Spinal cord structure

Spinal cord lies loosely in the vertebral canal. It extends from foramen magnum where it is continuous with medulla oblongata, above and up to the lower border of first lumbar vertebra below.

Coverings

Spinal cord is covered by sheaths called meninges, which are membranous in nature. Meninges are dura

mater, pia mater and arachnoid mater. These coverings continue as coverings of brain. Meninges are responsible for protection and nourishment of the nervous tissues.

Shape and Length

Spinal cord is cylindrical in shape. Length of the spinal cord is about 45 cm in males and about 43 cm in females.

Enlargements

Spinal cord has two spindle-shaped swellings, namely cervical and lumbar enlargements. These two portions of spinal cord innervate upper and lower extremities respectively.

Conus Medullaris and Filum Terminale

Below the lumbar enlargement, spinal cord rapidly narrows to a cone-shaped termination called conus medullaris. A slender non-nervous filament called filum terminale extends from conus medullaris downward to the fundus of the dural sac at the level of second sacral vertebra.

Segments

Spinal cord is made up of 31 segments. In fact, spinal cord is a continuous structure. Appearance of the segment is by nerves arising from spinal cord, which are called spinal nerve.

Spinal Nerves

Segments of spinal cord correspond to 31 pairs of spinal nerves in a symmetrical manne

Nerve Roots

Each spinal nerve is formed by an anterior (ventral) root and a posterior (dorsal) root. Both the rootsr either side leave the spinal cord and pass through the corresponding intervertebral foramina. The

first cervical spinal nerves pass through a foramen between occipital bone and first vertebra, which is

called atlas. Cervical and thoracic roots are shorter whereas, the lumbar and sacral roots are longer. Long nerves descend in dural sac to reach their respective intervertebral foramina. This bundle of descending roots surrounding the filum terminale resembles the tail of horse. Hence, it is called cauda equina.

Fissure and Sulci

On the anterior surface of spinal cord, there is a deep furrow known as anterior median fissure. Depth of this fissure is about 3 mm. Lateral to the anterior median fissure on either side, there is a slight depression called the anterolateral sulcus. It denotes the exit of anterior nerve root. On the posterior aspect, there is a depression called posterior median sulcus. This sulcus is continuous with a thin glial partition called the posterior median septum. It extends inside the spinal cord for about 5 mm and reaches the gray matter. On either side, lateral to posterior median sulcus, there is posterior intermediate sulcus. It is continuous with posterior intermediate septum, which extends for about 3 mm into the spinal cord. Lateral to the posterior intermediate sulcus, is the posterolateral sulcus. This denotes the entry of posterior nerve root.