Spinal Cord Anatomy PDF

Spinal cord I. External features - Site: It occupies the upper 2/3 of the vertebral canal. - Length: It is about 45 cm in length.  - Extent: a- Above: It begins as a continuation of the medulla oblongata. b- Below: the lower end of the cord tapers off into a conical end called conus medullaris at t...

Spinal cord I. External features - Site: It occupies the upper 2/3 of the vertebral canal. - Length: It is about 45 cm in length.  - Extent: a- Above: It begins as a continuation of the medulla oblongata. b- Below: the lower end of the cord tapers off into a conical end called conus medullaris at the level of the disc between L1/L2 vertebrae.  - It ends at the following levels : 1. By the 3rd month of intra-uterine life, it occupies the whole vertebral canal. 2. At birth, it ends at the level of L3 vertebra. 3. In adult, it ends at the level of the disc between L1/L2 vertebrae. These changes are due to differential growth between the spinal cord (slower) and vertebral column (faster). − Shape: The spinal cord is cylindrical and shows two enlargements: a- Cervical enlargement: gives origin to the brachial plexus of the upper limb. b- Lumbar enlargement: gives origin to lumbar and sacral plexuses of the lower limb. External features: ▪ The spinal cord is divided into segments. The external surface of the spinal cord shows 6 longitudinal grooves or sulci: Anterior median fissure or sulcus: is the deepest. Posterior median sulcus. Two anterolateral sulci: for exit of the ventral (motor) roots. Two posterolateral sulci: for entry of the dorsal (sensory) roots. Spinal Segments : ▪ The spinal cord is divided into 31 segments, each segment gives origin to a pair (right and left) of spinal nerves which are (8 cervical, 12 thoracic, 5 lumbar segments, 5 sacral segments and 1 (one) coccygeal). Spinal nerves: ▪ Each spinal nerve is attached to the side of its corresponding spinal segment by two roots: a. Ventral root (motor): It contains (motor) fibers. b. Dorsal root (sensory): It consists of (sensory) fibers. At the corresponding intervertebral foramen, the two roots (ventral and dorsal) unite together forming the spinal nerve trunk (mixed) which is very short and divides immediately after leaving the foramen into two primary rami (anterior and posterior). Both rami are mixed and supply the anterior and posterior aspects of the body respectively. Since the spinal cord is shorter than the vertebral column ; accordingly: - The roots of the spinal nerves have to descend for some distances to reach their intervertebral foramina. Therefore, the roots of the spinal nerves become progressively longer and more oblique as we go downwards. - There is gradual lengthening of the lumbar, sacral and coccygeal nerve roots which have to descend downward to reach their exit foramina as a bundle of fibers called cauda equine. Coverings of the spinal cord (Spinal Meninges):  The spinal cord, is surrounded by three membranes called the meninges; from inside outwards they are: the pia, the arachnoid and the dura maters. 1) Spinal Pia Mater: It is a delicate membrane which is closely adherent to the spinal cord. It extends down to the level of the disc between L1& L2 vertebrae (below). It is thickened to form two fibrous bands: i. Denticulate ligaments: Two lateral extensions which arise from the pia mater, at the side of the spinal cord. The lateral edge of each ligament is serrated and has teeth like processes. ii. Filum terminale: - It is a silvery glistening fibrous band of pia mater which extends downward from the tip of conus medullaris to the back of coccyx. 2) Spinal Arachnoid Mater: - It is a delicate membrane which surrounding the pia and deep to the dura maters. - It ends at the level of the S2. - It is separated from the pia mater by the subarachnoid space which is filled with the CSF. The subarachnoid space contains: Cerebro-spinal fluid (CSF). Spinal blood vessels. Spinal nerve roots surrounding the Filum terminale. Lumbar puncture is procedure done by introducing a needle into the subarachnoid space either to obtain a sample of CSF or to inject a drug. It is safe to be done below of the level (L2) vertebra to avoid injury of the spinal cord. 3) Spinal Dura Mater: − It is thick tough fibrous membrane representing the outermost of the three meninges, extending down to the level of (S2) vertebra. - It is separated from the arachnoid mater by the subdural space, and separated from the bone of the vertebral canal by the extradural (epidural space) which lodges the internal vertebral venous plexus. Fixation of the Spinal Cord: ii. Denticulate ligaments which fixes the spinal cord on both sides. i. Filum terminale which attaches the spinal cord down to the back of the coccyx. II. Arterial Supply of the spinal cord 1. Single Anterior Spinal Artery:  It arises from the vertebral artery.  It descends along the anterior median fissure of the spinal cord.  It gives central branches to supply the anterior 2/3 of the spinal cord. 2. Two Posterior Spinal Arteries:  Each artery arises from the vertebral artery.  Each artery descends downwards along the posterolateral sulcus.  They supply the posterior 1/3 of the spinal cord. Anastomosis between anterior and posterior spinal arteries. 3. Radicular Spinal Arteries: (Transverse arteries)  They enter the vertebral canal through the intervertebral foramina and divide into anterior and posterior branches which run along the ventral and dorsal spinal nerve roots; they are main source of blood supply to the lower 1/3 of the spinal cord.  They end by anastomosing with both anterior and posterior spinal arteries. One of the lower radicular arteries is very large and supplies the lumbar enlargement (the arteria radicularis magna).