The spinal cord is supplied by multiple radicular arteries, which form the anterior spinal artery (ASA) and the two posterior spinal arteries (PSAs). The radicular arteries arise from neighbouring arteries at the level of each vertebral segment and accompany the nerve roots via the intervertebral foramina. The smaller sized arteries (which comprise the majority) do not reach the spinal cord, and are therefore principally concerned with supplying the exiting nerve roots; while the larger sized ones (named the radiculomedullary arteries), supply not only the exiting nerve roots but also the spinal cord (and the covering meninges). The anterior radicular arteries (ARAs) and posterior radicular arteries (PRAs), are the radiculomedullary branches that contribute to the formation of the anterior spinal artery and the two paired posterior spinal arteries.
The ASA, coursing in the midline, usually originates in the upper cervical region, at the junction of the two descending anterior spinal branches of the vertebral arteries, and recieves contributions from six to ten ARAs throughout its length. The paired posterolaterally located PSAs arise as small branches of either the vertebral or the posterior inferior cerebellar arteries. Each PSA recieves contributions from ten to 23 PRAs. The ASA distributes blood to the anterior two thirds of the spinal cord, while each PSA distributes blood to the ipsilateral posterior one third of the cord.
Three areas of the spinal cord differ regarding the origin and quality of the blood supply of the ASA. These areas are as follows:
- The cervico-thoracic vascular area, comprising the cervical and the first two or three thoracic segments of the spinal cord, is supplied by the descending anterior spinal branches of the vertebral arteries in its proximal portion, and by two to four ARAs arising from the vertebral, deep cervical, and/or superior intercostal arteries and very rarely from the ascending cervical artery, in its distal part.
- The midthoracic region (extending from T3-T4 to the T7 spinal segments), usually recieves only one radicular artery (variably cited as accompanying the T4, T5 or T7 nerve roots).
- The thoracolumbosacral part of the cord (T8 to the conus medullaris), derives its main arterial supply from the artery of Adamkiewicz (AAD), which is also known as arteria radicularis magna.
The artery of Adamkiewicz may arise at any level between the 5th thoracic and the 4th lumbar arteries on either side, although it has a left-sided origin in 80% of individuals. Its most common site of origin has been cited as the left 10th intercostal artery. When its origin is relatively high or low, there is usually a supplementary supplying vessel to the ASA. The AAD has a large anterior and a smaller posterior radicular branch. On reaching the anterior aspect of the cord, the anterior radicular branch ascends a short distance and then makes a hairpin turn to give off a small ascending and a larger descending branch (in other words the terminal portion of the ASA), which drops to the level of the conus medullaris, where it forms an anastomotic circle with the terminal branches of the two PSAs. The cauda equina is accompanied and supplied by branches from the lumbar, iliolumbar, and lateral and median sacral arteries; these branches also contribute to the anastomotic arterial circle around the conus medullaris. This anastomotic circle, is potentially capable of supplying the lower cord if the AAD is occluded. The most important watershed area of the spinal cord is located at the periphery of the AAD.