Spinal Cord Anatomy

Original Editor - Naomi O'Reilly

Top Contributors - Naomi O'Reilly, Lucinda hampton, Jess Bell, Kim Jackson, Rucha Gadgil, Nikhil Benhur Abburi, Stacy Schiurring, Vidya Acharya, Admin, Tarina van der Stockt, Aminat Abolade and Ewa Jaraczewska  

Introduction

The spinal cord consists of a densely packed column of nerve tissue, which runs from the brainstem to around L1-2 in adults. The terminal end of the spinal cord is the conus medullaris. The cauda equina extends from the conus medullaris. The spinal cord plays a vital role in facilitating functional activities as it sends information to and from the brain and periphery.

Spinal Column

Spinal Column

The spinal cord sits inside the spinal or vertebral canal. This canal also contains the meninges (membranes that protect the spinal cord), blood vessels, spinal nerve roots, and surrounding fatty and connective tissues.

The spinal canal is formed by the vertebral foramina of the vertebral bodies. The anterior border of the canal is made up of the vertebral bodies, while the laminae (or vertebral arches) form the posterior border. The anterior longitudinal ligament prevents hyperextension of the vertebral column while the posterior longitudinal ligament prevents hyperflexion of the vertebral column. The diameter of the spinal canal varies, ranging from 12 to 22 mm in the cervical region and 22 to 25 mm in the lumbar region.

Adults typically have 33 vertebrae, including:

• seven cervical vertebrae
• twelve thoracic vertebrae
• five lumbar vertebrae
• five sacral vertebrae (fused in adults to form the sacrum)
• four coccygeal vertebrae (fused to form the coccyx)

An intervertebral disc sits between each vertebra. This provides a level of flexibility to the vertebral column. The cervical region is the most flexible area while the thoracic region is the least flexible.

The spinal cord extends from the foramen magnum to the lowest border of the first lumbar vertebra. It is enlarged at two sites: C3-T1 and L1-S2.

Spinal Cord

The spinal cord is the major pathway and reflex centre between the peripheral nerves and the brain:

• it transmits motor information from the brain to the muscles, tissues and organs
• it sends sensory information from these areas back to the brain

In adults, the spinal cord is around 45 cm long. It is cylindrical and slightly flattened anteriorly and posteriorly.

Structure of the Spinal Cord

The upper end of the spinal cord is continuous with the medulla, beginning just above the first pair of cervical nerves.

The spinal cord structure includes both fissures and sulci.

• Anterior structures:
  • the anterior median fissure is found centrally
  • the anterior nerve roots exit from the anterolateral sulcus
• Posterior structures:
  • the posterior median sulcus is found posteriorly with the posterolateral sulcus present on either side
  • the posterior nerve roots exit from the posterolateral sulcus

The lower end (i.e. the conus medullaris) ends at the L1-2 intervertebral disc in adults and at the L3 vertebra in newborns. Below this point, it becomes the cauda equina.

The cauda equina:

• is a bundle of filaments that resembles a horse's tail
• consists of:
  • three to five pairs of lumbar nerve roots
  • five pairs of sacral nerve roots
  • one pair of coccygeal nerves
• provides motor and sensory innervation to the legs, pelvis and pelvic organs

Grey and White Matter

Spinal Cord Sectional Anatomy

The spinal cord has grey matter and white matter:

• the mass of butterfly-shaped grey matter is made up of the cell bodies of motor and sensory neurons, interneurons and neuropils. The central canal runs through its centre. This central canal is filled with cerebrospinal fluid:
  • the grey matter is organised into regions called horns:
    • anterior horns: location of motor neurons
    • posterior horns: location of sensory neurons
    • lateral horns: location of autonomic neurons
      • the lateral horns are only present in the thoracic, upper lumbar and sacral regions of the spinal cord
    • there is more grey matter in the cervical and lumbar regions

• 

  Spinal cord regions: 1. Central canal, 2. Posterior median sulcus, 3. Grey matter, 4. White matter, 5. Dorsal root and dorsal root ganglion, 6. Ventral root, 7. Fascicles, 8. Anterior spinal artery, 9. Arachnoid mater, 10. Dura mater.

  white matter surrounds the grey matter:
  • divided into the posterior (dorsal) column, anterior (ventral) column and the lateral column
  • fibres decussate through the anterior white commissure
  • these tracts are largely made up of myelinated sensory and motor axons
    • ascending tracts: transmit information from the sensory receptors in the periphery for processing in the central nervous system
    • descending tracts: transmit information to the periphery from the central nervous system

Spinal Nerves

Spinal nerve.

Spinal nerves are "mixed nerves that interact directly with the spinal cord to modulate motor and sensory information from the body's periphery."

Humans have 31 pairs of spinal nerves:

• eight cervical
• twelve thoracic
• five lumbar
• five sacral
• one coccygeal

Each pair roughly corresponds to a segment of the vertebral column. While there are seven cervical vertebrae, there are eight pairs of cervical spinal nerves:

• the first pair of spinal nerves exits above the first cervical vertebra
• the eighth pair exits below the last cervical vertebra

Spinal cord and spinal nerves

From the thoracic region, each spinal nerve exits below its corresponding vertebra.

The spinal nerves are relatively large. They form from nerve fibres (fila radicularia) that extend from the posterior (dorsal) root and anterior (ventral) root of the spinal cord:

• posterior (dorsal) root:
  • contains sensory nerve fibres
  • synapses at the dorsal horn of the spinal cord, but the sensory cell bodies of these neurons are located in the dorsal root ganglia
  • enters the spinal cord via the posterolateral sulcus
• anterior (ventral) root:
  • contains motor and preganglionic autonomic fibres
  • cell bodies of motor fibres are situated in the anterior horn of the spinal cord (anterior and lateral grey columns)
  • exits the spinal cord from the anterolateral sulcus

The anterior and posterior roots join to form two spinal nerves, one on either side of the spine, which exit the vertebral canal through the intervertebral foramina. Once outside the intervertebral foramina, they form peripheral nerves.

Spinal Cord Muscle Innervation

Please see Spinal Cord Muscle Innervation for muscle innervation reference tables.

Spinal Motor Neurons

Alpha and Gamma motor neurons are both found in the anterior (ventral) horn. 

• Alpha motor neurons are the largest motor neurons in the nervous system and innervate skeletal muscle
• Gamma motor neurons innervate intrafusal muscle fibres of the muscle spindle

Motor neurons are arranged somatotopically across the anterior horn. The more medially placed motor neurons innervate proximal muscles while laterally placed motor neurons innervate distal muscles.

Associated Pathways

Brain and spinal cord tracts

The central nervous system uses ascending and descending pathways to communicate with the external environment. The tracts and pathways complete the circuit in the spinal cord.

Ascending tracts:

• dorsal column medial lemniscal pathway
  • carries fine touch, two-point discrimination, conscious proprioception, and vibration sensations from the periphery
• spinothalamic tracts
  • lateral spinothalamic tract carries information about pain and temperature
  • anterior spinothalamic tract carries information about crude touch and pressure
• spinocerebellar tracts
  • located in the lateral column and carry proprioceptive information

Descending tracts:

• corticospinal tract
  • one of the two main divisions of the pyramidal tract
  • responsible for voluntary movements of the trunk and limbs
• corticonuclear (or corticobulbar) tract
  • the other main division of the pyramidal tract
  • provides motor output for cranial nerves to the head and face
• vestibulospinal tract
  • involved in maintaining posture and balance and adjusting head movements
• tectospinal tract
  • not well understood, but likely involved in turning the head in response to visual stimuli
• reticulospinal tract
  • plays an important role in motor and postural control
• rubrospinal tract
  • involved in regulating flexor muscle tone

Please see Central Nervous System Pathways if you would like to learn more.

Clinical Pearls: Spinal Reflexes

Spinal reflexes are involuntary actions to stimuli involving simple circuits at the level of the spinal cord. They allow for near-instantaneous responses because they can bypass processing from the brain. Although spinal reflexes can bypass the cortical circuit, the descending circuit of the brainstem and cortex can still modulate the intensity of the response.

These include:

1. myotatic or stretch reflex (also known as the jerk reflex): occurs when muscle spindle stretch receptors are activated. Information is sent along the afferent neuron to the spinal cord where it synapses on the efferent neuron to the same muscle, causing that muscle to contract in response. At the same time, reciprocal inhibition occurs. The afferent neurons also synapse with inhibitory interneurons in the spinal cord that prevent contraction of the antagonistic muscles.
2. flexor withdrawal reflex: occurs in response to noxious stimuli. For example: when you touch a hot object, a pain receptor in your hand picks up the noxious stimulus and the afferent neuron carries the impulse to the spinal cord where it synapses on an interneuron. The interneuron then synapses with the efferent neuron, which sends a signal to the muscle to withdraw your hand.

Blood Supply

The blood supply to the spinal cord "is highly complex and anastomotic to ensure adequate supply to the entire structure. Variability in the vasculature is not uncommon."

Spinal arteries: The spinal arteries arise from the vertebral artery and run longitudinally down the spinal cord. Three major arteries supply the spinal cord:

• unpaired anterior spinal artery lies in the anterior median fissure of the spinal cord and extends from the level of the lower brainstem to the tip of the conus medullaris. It supplies the ventral medial surface of the medulla and anterior two-thirds of the spinal cord. Its diameter is narrower through the thoracic spine when compared to the cervical and lumbar spine. Therefore, much of the blood flow to the thoracic spine comes from segmental arteries that branch off from the dorsal aorta.
• paired posterior spinal arteries supply the posterior one-third of the spinal cord, the posterior (dorsal) columns and much of the dorsal horns (apart from their base).
• the anterior spinal artery and posterior spinal arteries join circumferentially through the pial plexus (also called the vasocorona) and supply the spinal cord's periphery.

Segmental arteries: the blood supply for the spinal cord is reinforced by segmental, medullary, and radicular arteries. These come from spinal branches of the cervical, posterior intercostal, and lumbar arteries. The segmental medullary arteries supply the anterior or posterior spinal arteries, while radicular arteries mainly supply the spinal nerve roots.

Occlusion of the anterior spinal artery can lead to a loss of power or paralysis and spinothalamic sensory deficit (i.e. pain and temperature sensation are affected). Fine touch, two-point discrimination, vibration and proprioception, which are transmitted by the posterior (dorsal) columns, are maintained. Occlusion of the posterior spinal artery affects the posterior (dorsal) column, leading to posterior cord syndrome. This presents with a loss of proprioception and vibration, lack of deep tendon reflexes, hypotonia, and ataxic gait. Anterior spinal artery occlusion is more common than posterior spinal artery blockage.

Posterior spinal cord, great posterior radiculomedullary artery

Spinal veins: blood from within the spinal cord travels through the intramedullary veins, to the anterior and posterior spinal veins. They form a reticulated network in the pia mater around the circumference of spinal cord:

• the anterior spinal vein drains a significant portion of the gray matter of the spinal cord.
• the posterior and lateral spinal veins run the length of the entire spinal cord and primarily drain the white matter and portions of the grey matter not drained by the anterior spinal vein. These vessels empty through the radicular veins into the external and internal vertebral venous plexuses, groups of valveless veins that extend from the coccyx to the base of the skull.

• the vertebral and deep cervical veins drain venous blood from the cervical spine into the superior vena cava; the posterior intercostal and lumbar veins drain venous supply from the thoracic and lumbar spine into the azygos and hemiazygos veins; and the median and lateral sacral veins drain venous supply from the sacrum into the common iliac vein.