Internal Structure of Spinal Cord

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