Spinal Cord – Anatomy, Types, Functions

The spinal cord is a tubular structure composed of nervous tissue that extends from the brainstem and continuing distally before tapering at the lower thoracic/upper lumbar region as the conus medullaris.  The spinal cord is anchored distally by the filum terminale, a fibrous extension of the pia mater anchoring the spinal cord to the coccyx.[rx] Protecting the spinal cord is the surrounding cerebrospinal fluid (CSF), supportive soft tissue membranes and meninges, and the osseous vertebral column.[rx] Basic knowledge of the structure and function of the spinal cord and spinal column is essential for healthcare providers as the recognition of characteristic clinical signs and symptoms is imperative for the appropriate treatment of patients presenting with a wide range of clinical symptoms.

Structure of Spinal Cord

Meninges and Spaces

• Epidural space – fatty space between the bony framework of the spinal vertebral column and the thick dura mater surrounding the spinal cord. It contains adipose tissue and blood vessels.
• Dura mater – Thick outermost covering (meninges) of the spinal cord, extending down to the level of the S2 vertebra
• Arachnoid mater – The middle covering of the spinal cord
• Subarachnoid space – Space between the arachnoid mater and the innermost covering of the spinal cord. It contains Cerebrospinal fluid.
• Pia mater – The innermost covering of the spinal cord, intimately adhering to its surface, it stabilizes the spinal cord through lateral extensions of the pia called the denticulate ligaments, extending between the ventral and dorsal roots unto the dura mater.

Spinal cord

• The length is about 45 cm in men and 43 cm in women[rx]
• Anatomic course originates in the brainstem before coursing through the foramen magnum.  The spinal cord continues distally through the cervical and thoracic regions of the spinal column before terminating as a tapered structure known as the conus medullaris
• The spinal cord proper terminates at the L1-L2 vertebral level and is anchored distally via the filum terminale, representing an extension of the pia mater with fibrous attachments to the coccyx
• The spinal cord comprises 5 segments, cervical, thoracic, lumbar, sacral and coccygeal
• Long, cylindrical structure with varying levels of thickness/width depending on the corresponding vertebral levels
• 31 total nerve root segments

  • 8 cervical
  • 12 thoracic
  • 5 lumbar
  • 5 sacral
  • 1 coccygeal
• Cord width ranges from 0.64-0.83 cm in the thoracic region to 1.27-1.33 cm in the cervical and lumbar regions

  • A 2016 systematic review of the literature demonstrated that, on average, the largest transverse diameter corresponded to the C5 neuronal segment (1.33 +/- 0.22 cm) and the smallest transverse diameter, on average, corresponded to the T8 segment (0.83 +/- 0.21 cm)[rx]
  • Enlarged regions of gray matter correspond to nerve root distribution to the upper and lower extremities
• A cross-sectional view of the spinal cord shows its organization into the gray and white matter. The anterior aspect of the cord is identifiable with the presence of anterior median fissure. The gray matter is organized into an H- shaped body of cell bodies. The anterior horn comprises motor nuclei while the posterior horn comprises sensory nuclei. The surrounding white matter is organized into anterior, posterior and lateral columns (funiculi), from neuronal axons organized into tracts that convey neural messages back and forth the CNS (the ascending and descending tracts). The anterolateral columns carry temperature and pain information, while the dorsal column communicates the sense of touch, proprioception, vibration. The cervical and thoracic spinal segments present an intermediolateral gray horn which gives off preganglionic sympathetic fibers onto the sympathetic trunk on both sides of the spinal cord.
• The spinal cord’s central canal is an extension of the 4th ventricle. It contains CSF, surrounding it is the gray commissure and the anterior white commissure. Decussation of the tracts of the white columns occurs at the anterior white commissure.

Functions of Spinal Cord

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.[rx] 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.[rx] 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.[rx] For experimental purposes, the grey matter divides into ten laminae. The laminae I to VI occupy the posterior grey column.[rx] 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.[rx] 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.[rx] 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.[rx]

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.[rx] 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. 

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.

Blood Supply and Lymphatics  of Spinal Cord

• Anterior spinal artery – branch of the vertebral artery, supplies the anterior 2/3 of the spinal segment[rx]
• Left and right posterior spinal arteries – The pair posterior branches of the vertebral artery supply the posterior 1/3 of the spinal segment

Radiculo-medullary branches from the anterior and posterior spinal arteries and the costocervical trunk anastomose to supply the cervical region of the spinal cord. The thoracolumbar segments of the spinal cord receive extra arterial blood supply through radiculo-medullary branches of segmental arteries from the aorta; of note is the great radicular artery called the Adamkiewicz artery.[rx] It has a variable origin arising anywhere between T9 and L5 vertebra.

Nerves

There are 31 spinal nerve pairs (mixed) that arise from the intervertebral foramen on both sides of the vertebral column.

• 8 Cervical nerves – C1 – C7 nerves arising from above corresponding cervical vertebra except for C8 which exits from between C7 and T1 Vertebrae
• 12 Thoracic nerves
• 5 Lumber nerves
• 5 Sacral nerves
• 1 Coccygeal segment

Nerves from the lower spinal segment exit terminal to the conus medullaris and form the cauda equina.

Ventral spinal nerve roots – make up the motor component of the spinal innervation with rootlets arising from the anterior horn of the spinal segment, sending motor impulses to muscles and joints, etc

Dorsal spinal nerve roots – make up the sensory component, with the dorsal ganglia sending sensory impulses via nerve rootlets to the posterior horn of spinal segments.

Surgical Considerations

Myelotomy:[rx][rx] Placing an incision on the surface of the spinal cord is known as myelotomy.

• A posterior median sulcus approach – for most intramedullary gliomas and ependymomas

Lateral myelotomy – for vascular tumors such as hemangioblastoma or cavernous malformation

• Limited midline myelotomy – for intractable visceral pain

Cordotomy:[rx]

• Cervical cordotomy – the nociceptive pathways in the lateral spinothalamic tract (anterolateral column) are surgically destroyed at the level of C1-C2. This is done for pain relief in unilateral malignancies and lower limb pathologies.

Clinical Significance

Spinal cord disorders, injuries (SCIs), or syndromes may include (but are not limited to)[rx][rx][rx]

• Traumatic (ground-level falls in the elderly, high-energy motor vehicle accidents in any age group)

  • The incidence and prevalence of traumatic SCI in the United States is higher compared to rates reported in the literature for other countries worldwide[rx][rx]
  • The average age at clinical presentation continues to increase, corresponding to the aging of the general population[rx]
  • Heightened clinical suspicion should be given to vertebral compression fractures which can occur spontaneously (i.e., in the absence of trauma)[rx][rx]
• Infection[rx]
• Tumors/malignancy (including metastatic disease)[rx][rx][rx]
• Disc herniations[rx][rx]
• Spinal stenosis/degenerative conditions/facet arthropathy[rx][rx]
• Syringomyelia[rx][rx]

In general, the extent of disability depends on the level of injury and the underlying degree of severity of the corresponding clinical pathology.  For example, injuries at the levels of the cervical segment affect both upper and lower limbs while lesions from thoracic segments downwards affect the lower limb.[31]

Spinal cord injury (SCI) patterns

• Descriptive

  • Tetraplegia

    • Cervical spine injury resulting in impaired arm/trunk/leg/pelvic organ function
  • Paraplegia

    • Thoracic/lumbar/sacral spinal injury leading to impaired trunk/leg/pelvic organ function
    • Preserved arm function
  • Complete injuries

    • By definition, a complete SCI yields no sparing of the motor or sensory function below the injured level

      • The patient must have already recovered from the acute phase of spinal shock (usually 48 hours from presentation)
      • Spinal shock: by definition, the temporary (typically 48 hours) loss of all spinal cord function (including reflex activity) below the level of injury

        • Absent bulbocavernosus reflex
        • Flaccid paralysis
        • Bradycardia/hypotension
• Spina bifida – Neural tube defect in which the neural tube does not completely close leaving a dorsal defect. Folate deficiency in early pregnancy is a risk factor.[rx] The severity of symptoms depends on the extent of the defect; myelomeningocele is the most severe variant with the spinal cord, meninges both exposed. Other variants include meningocele, which exposes only the meninges, and spina bifida occulta which is the mildest. Symptoms include loss of lower limb sensations, lower limb weakness, urinary incontinence, bowel incontinence

Incomplete spinal cord injuries (SCIs)

• Central cord syndrome[rx]

  • Most common incomplete SCI
  • Pathophysiology: central gray matter injury
  • Mechanism(s): hyperextension (i.e., from a fall) in a patient with underlying cervical spinal canal stenosis
  • Clinical presentation:

    • Upper extremity loss of motor function (lower extremity motor function no affected/minimally affected)
    • Sensory sparing variable
  • Prognosis: Good
• Anterior cord syndrome[rx]

  • Second most common incomplete SCI
  • Pathophysiology: injuries occur secondary to direct compression to the anterior spinal cord (e.g., hyperflexion injuries, anterior spinal artery occlusion, or disc prolapse)
  • Mechanism(s): hyperflexion injuries, anterior spinal artery occlusion, disc prolapse
  • Clinical presentation:

    • Loss of motor, pain, and temperature reception below the level of injury
    • Preserved dorsal column function (i.e., proprioception, vibration sensation, and deep pressure sensation)
  • Prognosis: Poor (for motor recovery specifically)
• Posterior cord syndrome[rx]

  • Very rare/least common incomplete SCI pattern of injury
  • Pathophysiology: injury to the dorsal column
  • Mechanism(s):

    • subacute combined degeneration
    • tabes dorsalis (i.e., secondary to syphilis)
    • multiple sclerosis
    • vascular malformations (arterio-venous malformation – AVM)
    • malignancy (e.g., compressive extramedullary tumors)
    • degenerative conditions (e.g., spondylosis)
  • Clinical presentation:

    • Loss of proprioception, vibration, and deep pressure sensation below the level of injury
    • Preserved ambulatory function

      • patients will complain of difficulty balancing in the dark or with his/her eyes closed
    • Classic exam consistent with a positive Romberg sign
  • Prognosis: Preserved ambulatory function; dorsal column recovery unpredictable
• Spinal cord hemisection/hemicord (Brown-Sequard syndrome)[rx]

  • Rare (2% to 4% of SCIs)[rx]
  • Pathophysiology: trauma to one side of the spinal cord (in cross-sectional reference)
  • Mechanism(s):

    • Penetrating trauma (knife, gunshot wound)
  • Clinical presentation:

    • Ipsilateral loss of motor and proprioception (directly below the level of injury)
    • Contralateral loss of pain/temperature

      • Classically the contralateral pain/temperature loss occurs one to two levels below the level of injury)
  • Prognosis: 90% recover ambulatory function
• Cauda equina syndrome[rx][rx]

  • Pathophysiology: Injury to (only) the nerve roots of the cauda equina itself (i.e., spares the spinal cord itself)
  • Mechanism(s):

    • disc herniations
    • burst fractures (e.g., associated hematoma from trauma)
  • Clinical presentation:

    • bilateral buttock/lower extremity pain
    • bowel/bladder dysfunction (urinary retention)
    • saddle anesthesia
    • loss of lower extremity motor/sensory function

      • differentiated from conus medullaris syndrome in that findings are asymmetrical, as opposed to symmetrical (i.e. conus medullaris motor symptoms are symmetrical on presentation)
  • Prognosis: surgical decompression within the first 48 hours appears to yield improved overall outcomes (although the overall prognosis remains guarded)
• Conus medullaris syndrome[rx]

  • Often confused with cauda equina syndrome, although this must be recognized as a separate clinical entity
  • Pathophysiology: injury to the spinal cord at L1-L2 level
  • Mechanism(s):

    • Direct spinal trauma to the thoracolumbar junction[rx]
  • Clinical presentation:

    • Saddle anesthesia
    • bowel/bladder dysfunction (often presents with dysfunction more acutely compared to cauda equina which can evolve over a variable period time prior to the patient’s presentation)
    • classically presents with mild, symmetrical motor symptoms (often mixed upper and lower motor neuron syndromes)

      • can present with both spasticity and flaccid paresis
      • hyperreflexia and/or hyporeflexia
    • Prognosis: guarded

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