Axon
SOURCE:
www.Mult-Sclerosis.org
The axon is a
long filament that extends from the cell body (the soma) in
nerve
cells (neurons). See the simplified diagram of a neuron
below.
The role of the axon is to carry nerve impulses away from
the soma to the
presynaptic terminals where the impulses are transmitted to other neurons or to muscles in the case of motor neurons.
Axons can be very long, in some cases over a meter and carry
impulses at a rate of anything up to 100 meters per second or
greater. The greater the diameter of the axon, the faster the
nerve impulses will travel along it. Many axons are clearly
visible to the naked eye.
Axons are encased in a semi-permeable membrane (the
phospholipid bilayer) which allows certain particles to pass
through it but restricts others. In relation to the
transmission of nerve impulses, this membrane selectively
restricts the passage of charged particles (ions).
The membrane contains special "gates" or "ion channels" that,
when they are open, selectively let positively charged ions
pass through. The two principle gates are the
sodium and
potassium channels. At rest, mechanisms in the neuron pump
sodium ions out of the cell to create an
electric
potential across the membrane. This is called the
resting potential. When the nerve is excited, a explosive
wave of depolarising current called an
action potential moves along the entire nerve through the
length of axon and out into the
presynaptic terminals. This is
achieved by allowing the sodium ions to flood back through the
sodium channels. This mechanism is discussed more fully in the
section on the
action potential.
Axons are sheathed in a smooth fatty protein called
myelin
which insulates the axon, prevents the wrong ion channels from
opening and considerably increases the speed that nerve
impulses travel along the axon. Without the myelin, the axons
would have to be about one hundred times their volume to
achieve the same speed of nerve transmissions. The myelin is
wrapped around the axon in many thin layers. The myelin does
not enclose the axon in one entire sheath, but has gaps at
intervals called the
nodes of Ranvier (The nodes of Ranvier are regular breaks in
the myelin sheath that surrounds the axonal extension of nerve
cells). The precise function of these nodes is
unknown but the nodes are major sites of sodium channels and
may serve to prevent the decay of nerve impulses by
effectively amplifying them. They may also act to anchor the
myelin sheath to the axon and to isolate each segment of
myelin from its neighbors. Work on rats with genetic
deformities in their nodes of Ranvier has shown that these
nodes are vital to efficient transmission of nerve impulses.
How the myelin sheath works is discussed more fully in the
section on
myelin.
In multiple sclerosis, the myelin sheath is stripped off
from the neuron which considerably reduces the speed of
conduction of nerve transmissions. This process is known as
demyelination. The effects of this is to considerably slow
down the speed of nerve transmissions along the demyelinated
axons. It is known that when demyelinated axons are excited in
the middle of their length, the action potential moves away
from the point of excitement in both directions. When two
neighboring axons are demyelinated the action potential can
"jump" from one axon into the other and generate all kinds of
havoc. In the phase of the disease known as
Relapsing/Remitting MS, the myelin is often laid down
again in a repair process known as
remyelination which can often restore near normal
functioning to the axon. However, and particularly in
progressive stages of the disease, the myelin can be replaced
with scar tissue or the axonal body itself can be damaged.
Functionally, neurons are classified as
afferent,
efferent, or interneurons (association neurons) according to the
direction in which they transmit impulses relative to the central nervous
system. Afferent, or sensory, neurons carry impulses from peripheral sense
receptors to the
central nervous system (CNS). They usually have long dendrites and relatively
short axons. Efferent, or motor, neurons transmit impulses from the CNS to
effector organs such as muscles and glands. Efferent neurons usually have
short
dendrites and long axons. Interneurons, or association neurons, are
located entirely within the CNS in which they form the connecting link
between the afferent and efferent neurons. They have short dendrites and
may have either a short or long axon.
Axon links:
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