Spinal Cord Anatomy and Function

Spinal Cord

• The spinal cord is a tubular bundle of nervous tissue and supporting cells.
• It extends from the brainstem to the lumbar vertebrae.
• Together, the spinal cord and brain form the central nervous system.
• It is a cylindrical structure, greyish-white in color.

Spinal Cord Enlargements

• There are two points of enlargement along the spinal cord: cervical and lumbar.
• The cervical enlargement extends from C4 to T1 segments. Most anterior rami of spinal nerves arising from this area form the brachial plexus, which innervates the upper limbs.
• The lumbosacral (lumbar) enlargement extends from L1 to S3 segments. Anterior rami of these nerves form the lumbar and sacral plexuses, innervating the lower limbs.

Cauda Equina

• At the L2 vertebral level, the spinal cord tapers off, forming the conus medullaris.
• The spinal nerves arising from the cord's lower end form the cauda equina.
• This structure occupies about two-thirds of the vertebral canal.

Spinal Meninges

• The spinal cord is surrounded by three membranes called meninges: dura mater, arachnoid mater, and pia mater.
• These meninges contain cerebrospinal fluid, acting as support and protection to the spinal cord.
• The dura mater is the outermost layer, forming a dural sac.
• It extends from the foramen magnum to the second sacral vertebra.
• The arachnoid mater is between the dura and pia mater. The space between them is called the subarachnoid space, which contains cerebrospinal fluid.
• The pia mater is the innermost layer directly covering the spinal cord.
• Distally, the meninges attach to the coccyx via the filum terminale.

Epidural Space

• The epidural space lies between the inner walls of the vertebral canal and dura mater.
• It contains fat and the internal vertebral venous plexus.
• The venous plexus extends throughout the length of the epidural space and connects to the dural sinuses in the cranial cavity.

Subarachnoid Space

• Located between the arachnoid mater and pia mater.
• Contains cerebrospinal fluid (CSF).
• Distal to the conus medullaris, the subarachnoid space forms the lumbar cistern, used for lumbar puncture.
• The lumbar cistern extends from the L2-L3 vertebral level to the S2 vertebral level.

Spinal Nerves

• 31 pairs of spinal nerves attach to the spinal cord.
• These nerves connect the CNS with different parts of the body.
• Each spinal nerve is composed of a posterior (sensory) and an anterior (motor) root.
• These roots unite at the intervertebral foramina to form a single spinal nerve.
• Spinal nerve roots consist of motor fibers connecting to skeletal muscle and many presynaptic autonomic fibers, whose cell bodies lie in the anterior horns of gray matter.
• Posterior roots contain axons from sensory neurones whose cell bodies reside in the posterior root ganglia, outside of the spinal cord.
• Dorsal rami innervate parts of the back and neck.
• Ventral rami innervate the skin of the anterolateral trunk and limbs and the skeletal muscles in this area and form the brachial and lumbosacral plexuses.

Lumbar Puncture

• Used to inject anesthetic or withdraw CSF from the subarachnoid space.
• Typically performed at the L4-L5 interspace, marked by a horizontal line drawn at the top of the iliac crest.

Epidural Anesthesia

• Anesthetic agent injection into the epidural space.
• Anesthetizes spinal nerve roots of the cauda equina.

Spinal Cord Blood Supply

• The spinal cord is supplied by arteries (three major arteries: Anterior spinal artery, Posterior spinal arteries).
• The anterior spinal artery branches off vertebral arteries, carrying blood to the anterior median fissure of the spinal cord.
• The posterior spinal arteries arise from the vertebral artery or the posteroinferior cerebellar artery, anastomosing to one another within the pia mater.
• Additional supply is via the anterior and posterior segmental medullary arteries that originate from the spinal nerve roots.

Spinal Cord Venous Drainage

• Venous drainage occurs via three anterior and three posterior spinal veins.
• They form an anastomosing network and drain into the internal and external vertebral plexuses.
• These plexuses empty into the segmental veins or dural venous sinuses.