The spinal cord is the lower elongated part of the central nervous system (CNS). It is cylindrical in shape, slightly flattened anteroposteriorly and occupies the upper two-third of the vertebral canal.
Spinal cord measures about 45 cm in adult male and 42 cm in adult female, and weighs about 30 g. It extends as a downward continuation of medulla oblongata from the upper border of the posterior arch of first cervical vertebra (C1) to the lower border of the first lumbar vertebra (L1). Its lower tapering extremity is called conus medullaris. The apex of conus medullaris continues downwards as a thin, thread-like filament called filum terminale. Spinal cord provides attachment to 31 pairs of spinal nerves which connect it to the tissues of the trunk, girdles, limbs, and the viscera.
The spinal cord contains large number of ascending and descending pathways, which serve as conduits for nervous information, passing to and fro between different parts of the body and the brain.
The spinal cord is surrounded by three protective membranes called spinal meninges. From outside inwards these are: dura mater, arachnoid mater, and pia mater.
The spinal dura extends from foramen magnum to the lower border of second sacral vertebra (S2). The space between spinal dura and vertebral canal is termed epidural space. This space contains loose areolar tissue, semiliquid fat and internal vertebral venous plexus. The space between dura and arachnoid is termed subdural space. It contains capillary layer of fluid. The spinal dura differs from cranial dura. Arachnoid mater is a thin delicate transparent avascular membrane which invests the spinal cord loosely. Above it is continuous with the arachnoid mater of the brain and below it extends up to the lower border of the second sacral vertebra (S2). Pia mater is a thin highly vascular membrane that closely invests the spinal cord and continues below the spinal cord as a thin thread-like prolongation, the filum terminale.
The subarachnoid space between the pia mater and the arachnoid mater is filled with cerebrospinal fluid (CSF). Above it is continuous with subarachnoid space around the brain. Distal to the termination of spinal cord, the subarachnoid space around the filum terminale, becomes roomy, forming a pool of CSF called lumbar cistern. The lumbar puncture is done at this site to take out the CSF.
Up to the 3rd month of intrauterine development, the spinal cord extends through the entire length of the vertebral canal and the spinal nerves pass through the intervertebral foramina at their level of origin. Thereafter, the vertebral column grows faster than the spinal cord and the terminal end of the spinal cord gradually shifts to a higher level. Consequently at birth spinal cord ends at the level of L3 vertebra while in adult, it terminates at the level of lower border of L1 (or the intervertebral disc between the L1 and L2). The knowledge of these variations in the vertebral level of the lower end of the cord is important to avoid injury to the cord while performing lumbar puncture, especially in children.
The spinal cord is roughly cylindrical in shape with an average diameter of about 1.25 cm. However, opposite to the attachments of the nerve roots contributing to the formation of brachial and lumbosacral plexuses, the spinal cord presents definite fusiform swellings called cervical and lumbar enlargements respectively. These enlargements are produced due to the presence of large number of large motor neurons in these regions to supply the musculature of the upper and lower limbs and associated girdles.
The cervical enlargement extends from C5 to T1 spinal segments whereas lumbar enlargement extends from L2 to S3 spinal segments. The vertebral levels of these enlargements are quite different from those of spinal segments, viz. the cervical enlargement lies opposite C3 to T1 vertebrae and lumbar enlargement lies opposite T9 to L1 vertebra.
Like the vertebral column, the spinal cord is also segmented though the segments are not visible externally. The part of spinal cord to which a pair of spinal nerves (right and left) is attached is known as spinal segment. The number of spinal segments corresponds to the number of vertebrae in thoracic, lumbar and sacral regions, but in cervical region, one segment is more than the number of vertebrae, whereas in coccygeal region there is only one segment for four coccygeal vertebrae. Thus, the spinal cord is made up of 31 spinal segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and one coccygeal. Since, the length of spinal cord (45 cm) is smaller than the length of vertebral column (65 cm), the spinal segments are, short and crowded, especially in the lower part of the cord. Thus, the spinal and vertebral segments (spines) do not lie at the same level. The spinal segments as a rule always lie above their numerically corresponding vertebral spines. In the lower part of the spinal cord, they lie well above their corresponding vertebrae. For example, the lumbar segments are related to the thoracic vertebrae. These levels, although not strictly accurate, give a reliable assessment of the level of cord compression following an injury or disease of the surrounding vertebrae.
In cross-section of cord the grey matter is seen as H-shaped (or butterfly-shaped) fluted column, extending throughout the length of the spinal cord. It is divided into symmetrical right and left comma shaped masses which are connected across the midline by a transverse grey commissure. The central canal of the cord passes through the centre of grey commissure. The canal is surrounded by substantia gelatinosa centralis. The lateral comma shaped mass of grey matter is further divided by a transverse grey commissure into a narrow elongated posterior horn, and a broad anterior horn.
The white matter of the spinal cord surrounds the central 'H-shaped mass of grey matter, and mainly consists of ne fibres, the large proportion of them being myelinated, giving it a white appearance.
The anterior spinal artery is formed by the union of two small spinal branches of the right and left vertebral arteries in the upper cervical canal. It runs caudally in the anterior median fissure of the spinal cord and terminates along the filum terminale.
There are two posterior spinal arteries each arising as a small branch from either the vertebral or posterior inferior cerebellar artery. Each posterior spinal artery runs down on the posterolateral aspect of the cord in the posterolateral sulcus along the line of attachment of posterior nerve roots and usually divides into two collateral arteries along the medial and lateral sides of the posterior nerve roots. Thus, there are five longitudinal arteries around the spinal cord.
These arteries are reinforced by the segmental arteries to form 5 longitudinal arterial trunks. These arterial trunks communicate around the cord forming a pial plexus, the arterial vasocorona/arteriae coronae. The arteriae coronae give peripheral branches which supply the superficial regions of the cord. The anterior spinal artery supplies the anterior two-third of the cord, while two posterior spinal arteries together supply the posterior one-third of the cord.