Internal Structure of Spinal Cord
Neural substance of spinal
cord is divided into inner gray matter and outer white
matter.
GRAY MATTER OF SPINAL CORD
Gray matter of spinal cord
is the collection of nerve cell bodies, dendrites and
parts of axons. It is placed
centrally in the form of wings of the butterfly and it resembles the letter ‘H’. Exactly in the center of gray
matter, there is a canal
called the spinal canal. Ventral and the dorsal portions of each
lateral half
of gray
matter are called ventral (anterior) and dorsal (posterior) gray horns respectively.
In addition, the gray matter forms a small projection in between the anterior and posterior horns in all thoracic and first two lumbar segments.
It is called the lateral gray horn. Part of the gray matter anterior to
central canal is called the anterior gray commissure and part of gray matter posterior
to the central canal is called posterior
gray commissure.
Neurons in Gray
Matter of Spinal Cord
Gray matter contains two
types of multipolar neurons:
1. Golgi
type I neurons
Golgi type I neurons have long axons
and are usually found in anterior horns.
Axons of these neurons form the long tracts
of spinal cord. Golgi type II neurons Golgi type II neurons have short
axons, which are found mostly in posterior horns.
Axons of these neurons pass towards the
anterior horn of same side or opposite side.
Organization of
Neurons in Gray Matter
Organization of neurons in the gray
matter of spinal cord is described in two methods:
1. Nuclei
or columns
2. Laminae
or layers
NUCLEI
Clusters
of neurons are present in the form of nuclei or cell columns in gray matter. Advantage
of this method is that different nuclei are easily distinguished. Disadvantage is that
some neurons like internuncial neurons, which are
outside the distinct nuclei are not included.
Nuclei in Posterior
Gray Horn
Posterior
gray horn contains the nuclei of sensory neurons, which receive impulses from
various receptors of the body through posterior nerve
root fibers. There are four types of nuclei of sensory neurons:
1. Marginal
nucleus
Marginal
nucleus is also called posteromarginal nucleus, marginal zone nucleus or border
nucleus. It
covers the very tip of
posterior gray horn and it is found in all levels of
spinal cord.
2. Substantia
gelatinosa of Rolando
Substantia gelatinosa of Rolando
is a cap-like gelatinous material at the apex of posterior horn situated in all
levels of spinal cord. It is formed by small neurons.
3. Chief
sensory nucleus or nucleus proprius
Chief sensory
nucleus is situated in the posterior gray horn ventral to substantia
gelatinosa. It is a poorly
defined
cell column located in all segments of spinal cord.
4. Dorsal
nucleus of Clarke
Clarke
nucleus is also called Clarke column of cells and it is the collection of well-defined neurons. It occupies the basal portion of posterior horn. This nucleus is found in spinal segments between C8 and L3 only.
Nuclei in Lateral
Gray Horn
Lateral
gray horn has cluster of neurons called intermediolateral nucleus. The neurons
of this nucleus
give rise
to sympathetic preganglionic fibers, which leave the spinal cord through the
anterior nerve root.
Inter m
ediolateral nucleus extends between T1 and L2 segments of spinal cord.
Nuclei in Anterior
Gray Horn
Anterior gray horn contains
the nuclei of lower motor neurons, which are involved
in motor function. These nuclei are present in almost all the
levels of spinal cord.
Three types of motor
neurons are present in lower motor neuron nuclei:
1. Alpha
motor neurons
Alpha
motor neurons are large and multipolar cells. Axons of these neurons
leave the spinal cord through
the anterior root and end
in groups of skeletal muscle fibers called extrafusal fibers.
2. Gamma
motor neurons
Gamma motor neurons are
smaller cells scattered among alpha motor neurons. These
neurons send
axons to intrafusal fibers of the muscle
spindle.
3. Renshaw
cells
These
cells are also smaller in size. Renshaw cells are the inhibitory neurons, which play an important role in synaptic inhibition at the spinal cord.
LAMINAE
Neurons of gray matter are
distributed in laminae or layers. Each lamina consists
of neurons of different
size and
shape. This cytoarchitectural lamination was identified in 1950 by Brian
Burke and Rexed.
He
classified the neurons in 10 laminae based on his observation on sections of
brain in a neonatal cat.
Laminae
are also called Rexed laminae. Advantage of this method is that
all the
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