WHITE MATTER OF SPINAL CORD
White
matter of spinal cord surrounds the gray matter. It is formed by the
bundles of both myelinated and nonmyelinated fibers,
but predominantly the myelinated fibers. Anterior median fissure and posterior
median septum divide the entire mass of white matter into two lateral halves. The band of white matter lying in front of
anterior gray commissure is called anterior white commissure.
Each half of the white
matter is divided by the fibers of anterior and posterior
nerve roots into three
white columns or funiculi:
I. Anterior or
Ventral White Column
Ventral white column lies
between the anterior median fissure on one side and
anterior nerve root and anterior gray horn
on the other side. It is also called anterior or ventral
funiculus.
II. Lateral White
Column
Lateral
white column is present between the anterior nerve root and anterior
gray horn on one side and
posterior nerve root and posterior
gray horn on the other side. It is also called lateral
funiculus.
III. Posterior or
Dorsal White Column
Dorsal white column is
situated between the posterior nerve root and posterior gray horn on one side
and
posterior
median septum on the other side. It is also called posterior
or dorsal funiculus.
TRACTS IN SPINAL CORD
Groups of
nerve fibers passing through spinal cord are known as tracts of the spinal
cord. The spinal tracts are divided into two main
groups. They are:
1. Short
tracts
2. Long
tracts.
1. Short Tracts
Fibers of the short tracts
connect different parts of spinal cord itself.
Short tracts are of two
types:
i. Association
or intrinsic tracts, which connect adjacent segments of spinal cord on the same side
ii. Commissural
tracts, which connect opposite halves of same segment
of spinal cord.
2. Long Tracts
Long
tracts of spinal cord, which are also called projection tracts, connect
the spinal cord with other
parts of
central nervous system. Long tracts are of two types:
i.
Ascending tracts, which carry sensory impulses from the spinal cord to
brain
ii.
Descending tracts, which carry motor impulses from brain to the spinal cord.
ASCENDING TRACTS OF SPINAL CORD
Ascending tracts of spinal
cord carry the impulses of various sensations to the
brain.
Pathway
for each sensation is formed by two or three groups of neurons, which
are:
1. First
order neurons
2. Second
order neurons
3. Third
order neurons.
First Order Neurons
First order neurons receive sensory impulses from the receptors and send them to
sensory neurons present in the posterior gray horn of spinal cord through their
fibers. Nerve cell bodies of these neurons are located in the posterior
nerve root ganglion.
Second Order Neurons
Second order neurons are
the sensory neurons present in the posterior gray horn.
Fibers from these
neurons
form the ascending tracts of spinal cord. These fibers carry sensory impulses
from spinal
cord to different brain
areas below cerebral cortex (subcortical areas) such as thalamus.
All the
ascending tracts are formed by fibers of second order neurons of the
sensory pathways except
the ascending tracts in
the posterior white funiculus, which are formed by the
fibers of first order neurons.
Third Order Neurons
Third order neurons are in
the subcortical areas. Fibers of these neurons carry the
sensory impulses from subcortical areas to cerebral cortex.
ANTERIOR
SPINOTHALAMIC TRACT
Anterior
spinothalamic tract is formed by the fibers of second order neurons of
the pathway for crude touch sensation.
Situation
Anterior spinothalamic
tract is situated in anterior white funiculus near the
periphery.
Origin
Fibers of anterior spinothalamic
tract arise from the neurons of chief
sensory nucleus of posterior gray
horn, which form the second
order neurons of the crude touch pathway. First order
neurons are situated in the posterior nerve
root ganglia. These neurons receive the impulses of crude touch
sensation from the pressure receptors. Axons of the first
order neurons reach the chief sensory nucleus through the
posterior nerve root.
Course
Anterior spinothalamic
tract contains crossed fibers. After taking origin, these fibers cross obliquely in the
anterior white commissure
and enter the anterior white column of opposite side. Here,
the fibers ascend
through other segments of
spinal cord and brainstem (medulla, pons and midbrain)
and reach thalamus.
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