Spinal cord | Anatomy2Medicine
Spinal cord

Spinal cord

 

  • Spinal Cord

 

      • weighs about 30 g, comprising 2% of the weight of the adult brain.

 

  • averages, in length, 45 cm in males and 42 cm in females.
  • extends, in adults, from the foramen magnum to the lower border of the first lumbar vertebra (MCQ)

 

      • extends in newborns, it extends to the third lumbar vertebra(MCQ)

 

  • lies within the subarachnoid space, which extends caudally to the level of the second sacral vertebra

 

      • has cervical (C5–T1) and lumbar (L1–S2) enlargements for the nerve supply of the upper and lower extremities (the brachial and lumbosacral plexuses).
      • terminates caudally as the conus medullaris.
    • Spinal nerves
      • consist of 31 pairs of nerves that emerge from the spinal cord:
        • 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
      • contain both motor and sensory fibers.
      • The first cervical nerve and the coccygeal nerve usually have no dorsal (sensory) roots and no corresponding dermatomes.
      • The first cervical nerve passes between the atlas and the skull.
      • The second cervical nerve passes between the atlas and the axis.

 

  • With the exception of C1, spinal nerves exit the vertebral canal via intervertebral or sacral foramina.
  • Functional components of spinal nerve fibers

 

        • General somatic afferent (GSA) fibers
          • convey sensory input from skin, muscle, bone, and joints to the central nervous system (CNS).
        • General visceral afferent (GVA) fibers
          • convey sensory input from visceral organs to the CNS.
        • General somatic efferent (GSE) fibers
          • convey motor output from ventral horn motor neurons to skeletal muscle.
        • General visceral efferent (GVE) fibers

 

  • convey motor output from intermediolateral cell column neurons, via paravertebral or prevertebral ganglia, to glands, smooth muscle, and visceral organs (sympathetic divisions of the autonomic nervous system).
  • convey motor output from the sacral parasympathetic nucleus to the pelvic viscera via intramural ganglia.
  • Components and branches of spinal nerves

 

      • The spinal nerve is formed by the union of dorsal and ventral roots within the intervertebral foramen, resulting in a mixed nerve.

 

  • Dorsal root

 

        • enters the dorsal lateral sulcus as dorsal rootlets
        • conveys sensory input from the body via the dorsal root ganglion.

 

  • contains, distally, the dorsal root ganglion.

 

        • joins the ventral root distal to the dorsal root ganglion and within the intervertebral foramen to form the spinal nerve.

 

  • Dorsal root ganglion

 

        • is located within the dorsal root and within the intervertebral foramen.

 

  • contains pseudounipolar neurons of neural crest origin, which transmit sensory input from the periphery (GSA and GVA) to the spinal cord via the dorsal roots.
  • Ventral root

 

        • emerges as ventral rootlets from the ventral lateral sulcus
        • conveying motor output from visceral and somatic motor neurons.
        • joins the dorsal roots distal to the dorsal root ganglion and within the intervertebral foramen to form the spinal nerve.

 

  • Cauda equina

 

        • consists of lumbosacral (dorsal and ventral) nerve roots (L2–Co)
        • descend from the spinal cord through the subarachnoid space to exit through their respective intervertebral or sacral foramina.

 

  • Spinal nerve rami
  • Dorsal primary ramus

 

          • innervates the skin and muscles of the back.

 

  • Ventral primary ramus

 

          • innervates the ventral lateral muscles and skin of the trunk, extremities, and visceral organs.

 

  • Meningeal ramus

 

          • innervates the meninges and vertebral column.

 

  • Gray communicating rami

 

          • contain unmyelinated postganglionic sympathetic fibers.

 

  • are associated with all spinal nerves. (MCQ)
  • White communicating rami
  • contain myelinated preganglionic sympathetic fibers and myelinated GVA fibers (splanchnic nerves).

 

          • are found only in thoracolumbar segments of the spinal cord (T1–L3) (MCQ)

 

  • Internal Morphology of Spinal cord
  • In transverse sections, the spinal cord consists of central gray matter and peripheral white matter.
  • Gray matter

 

        • is in the center of the spinal cord.
        • is butterfly- or H-shaped in a configuration
        • contains a central canal.
        • is divided into cytoarchitectural areas called Rexed laminae
        • is divided into three horns or cell columns on each side:

 

  • Dorsal horn (column)

 

          • receives and processes sensory input
          • is found at all levels.

 

  • Dorsomarginal nucleus (Rexed lamina I)

 

            • is found at all cord levels.

 

  • is associated with light touch, pain, and temperature sensation.
  • is one site of origin of the ventral and lateral spinothalamic tracts.
  • Substantia gelatinosa (Rexed lamina II)

 

            • is found at all cord levels.
            • is homologous to the spinal trigeminal nucleus.
            • is associated with light touch, pain, and temperature sensation
            • integrates input for the ventral and lateral spinothalamic tracts.

 

  • Nucleus proprius (Rexed laminae III and IV)

 

            • is found at all cord levels.

 

  • is associated with light touch, pain, and temperature sensation.

 

            • gives rise (from Rexed lamina IV) to the ventral and lateral spinothalamic tracts.

 

  • Nucleus dorsalis of Clarke (Rexed lamina VII)

 

            • is found at the base of the dorsal horn.
            • extends from (C8) T1 to L3.

 

  • is homologous to the accessory cuneate nucleus of the medulla.
  • subserves unconscious proprioception from muscle spindles and Golgi tendon organs (GTOs).

 

            • is the origin of the dorsal spinocerebellar tract.

 

  • Lateral horn (column) (Rexed lamina VII)

 

          • receives viscerosensory input.
          • is found between the dorsal and ventral horns.
          • extends from (C8) T1 to L3.

 

  • contains the intermediolateral nucleus (column)

 

            • a visceromotor nucleus that extends from T1 to L3
          • contains preganglionic sympathetic neurons (GVE).
          • the ciliospinal center of Budge
            • present at T1–T2
            • provide sympathetic innervation of the eye).

 

  • Ventral horn (column) (Rexed laminae VII, VIII, and IX)

 

        • contains predominantly motor nuclei.
        • is found at all levels.
        • includes the following nuclei:
          • Spinal border cells (Cooper-Sherrington border cells)
            • extend from L2 to S3.
            • subserve unconscious proprioception from GTOs and muscle spindles.
            • are the origin of the ventral spinocerebellar tract.

 

  • Sacral parasympathetic nucleus (Rexed lamina VII)

 

            • extends from S2 to S4.

 

  • gives rise to preganglionic parasympathetic fibers that innervate the pelvic viscera via the pelvic nerve.
  • Somatic motor nuclei (Rexed lamina IX)

 

            • are found at all levels.

 

  • are subdivided into medial and lateral groups that innervate axial and appendicular muscles, respectively.
  • Spinal accessory nucleus (Rexed lamina IX)

 

            • extends from C1 to C6.

 

  • gives rise to the spinal root of the spinal accessory nerve (cranial nerve [CN] XI).

 

            • innervates the sternocleidomastoid and trapezius muscles.

 

  • Phrenic nucleus (Rexed lamina IX)

 

            • extends from C3 to C6.
            • innervates the diaphragm
      • White matter
        • consists of bundles of myelinated fibers that surround the central gray matter.
        • consists of ascending and descending fiber pathways called tracts.
        • is divided bilaterally by sulci into three major divisions.

 

  • Dorsal funiculus (dorsal column)

 

            • is subdivided above T6 into two fasciculi:

 

  • Fasciculus gracilis

 

              • is found at all cord levels

 

  • Fasciculus cuneatus

 

              • is found only at C1–T6

 

  • Lateral funiculus

 

            • is located between the dorsal lateral and ventral lateral sulci.

 

  • Ventral funiculus

 

            • is located between the central canal and the ventral medial fissure
            • contains decussating spinothalamic tracts (involved in damage by Syringomyelia)
      • Determination of spinal cord levels
      • Cervical cord
        • ventral horns are massive from C3 to C8.
      • Thoracic cord

 

  • The nucleus dorsalis of Clarke is present at all thoracic levels but is most prominent at T11 and T12

 

        • Dorsal and ventral horns are typically slender  and H-shaped
      • Lumbar cord

 

  • The nucleus dorsalis of Clarke is very prominent at L1 and L2.

 

        • contains massive ventral and dorsal horns from L2 to L5
        • the substantia gelatinosa is greatly enlarged.
        • The lateral horn is prominent only at L1.
      • Sacral cord
        • contains massive ventral and dorsal horns
        • the substantia gelatinosa is greatly enlarged.

 

  • is greatly reduced in diameter from S3 to S5.

 

      • Coccygeal segment
        • contains dorsal horns that are more voluminous than the ventral horns.

 

  • Myotatic Reflex

 

        • is a monosynaptic and ipsilateral muscle stretch reflex (MSR
        • is incorrectly called a deep tendon reflex.
        • has an afferent and an efferent limb, like all reflexes.
        • Interruption of either limb results in areflexia.
        • Afferent limb includes
          • muscle spindle (receptor)
          • a dorsal root ganglion neuron and its Ia fiber.
        • Efferent limb includes
          • a ventral horn motor neuron that innervates striated muscle (effector).
        • The five most commonly tested MSRs

 

  • Ankle jerk (Achilles reflex):

 

            • cord segment S 1
            • gastrocnemius muscle

 

  • Knee jerk

 

            • cord segments L2 to L4
            • quadriceps muscle

 

  • Biceps jerk

 

            • cord segments C5 and C6
            • biceps muscle

 

  • Forearm jerk

 

            • cord segments C5 and C6
            • brachioradialis muscle

 

  • Triceps jerk

 

          • cord segments C7 and C8
          • triceps muscle