schwann cells.
•
As
the diagram shows, the nerve cell invaginates the schwann cell…
•
The
schwann cell wraps around the axon in concentric spirals.
•
Collectively,
the various layers form the myelin sheath (a patch of myelin might be made of
upto 300 layers of wrapped lipid bilayers)
Nodes of Ranvier
•
In
myelinated nerve fiber, the myelin sheath is not a continuous sheath,
but is deficient at regular intervals.
•
Between
the myelinated regions, at the NODES OF RANVIER, the axonal membrane is
bare and exposed to the ECF.
•
Current
can flow across the membrane only at these bare spaces to produce action
potentials.
•
Voltage-gated
Na+ channels are concentrated at these regions.
Fibers
OUTSIDE the CNS
•
Myelinated (WHITE MATTER)
–
Only
single nerve fiber invaginates single cell
–
Concentric
layers of schwann cells wrapped around the fiber
–
No
cytoplasm as all squeezed out- process called myelination
–
Outermost
layer called Neurilemma or sheath of schwann
–
White appearance (white matter)
•
Unmyelinated (GREY MATTER)
–
Small
diameter fibers
–
The
nerve fiber only invaginates
–
No
concentric layers or wrapping
–
A
single schwann cell is invaginated by multiple nerve fibers
–
Nerve
fibers surrounded by Schwann cell cytoplasm
–
Gray appearance
(gray matter)
Nerve
fibers lying WITHIN the CNS
•
Myelinated fibers
–
Myelin
sheath produced by Oligodendrocytes
–
Myelinates
upto 6 nerve fibers at a time.
–
Do
not aid in regeneration
•
Unmyelinated fibers
–
Not
supported by Oligodendrocytes
–
Indirectly
supported by mass of surrounding tissues.
–
Do
not aid in regeneration.
SALTATORY
CONDUCTION
In a
myelinated nerve fiber, the nerve impulse “jumps” from node to node skipping
over the myelinated sections of the axons. This process is called Saltatory
conduction.
Basis: Saltatory conduction propagates nerve impulse more
rapidly because the nerve impulse has to be generated only at the nodes of
ranvier and not repeatedly. Thus, it is faster.
In
unmyelinated fibers, the nerve impulse is like a grasshopper walking while in a
myelinated fiber, the nerve impulse is like grasshopper jumping.
Layers of
nerve fibers
•
Endoneurium:
finely reticular tissue lying just next to neurilemma.
Surrounds
individual fibers separating them from each other.
Forms the
endoneurial tube.
•
Perineurium:
Several nerves surrounded by layer of connective tissue.
•
Epineurium:
Nerve trunk itself surrounded by a loose layer of elastic tissue and CT.
