1. Definition
The spinal cord is part of the central nervous system (CNS). It lies within the spinal canal and serves as the connection between the brain and most peripheral nerves. The spinal cord contains nerve cell bodies, glial cells (gray matter), and their axons (white matter). Its primary function is to transmit information between the brain and various parts of the body, such as relaying commands from the brain to move your fingers.
2. Anatomy
2.1 Structure
The spinal cord is approximately 45 cm long and sits in the spinal canal within the vertebral foramen of the vertebrae. Spinal nerves exit through the intervertebral foramen. The spinal cord is divided into sections, named from cranial to caudal as follows:
- Cervical spinal cord (C1 to C8)
- Thoracic spinal cord (T1 to T12)
- Lumbar spinal cord (L1 to L5)
- Sacral spinal cord (S1 to S5)
A spinal cord segment is the section where a pair of spinal nerves emerges. Each segment is about 1–2 cm thick. In total, there are 31 spinal cord segments, corresponding to 31 pairs of spinal nerves.
Macroscopically, the spinal cord has two enlargements (intumescences):
- Cervical enlargement (C5 to T1)
- Lumbar enlargement (L2 to S2)
On the ventral side, the spinal cord has an indentation called the anterior median fissure, while on the dorsal side, there is a similar groove known as the posterior median sulcus.
The spinal cord enters the spinal canal through the foramen magnum and terminates at the conus medullaris, around the level of L1. Below this, the exiting nerves form the cauda equina. This is why spinal punctures are usually performed at the level of L4, where there is no risk of puncturing the spinal cord.
2.2 Meninges
The spinal cord is protected by layers of meninges. From external to internal, these layers are:
- Dura mater (Pachymeninx) with external and internal layers. The dural sac ends at the level of S2.
- The space between the external layer and the vertebrae is the epidural space, containing the internal vertebral venous plexus and adipose tissue.
- The space between the internal dura layer and the arachnoid mater is the subdural space, a potential space.
- Arachnoid mater (Leptomeninx), which has a spiderweb-like appearance. Beneath this is the subarachnoid space, containing cerebrospinal fluid (CSF) and the spinal roots.
- Pia mater, which directly adheres to the surface of the spinal cord and spinal roots.
The pia mater and arachnoid mater are anchored by the denticulate ligaments (ligamentum denticulatum).
2.3 Blood Supply
2.3.1 Arteries
The blood supply to the spinal cord originates from the vertebral arteries, which give rise to the anterior and posterior spinal arteries:
- The anterior spinal artery supplies the anterior two-thirds of the spinal cord, including the anterior gray horn and central gray matter. It runs in the anterior median fissure.
- The posterior spinal arteries (paired) supply the posterior one-third of the spinal cord, including the dorsal horn, posterior funiculus, and dorsal root.
Additional spinal branches contribute to the blood supply at different segments:
- Subclavian artery → costocervical artery and thyrocervical trunk
- Thoracic aorta → anterior intercostal artery – includes the artery of Adamkiewicz, a major artery between T9 and L1 (can also arise from a suprarenal artery)
- Abdominal aorta → lumbar arteries
- Internal iliac artery → lateral sacral arteries
2.3.2 Veins
The venous drainage of the spinal cord involves the internal vertebral venous plexus, which communicates with the external vertebral venous plexus. These plexuses also drain into the basisvertebral vein and dural venous sinuses. Ultimately, the external plexus drains into the azygos vein and hemiazygos vein, which in turn drain into the superior vena cava.
2.4 Cross-Section
In a cross-section of the spinal cord, the gray matter is shaped like a butterfly, surrounded by white matter. The central canal, lined by ependymal cells, is in the middle, surrounded by central gray matter.
- The dorsal horn of gray matter is primarily responsible for sensory transmission (somatosensory and viscerosensory).
- The ventral horn is responsible for motor transmission (somatic motor neurons and interneurons).
- In the thoracic spinal cord (T1 to L2), there is a lateral horn responsible for autonomic (visceromotor) transmission.
The white matter is divided into:
- Anterior funiculus
- Lateral funiculus
- Posterior funiculus
These areas contain axons that transmit information either to the brain (ascending tracts) or from the brain (descending tracts).