WHITE MATTER OF SPINAL CORD

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.