External Features of Spinal Cord by Dr. Rajitha Vanga | NEET PG | USMLE | FMGE | Neuroanatomy
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Spinal cord is the terminal part of hind brain (myelencephalon)and is the caudal continuation of brainstem (medulla) below foramen magnum.
Location: Upper 2/3rd vertebral canal
Extent
• Adults - from upper border of C1 vertebra to lower border of L1 vertebra. where it terminates as the conus medullaris (medullary cone). A thin thread called filum terminale extends from the tip of the conus medullaris all the way to the 1st coccygeal vertebra (Co1) and anchors the spinal cord in place.
• At birth - from upper border of C1- lower border of L3 vertebra and it reaches to L1 at 2 years after birth.
Coverings
Duramater
• Extends from foramen magnum to S2 vertebra beyond this level it blends with the periosteum as a thin layer surrounding the filum terminale where it terminates by attaching to the dorsal surface of coccyx.
• The space between the inner surface of vertebral canal and duramater is known as Epidural space. It is closed above by fusion of the spinal dura with the edge of the foramen magnum, and below by the posterior sacrococcygeal ligament that closes the sacral hiatus.
• Sub-dural space is the space between duramater and arachnoid mater and it ends at the lower border of S2.
Arachnoid mater
• Is a thin avascular membrane which is continuous above with the arachnoid mater of brain and below it extends till the level of S2 vertebra.
• Sub arachnoid space/leptomeningeal space is the space between arachnoid mater and piamater contain CSF, blood vessels supplying spinal cord and arachnoid trabeculae.
Piamater
• Is a thin & highly vascular membrane closely investing the spinal cord and it continuous below as filum terminale.
Processes or modifications of piamater
• Filum terminale- Thin glistening white thread like structure extends from tip of conus medullaris to dorsal surface of first coccygeal vertebra. Can be differentiated into of filum terminale internum (15cm) present within dural sac and filum terminale externum (5cm) present outside Dural sac.
• Subarachnoid septum- Is the pial septum which connects the dorsal surface of spinal cord to arachnoid mater
• Ligamentum denticulatum/Denticulate ligament- is the transparent ribbon like thickened bands between motor and sensory roots. They are tooth like projections which pierce the arachnoid mater to attach the inner surface of duramater and present on either side of the spinal cord. There are 21 pairs where the first pair is at foramen magnum and last pair is fork like, it is between T12 and L1 spinal nerves.
Importance
Anchors spinal cord in the middle of subarachnoid space
Helpful for surgeons during cordotomy (section of sensory nerves to relieve pain) These tooth-like projections separate sensory roots from motor roots.
• Linea splendens thickening of piamater in anterior median fissure.
External features
Fissures and sulci
• Anterior median fissure contains anterior spinal artery.
• Posterior median sulcus contains a septum of neuroglial tissue extends into spinal cord known as posterior median septum.
• Two anterolateral sulci from where the motor nerve roots emerge from spinal cord
• Two posterolateral sulci from where the sensory nerve roots enter into spinal cord
Spinal nerves- There are 31 pairs of spinal nerves
• Cervical- 8
• Thoracic-12
• Lumbar- 5
• Sacral- 5
• Coccygeal-1
Each spinal nerve has an anterior and posterior root. Anterior roots transmit motor information, and they originate from the anterior horns of the gray matter and exit the spinal cord through the anterolateral sulcus. The posterior roots transmit sensory information and have sensory ganglion attached to them. They originate from the posterior horns of gray matter and exit through the posterolateral sulcus of the spinal cord.
The anterior and posterior roots merge just before the intervertebral foramen and form the trunk of the spinal nerve. The trunk is very short, and soon after exiting the vertebral column, it divides into four branches: anterior ramus, posterior ramus, communicating ramus, and meningeal ramus.
Spinal segments
• Is the part of spinal cord from which a pair of spinal nerves are originated.
• Spinal segments and vertebra level do not lie at the same level.
Enlargements
• Cervical enlargement- extends from C3 to T2 spinal segments supplies shoulder girdle and upper limb forming brachial plexus. It has the maximum diameter is at C5 level.
• Lumbar enlargement- extends from L2 to S3 – supplies pelvic girdle lower limb forming lumbosacral plexus. It has the maximum diameter is at lower part of T12 level.
Cauda equina (Resembles as tail of horse)
• lumbosacral spinal nerves form a bundle called the cauda equina (meaning horse’s tail).
• Consists of roots of pairs of spinal nerves from L2 to L5, S1 to S5 and CO1 nerves