Internal structures of spinal cord

Substance of spinal cord is divided into inner gray matter and outer white matter. Gray matter 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. White matter is the collection of myelinated and non-myelinated nerve fibers (Fig. 18).

GRAY MATTER OF SPINAL CORD

Ventral and the dorsal portions of each lateral half of gray matter are called ventral (anterior) and dorsal (posterior) gray horns respectively. Lateral horns of gray matter are present in the thoracic segments and first two lumbar segments only. The part of the gray matter anterior to central canal is called the anterior gray commissure and the part of gray matter posterior to the central canal is called the posterior gray commissure.

Nerve cells present in the gray matter are multipolar type. Golgi I type cells with long axons are usually found in anterior horns and Golgi II type cells with short axons are found mostly in posterior horns. Axons of Golgi I type cells form the tracts of spinal cord and short axons from type II cells pass towards the anterior horn of same side or opposite side.

Neurons in Gray Matter of Spinal Cord

In the gray matter of spinal cord, the neurons are arranged in different groups. Following are the important neurons in the gray matter (Fig.19).

Neurons in Anterior Gray Horn

The neurons of the anterior gray horn are involved in motor function and send motor nerve fibers to muscles and other effector organs.

Fig.18.Section of spinal cord — thoracic segment.

Fig.19.Neurons in gray horn of spinal cord — thoracic segment.

Three types of motorneurons are located in anterior gray horn.

Alpha motorneurons: 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, i.e. extrafusal fibers.

Gamma motor neurons: Gamma motor neurons are smaller cells scattered among alpha motor neurons. These neurons send axons to the intrafusal fibers of the muscle spindle.

Renshaw cells: These cells are also smaller cells. Renshaw cells are the inhibitory neurons playing an important role in synaptic inhibition at the spinal cord.

Neurons in Lateral Gray Horn

in thoracic and upper two lumbar segments, the gray matter forms a small projection in between the anterior and posterior horns. This is called the lateral gray horn. This has cluster of nerve cells called intermediolateral horn cells. These cells give rise to sympathetic preganglionic fibers, which leave the spinal cord through the anterior nerve root.

Neurons in Posterior Gray Horn

The posterior gray horn contains the sensory neurons, which receive impulses from various receptors of the body through posterior nerve root fibers. Four groups of neurons are present in the posterior gray horn.

Substantia gelatinosa of Rolando: This is a cap like gelatinous material at the apex of posterior horn. This is formed by smaller nerve cells.

Marginal cells: Marginal cells cover the substantia gelatinosa at the very tip of the posterior gray horn. Sometimes the marginal cells are also called the border cells.

Chief sensory cells: The chief sensory neurons are situated in the remaining parts of posterior column.

Clarke's column of cells: The Clarke's column of cells is the collection of well-defined cells and occupies the inner part of posterior horn.

WHITE MATTER OF SPINAL CORD

White matter of spinal cord surrounds the gray matter. It is formed by the bundles of both myelinated and non-myelinated fibers, but predominantly the myelinated fibers. The anterior median fissure and the 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 the 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.

1. Anterior or Ventral White Funiculus

This lies between the anterior median fissure on one side and anterior nerve root and anterior gray horn on the other side.

2. Lateral White Funiculus

This 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.

3. Posterior or Dorsal White Funiculus

It is situated between the posterior nerve root and posterior gray horn on one side and posterior median septum on the other side.