The study of the spinal cord is one of the most complexes, yet quite fascinating part of the nervous system. Its complex connections, the development defects, the lesions, and their clinical presentation are quite overwhelming and warrants a better understanding of its anatomical and physiological nature. This topic has received extensive study and revealed many minute details. But it quite acknowledgeable that still a lot is unknown, and discussing these details is beyond this article. This article is a humble effort to present the sophisticated anatomical features of the spinal cord briefly and in a straightforward manner. 

The spinal cord is an integral aspect of the central nervous system, which is cylindrical and elongated in structure. It is one of the most important contents of the vertebral canal. It presents as an extension of the lower part of the brain stem, i.e., the medulla oblongata. In an adult, it begins at the level of the first cervical vertebra and ends at the first lumbar vertebra.[1] But in the case of the young children, the spinal cord is relatively longer relative to the length of the vertebral column. Here it usually ends much lower at the level of the upper border of the third lumbar vertebrae.

Just like the brain, the very delicate and sensitive spinal cord has strong protection from a strong bony-cage formed by the vertebral arch and body of the vertebras. Together they form a long spinal column (spinal cavity) within which the spinal cord will be present. The spinal cord houses millions of neurons, and their bundled fibers are running as tracts that are either ascending or descending.[2] The three meninges completely cover the spinal cord - the dura mater, arachnoid mater, and pia mater (from outside to inside). Further protection and the cushioning effect is provided by the presence of the cerebrospinal fluid (CSF) present within the subarachnoid space, 

The spinal cord gives rise to 31 pairs of spinal nerves, which supply mainly the trunk and the limbs. These spinal nerves are mixed nerves having a motor component that helps in controlling all the voluntary muscles of the limbs and trunk. They also have a sensory component that helps in receiving the sensory information from these areas. Hence it very important to understand the basic anatomy to better interpret the clinical presentation and the pathology behind it in case of any spinal cord lesion.[3]

External Features

The spinal cord is a long cylindrical extension of the central nervous system within the cavity of the vertebral column. It is approximately 42.3 cm in length in males and 38.9 cm in females.[1] Even though the spinal cord is almost symmetrical, it is enlarged in a fusiform shape in two regions in the form of cervical and lumbosacral enlargements. In the cervical area, this fusiform enlargement is due to the exit of many nerves forming a plexus (the brachial plexus) supplying the muscles of the upper limb. This enlargement spreads from the fourth cervical segment to the second thoracic segment. The maximum transverse diameter is present at the level of the C5 segment.[4] Similarly, the lower thoracolumbar enlargement gives rise to a plexus of nerves supplying the muscles of the lower limb in the form of the lumbosacral plexus. This second enlargement extends from the second lumbar segment to the third sacral segment of the spinal cord.

The lowermost part of the spinal cord tapers in a cone-like fashion and thus termed as the conus medullaris. From the apex of this conus medullaris, an extension of the pia mater, i.e., the filum terminale, descends and gets attached to the posterior surface of the coccyx. In midline anteriorly, the cord shows the presence of a deep longitudinal cleft, which is the anterior median fissure. This fissure is the site of the presence of the anterior spinal artery. Similarly, the posterior surface demonstrates the presence of a shallow furrow in the midline, which is the posterior median sulcus. In combination, the anterior median fissure along with the posterior median sulcus will divide the whole of the spinal cord into two symmetrical halves - the right and left half, which one can better appreciate in a transverse section.

Further, we can appreciate the shallow depressions on either side as anterolateral and posterolateral sulci. The anterolateral sulcus will be the site of exit of the ventral (motor) roots of the spinal cord. The posterolateral sulcus will be the site for the entry of dorsal (sensory) roots of the spinal cord. 

Transverse section of the spinal cord: 

On sectioning of a spinal cord, two distinct areas are evident with the naked eyes, an inner grey matter, and an outer white matter. The grey matter indicates the collection of cell bodies of neurons. The white matter is the collection of the fibers running through the spinal cord. 

Grey Matter: When taking a transverse section of the spinal cord, in the middle is the grey matter, which appears roughly in the form of an H (butterfly) shaped mass. The grey matter is divisible on each half of the cord, into a larger ventral mass –  ventral grey column (anterior/ventral horn), and an elongated dorsal grey column (posterior/dorsal horn). In some parts of the cord (thoracic and upper lumbar regions), a small lateral projection of a grey mater is present between the ventral and dorsal columns. This structure is a lateral grey column (lateral horn). The cells in the anterior horn are called anterior horn cells. The grey matter on both the left and right halves connect by the grey commissure, which contains the central canal. This canal communicates above into the fourth ventricle. The central canal expands at the lower end in the conus medullaris to form the terminal ventricle. The central canal contains CSF and lined by ependymal cells.[5] For experimental purposes, the grey matter divides into ten laminae. The laminae I to VI occupy the posterior grey column.[6] The lamina VII is in the intermediate region, lamina VIII remains confined to the medial part of the anterior grey column, and lamina IX occupy the lateral part of the anterior grey column. The area around the central canal is distinguished as lamina X.[7] Each of these laminae has unique connections and functions.

White Matter: Surrounding the grey mater superficially, we find the white mater. On either side, the white matter, which is medial to the posterior grey column, forms the dorsal (posterior) funiculus. The white matter that is medial and ventral to the ventral (anterior) grey column forms the ventral (anterior) funiculus. While the white matter lateral to the anterior and posterior grey column forms the lateral funiculus. Each of these areas contains bundles of nerves in the form of tracts that can be ascending or descending.[8] This region is called white because it looks pale due to the presence of myelinated nerve fibers.

There is variation in the amount of the areas occupied by the white and grey matter in different levels of the spinal cord. The space occupied by the grey matter increases progressively, and the region for white matter decreases as we go craniocaudally.

Even the arrangement of fibers in each funiculus of the white matter is unique. When observing a transverse section of the spinal cord, we find that the posterior funiculus will have ascending sensory fibers arranged into two regions called fasciculus gracilis and fasciculus cuneatus. The anterior funiculus will contain descending motor fibers of olivospinal, tectospinal, lateral vestibulospinal, medial vestibulospinal, anterior corticospinal, anterior spinothalamic, spino-olivary, spino-tectal, and reticulospinal tracts. The lateral funiculi will contain, anterior and posterior spinocerebellar, lateral spinothalamic, spino-reticular, lateral corticospinal, rubrospinal, and lateral reticulospinal tracts. 

Arrangement of the spinal nerves and segments:

The spinal cord gives rise to a series of spinal nerves on either side. The spinal nerves are mixed nerves and have two roots, an anterior and posterior root. Sometimes they are also termed as ventral and dorsal root, respectively. The anterior or ventral root is motor in nature and gives efferent fibers to skeletal muscles. Sometimes they also contain preganglionic autonomic fibers to blood vessels and internal organs. The posterior or dorsal root is sensory and contains afferent fibers from peripheral receptors coming from the skin, bones, joints, muscles, and internal organs. Researchers discovered this in the 19th century, and it is known as Bell-Magendie Law.[9]

Each root is an aggregation of the number of rootlets arising from a part of the spinal cord, and that area is called the spinal segment. The spinal nerves are 31 pairs – 8 pairs of cervical, 12 pairs of thoracic, five pairs of lumbar, five pairs of sacral, and one pair of coccygeal nerves.[10] Thus there are 31 pairs of spinal segments. The dorsal nerve roots enter the spinal cord along postero-lateral sulcus and ventral roots along the anterolateral sulcus. The dorsal root shows a swelling called dorsal nerve root ganglion (spinal ganglion). This ganglion contains cell bodies of pseudounipolar sensory neurons. 

Due to the developmental differences in the vertebral column and spinal cord, spinal segments do not lie opposite to their corresponding vertebrae. The length and obliquity of the spinal nerves increase progressively caudally before their exit through the intervertebral foramina. This knowledge is vital in laminectomy to relieve spinal cord compression due to traumas and tumors. At the conus medullaris, these roots of nerves in pairs coming from the lower lumbar, sacral, and coccygeal nerves form a bundle of nerves resembles the tail of a horse and thus known as cauda equina.

Covering of Spinal Cord

Three meninges surround the spinal cord. From outside inwards, they are the dura mater (thick fibrous membrane), the arachnoid mater, and the pia mater. The space present between the dura and the wall of the vertebral canal is called the extradural (epidural) space. The space between the dura and arachnoid mater is said to be subdural space. Subarachnoid space is present between the pia mater and arachnoid mater. This space is filled with cerebrospinal fluid. The pia mater is adherent to the spinal cord and extends along with the cord up to the lower border of the L1 vertebra where the spinal cord end. But dura and arachnoid mater extend up to second sacral vertebra. Between these two vertebral levels, subarachnoid space has spinal nerve roots forming cauda equina. One should remember that the pia mater does not end at L1 but extends beyond this as the thin midline structure called filum terminale, and finally attached to the posterior surface of the coccyx. 

The most important function of the spinal cord is to receive, integrate, and association of the input and produce a reflex response.

The ectoderm of a trilaminar staged embryo will thicken at the midline to be called the neural plate. This plate depresses to form a neural groove whose ends later fuse to convert it into a neural tube. This tube enlarges at the cranial end and remains tubular at the caudal part. The cranial part forms the brain. The spinal cord develops from the caudal tubular part of this neural tube.[11][12] Maternal dietary supplement of foliate plays a vital role in the closure of the neural tube in the early stages.[13] The neural tube differentiates itself into the ependymal, mantle, and marginal layers from inside out. The mantle proliferates rapidly to form the gray matter, which is divided into a ventral basal lamina and dorsal alar lamina by the sulcus limitans. The basal lamina develops into the motor area that gives rise to anterior and lateral horns and all the neurons within. The alar lamina develops as a sensory area that gives rise to the posterior horn and all the neurons within it. The cavity within will narrow down to form the central canal. The neural crest cells help in the formation of the ventral sympathetic ganglion and dorsal root ganglion. 

The axons of the neuron start occupying the marginal zone during the fourth week of gestation. The axons now bundle up to form ascending and descending tracts within the marginal zone, which further subdivide this region into an anterior, posterior, and lateral white columns. With the development of limb buds, there is a simultaneous development of the cervical and lumbar enlargement at the fifth to sixth weeks of gestation. The tracts start forming during the third month, and the corticospinal tract appears by the fifth month of intra-uterine life. The myelination begins at the fourth month of embryonic life from short intersegmental tracts of the spinal cord and continues up to two to three years after birth till the child starts walking. 

The vertebral bodies and the muscles surrounding it develop from the surrounding dermatomes. The incomplete development of vertebras leads to several congenital anomalies that this article describes in the section of Clinical Significance. The length of the spinal cord and the vertebral column are equal in size up to the third month of fetal life. Later the vertebral column develops rapidly in comparison to the spinal cord. Thus the spinal cord ends at a higher level within the vertebral canal. At birth, it has receded to the third lumbar vertebra and reaches the first lumbar vertebra by puberty. Thus the spinal nerves arise from the spinal cord at a much higher level compared to their exit outside through the intervertebral foramina. 

Many studies indicate a crucial role of some of the critical signaling factors in the development of the spinal cord like retinoic acid, bone morphogenetic proteins, Wnt, fibroblast growth factors, and Hedgehog proteins.[14][15] It is now known whether vascular endothelial growth factors (VEGFs) play a significant role in the development of the vessels supplying the spinal cord.[16]

Arterial Supply of the Spinal Cord

The spinal cord receives its blood supply from three longitudinal channels that extend along the length of the spinal cord, i.e., one anterior spinal artery and two posterior spinal arteries. The anterior spinal artery runs along the anterior median fissure, and the smaller two posterior spinal arteries pass through the posterolateral sulcus on either side. The vertebral arteries are the main source from where these spinal arteries branch. But it should be remembered that the blood coming from the vertebral arteries supply only the cervical segments of the cord. In the lower down region, the spinal arteries receive blood through radicular arteries that reach the cord along with the roots of the spinal nerves. The radicular arteries are actually branches from many arteries like the vertebral, cervical, intercostal, lumbar, and even sacral arteries. The largest radicular feeder from the left posterior intercostal artery to the anterior spinal artery is between T9 and T12, called the artery of Adamkewicz. All the radicular and spinal arteries anastomose with each other to form an anastomotic pial plexus called vasocorona.[17] But the better part of the spinal cord is supplied by the anterior spinal artery and its branches.

Veinous Drainage of the Spinal Cord

The veins draining the spinal cord are arranged in the form of a network of six longitudinal channels, i.e., an anteromedian vein, a posteromedian vein, and a pair of anterolateral and posterolateral veins.[17] As the name indicates, the anteromedian and posteromedian channels will be present in the midline anteriorly and posteriorly, respectively, relative to the spinal cord. The anterolateral and posterolateral channels are paired and lie as per their names on either side of the spinal cord. All these channels are interconnected to form a plexus of veins of venous vasocorona. The venous blood from these veins drains into radicular veins. These radicular veins finally drain into the segmental veins.

Spinal Nerves

The nerves are of two types based on their origin - the cranial and spinal nerves. Those nerves which begin in the cerebrum or the brain stem are called cranial nerves. There are twelve pairs of cranial nerves. The nerves coming from the spinal cord are called the spinal nerves. There are 31 pairs of spinal nerves coming from the spinal cord and spread on either side of the vertebral column. They are classified into cervical, thoracic, lumbar, sacral, and coccygeal nerves.  The cervical nerves are in eight pairs; the thoracic are twelve pairs, the lumbar and sacral nerves are in five pairs, and one pair of coccygeal nerves.

Spinal tracts:

When we study the transverse section of a spinal cord, we can appreciate many spinal tracts within the white matter. They categorize into two types - ascending and descending tracts. The descending tracks are mainly motor, and ascending tracks are sensory.

Some of the most essential descending (motor) tracts passing through the spinal cord are as follows:

1. Cortico-spinal tract (This is also known as the pyramidal tract which is the most important of the motor tracts)
2. Tectospinal tract
3. Rubrospinal tract
4. Vestibulospinal tract
5. Reticulospinal tracts
6. Olivospinal tract

Some of the most important ascending (sensory) tracts are as follows:

1. Lateral spinothalamic tract (spinal lemniscus)
2. Anterior spinothalamic tract (medial lemniscus)
3. Trigeminal lemniscus
4. Lateral lemniscus
5. Spinotectal tract
6. Spino-olivary tract
7. Ventral spinocerebellar tract
8. Dorsal spinocerebellar tract
9. Dorsolateral tract (of Lissauer)
10. Spino-cervico-thalamic tract

Here we will explain one descending tract in detail to better understand the whole course. 

The cortico-spinal tract arises mainly from the motor area of the cerebral cortex (area 4) with some contribution from the premotor area (area 6), somatosensory area (area 3, 2, 1), and parietal cortex (area 5). The fibers run through the posterior limb of the internal capsule and then occupy the middle part of the crus cerebri of the midbrain. The tract then occupies the ventral part of the pons and descends through the pyramids in the medulla. At the lower end of the medulla, almost 70 to 80% of the fibers cross to the opposite side of the spinal cord, which is called the pyramidal (motor) decussation. Now, these crossed fibers enter the lateral funiculus of the spinal cord and continue as the lateral corticospinal tract. They end at various levels of the grey matter of the spinal cord by synapsing with internuncial neurons of dorsal and ventral columns. The remaining 20 to 30% fibers of the corticospinal tract, which have not crossed, will descend within the anterior funiculus as the anterior corticospinal tract. Finally, these anterior corticospinal tract fibers too cross to the opposite side at the appropriate levels. Thus all fibers of the corticospinal tract eventually cross to the opposite side and connect the cerebral cortex of one side with the ventral horn cells of the opposite part of the spinal cord. These fibers are called upper motor neurons. The fibers starting in the ventral horn cells and descending downwards will be called lower motor neurons. This knowledge is vital as the clinical presentation of upper and motor neuron lesions are quite different.[18] Any minor injury of these fibers will lead to widespread paralysis of the muscles supplied.

Spinal cord lesions:

Lesions of the spinal cord fall into the following categories:

1. Lesions of the afferent system:
   1. Dorsal nerve roots
   2. Spinothalamic tract
   3. Posterior white funiculus
   4. Syringomyelia
2. Lesions of the efferent system:
   1. Upper motor neuron lesions
   2. Lower motor neuron lesions
3. Lesions involving upper and lower motor neurons:
4. Lesions involving posterior and lateral funiculi
5. Thrombosis of the spinal artery
6. Hemisections (Brown-Sequard syndrome)
7. Transections 
8. Hereditary diseases: Friedreich ataxia

Details of some of these lesions will be discussed in the section of Clinical Significance.