The spinal cord is a tubular bundle of nervous tissue and supporting cells that extends from the brainstem to the lumbar vertebrae. Together, the spinal cord and the brain form the central nervous system. In this article, we shall examine the macroscopic anatomy of the spinal cord - its structure, membranous coverings and blood supply.
The spinal cord is a cylindrical structure, greyish-white in colour. It has a relatively simple anatomical course- The spinal cord arises cranially as a continuation of the medulla oblongata (part of the brainstem). It then travels inferiorly within the vertebral canal, surrounded by the spinal meninges containing cerebrospinal fluid. At the L2 vertebral level the spinal cord tapers off, forming the conus medullaris. As a result of the termination of the spinal cord at L2, it occupies around two thirds of the vertebral canal. The spinal nerves that arise from the end of the spinal cord are bundled together, forming a structure known as the cauda equina. During the course of the spinal cord, there are two points of enlargement. The cervical enlargement is located proximally, at the C4-T1 level. It represents the origin of the brachial plexus. Between T11 and L1 is the lumbar enlargement, representing the origin of the lumbar and sacral plexi.
The spinal cord is marked by two depressions on its surface. The anterior median fissure is a deep groove extending the length of the anterior surface of the spinal cord. On the posterior aspect there is a slightly shallower depression – the posterior median sulcus.
The cauda equina is a bundle of spinal nerves that arise from the distal end of the spinal cord. They run in the subarachnoid space, before exiting at their appropriate vertebral level. Compression of these nerves produces a range of signs and symptoms collectively termed cauda equina syndrome. There are many causes of compression, including intervertebral disc prolapse, extrinsic or primary cord tumours, spinal stenosis, trauma and abscess formation. Suspected cauda equina patients should be assessed with a full lower limb neurological assessment. The main signs to assess for are saddle-area anaesthesia, incontinence/retention of urine or faeces, reduced anal tone, and paralysis +/ - sensory loss.
The spinal meninges are three membranes that surround the spinal cord – the dura mater, arachnoid mater, and pia mater. They contain cerebrospinal fluid, acting to support and protect the spinal cord. They are analogous with the cranial meninges.
Distally, the meninges form a strand of fibrous tissue, the filum terminale, which attaches to the vertebral bodies of the coccyx. It acts as an anchor for the spinal cord and meninges.
The spinal dura mater is the most external of the meninges. It extends from the foramen magnum to the filum terminale, separated from the walls of the vertebral canal by the epidural space. This space contains some loose connective tissue, and the internal vertebral venous plexus. As the spinal nerves exit the vertebral canal, they pierce the dura mater, temporarily passing in the epidural space. In doing so, the dura mater surrounds the nerve root, and fuses with the outer connective tissue covering of the nerve, the epineurium.
The spinal arachnoid mater is a delicate membrane, located between the dura mater and the pia mater. It is separated from the latter by the subarachnoid space, which contains cerebrospinal fluid.
The spinal pia mater is the innermost of the meninges. It is a thin membrane that covers the spinal cord, nerve roots and their blood vessels. Inferiorly, the spinal pia mater fuses with the filum terminale. Distal to the conus medullaris, the subarachnoid space expands, forming the lumbar cistern.
In addition to the membranous coverings, the spinal cord has a complex vasculature which is supplied by segmental vertebral arteries. The blood supply is paramount to the survival of the spinal cord. However, when the spinal cord is injured, significant disruption of the blood supply may occur.
It is important to be aware of the anatomy and the vulnerable vascular anatomy of the spinal cord. Compromise of the blood supply can result in significant structural damage to the spinal cord, and the resultant neurological deficits can be severe.
The spinal nerves, which make up the peripheral nervous system, originate from the spinal cord. Each spinal nerve consists of an anterior (motor) and a posterior (sensory) nerve root, which arise from the spinal cord and unite at the intervertebral foramina, forming a single spinal nerve. The spinal nerve then leaves the vertebral canal via the intervertebral foramina, and then divides into two: a posterior ramus, which supplies nerve fibres to the synovial joints of the vertebral column, deep muscles of the back and the overlying skin; and an anterior ramus, which supplies nerve fibres to the rest of the body, both motor and sensory.
From the distal end of the spinal cord, nerve roots L2-S5 form a bundle known as the cauda equina, which is the lowermost part of the spinal cord. It is located at the level of the L2 vertebra, and is where the spinal cord tapers off and occupies two thirds of the vertebral canal.
The arterial supply to the spinal cord is provided by three longitudinal arteries: the anterior spinal artery, formed from branches of the vertebral arteries, and a pair of posterior spinal arteries, originating from the vertebral or posteroinferior cerebellar artery. There is also additional arterial supply from the anterior and posterior segmental medullary arteries, which enter through the nerve roots. The largest anterior segmental medullary artery is the artery of Adamkiewicz, which arises from the inferior intercostal or upper lumbar arteries and supplies the inferior 2/3 of the spinal cord.
Venous drainage is via three anterior and three posterior spinal veins. These veins are valveless and anastomose with one another to form a network on the surface of the spinal cord. They also receive venous blood from the radicular veins, and empty into the internal and external vertebral plexuses, and then into the systemic segmental veins. The internal vertebral plexus also empties into the dural venous sinuses superiorly.
Between the nerve roots, the pia mater thickens, forming the denticulate ligaments. These ligaments attach to the dura mater and suspend the spinal cord in the vertebral canal.
The spinal meninges are three membranes that surround the spinal cord - the dura mater, arachnoid mater and pia mater. These contain cerebrospinal fluid which acts to support and protect the spinal cord. Distally, the meninges form a strand of fibrous tissue, the filum terminale, which attaches to the vertebral bodies of the coccyx. This acts as an anchor for the spinal cord and meninges.
The dura mater is the most external of the meninges and extends from the foramen magnum to the filum terminale. It is separated from the walls of the vertebral canal by the epidural space, which contains some loose connective tissue and the internal vertebral venous plexus.
The spinal cord is marked by two depressions on its surface. The anterior median fissure is a deep groove extending the length of the anterior surface of the spinal cord. On the posterior aspect there is a slightly shallower depression - the posterior median sulcus.
The spinal cord has two points of enlargement. At the C4-T1 level is the cervical enlargement, which is the origin of the brachial plexus. Between T11 and L1 is the lumbar enlargement, which is the origin of the lumbar and sacral plexi.
Spinal cord infarction (also known as a spinal stroke) is the death of nervous tissue caused by an interruption of the arterial supply. Signs of spinal cord infarction include muscle weakness and paralysis with loss of reflexes. The most common causes of infarction are vertebral fractures or dislocations, vasculitic disease, atheromatous disease, or external compression (e.g. abdominal tumours). Around 95% of spinal cord ischaemic events are to the anterior aspect of the spinal cord, with the posterior columns preserved. Treatment involves reversing any known cause.
As the spinal nerves exit the vertebral canal, they pierce the tough and protective dura mater, a membrane that surrounds and fuses with the outer connective tissue covering of the nerve, known as epineurium, before temporarily passing through the epidural space.
The arachnoid mater is the second outer membrane surrounding the spinal nervous system. This delicate membrane is located between the dura mater and the pia mater, with the subarachnoid space containing cerebrospinal fluid between them.
The innermost of the three meninges is the spinal pia mater, a thin membrane that covers the spinal cord, nerve roots, and their associated blood vessels. At the inferior end, the spinal pia mater fuses with the filum terminale, while between the nerve roots it thickens to form the denticulate ligaments. These ligaments attach to the dura mater and suspend the spinal cord in the vertebral canal.
The spinal nerves are mixed nerves that are part of the peripheral nervous system, and which originate from the spinal cord. Each nerve root takes the form of an anterior (motor) and a posterior (sensory) nerve, which come together at the intervertebral foramina to form a single spinal nerve. The spinal nerve then exits the vertebral canal and divides into two: the posterior rami and the anterior rami.
The posterior rami of the spinal nerve provide nerve fibres for the synovial joints, deep muscles, and overlying skin of the vertebral column. On the other hand, the anterior rami supply nerve fibres to much of the rest of the body, both for motor and sensory functions. The nerve roots L2-S5 arise from the distal end of the spinal cord and form a bundle known as the cauda equina.
The arterial supply to the spinal cord comes from three main sources. Firstly, the anterior and posterior spinal arteries, which are formed from branches of the vertebral arteries which travel in the anterior median fissure. Secondly, the anterior and posterior segmental medullary arteries, which are small vessels that enter through the nerve roots. Finally, the largest anterior segmental medullary artery is the artery of Adamkiewicz, which arises from the inferior intercostal or upper lumbar arteries and supplies the inferior two-thirds of the spinal cord.
Venous drainage from the spinal cord originates from three anterior and three posterior spinal veins. These veins form an anastomosing network along the surface of the spinal cord before continuing to the internal and external vertebral plexuses, which in turn empty into the systemic segmental veins. The internal vertebral plexus also empties into the dural venous sinuses at a superior point.
Spinal cord infarction (also known as a spinal stroke) is a dangerous condition in which the death of nervous tissue occurs due to the interruption of arterial supply. Clinical signs of spinal cord infarction include muscle weakness, paralysis, and the loss of reflexes.
The most common causes of infarction are vertebral fractures or dislocations, vasculitic disease, atheromatous disease, and external compression from tumours, among other factors. 95% of spinal cord ischemic events are to the anterior aspect of the spinal cord, while posterior columns tend to be preserved. Treatment for spinal cord infarction depends on addressing the underlying cause.
Understanding the structure of the spinal nerves, the vasculature which serves them, and the potential clinical implications of damage to the spinal nerves is important in providing more comprehensive medical care for those who are in need.
