TRACTS OF THE SPINAL CORD | Anatomy2Medicine
tracts-of-spinal-cord

TRACTS OF THE SPINAL CORD

 

  • Tracts of the Spinal Cord

 

      • consist of fiber bundles that have a common origin and a common termination
      • are somatotopically organized.
      • are divided into ascending and descending pathways.

 

  • Ascending Spinal Tracts
  • represent functional pathways that convey sensory information from soma or viscera to higher levels of the neuraxis.

 

        • usually consist of a chain of three neurons: first-, second-, and third-order neurons.
        • The first- order neuron is always in the dorsal root ganglion.
        • may decussate before reaching their final destination.
        • give rise to collateral branches that serve in local spinal reflex arcs.
        • include six major tracts
        • Dorsal column–medial lemniscus pathway

 

  • mediates tactile discrimination, vibration, form recognition, and joint and muscle sensation.

 

          • mediates conscious proprioception.
          • receives input from Pacini and Meissner corpuscles, joint receptors, muscle spindles, andGolgi tendon organs (GTOs)

 

  • First-order neurons

 

            • are located in dorsal root ganglia at all levels.
            • give rise to the fasciculus gracilis from the lower extremity.
            • give rise to the fasciculus cuneatus from the upper extremity.

 

  • give rise to axons that ascend in the dorsal columns and terminate in the gracile and cuneate nuclei of the medulla.
  • Second-order neurons

 

            • are located in the gracile and cuneate nuclei of the caudal medulla.
            • give rise to axons, internal arcuate fibers that decussate and form a compact fiber bundle, the medial lemniscus.
            • The medial lemniscus ascends through the contralateral brainstem to terminate in the ventral posterolateral (VPL) nucleus of the thalamus.

 

  • Third-order neurons

 

            • are located in the VPL nucleus of the thalamus.
            • project via the posterior limb of the internal capsule to the postcentral gyrus, the

 

  • somatosensory cortex (areas 3, 1, and 2).

 

        • Ventral spinothalamic tract

 

  • along with lateral spinothalamic tract comprises anterolateral system.

 

          • is concerned with light touch, the sensation produced by stroking glabrous skin with a wisp of cotton.
          • receives input from free nerve endings and from Merkel tactile disks.

 

  • First-order neurons

 

            • are found in dorsal root ganglia at all levels.
            • project axons into the medial root entry zone to second-order neurons in the dorsal horn.

 

  • Second-order neurons
  • are located in the dorsal horn.
  • give rise to axons that decussate in the ventral white commissure and ascend in the contralateral ventral funiculus.

 

            • terminate in the VPL nucleus of the thalamus.

 

  • Third-order neurons

 

            • are found in the VPL nucleus of the thalamus.
            • project via the posterior limb of the internal capsule and corona radiata to the post- central gyrus (areas 3, 1, and 2)
        • Lateral spinothalamic tract
          • mediates itch, pain, and temperature sensation.
          • receives input from free nerve endings and thermal receptors.
          • receives input from A- delta and C fibers (i.e., fast- and slow-conducting pain fibers, respectively).

 

  • is somatotopically organized with sacral fibers dorsolaterally and cervical fibers ventromedially.
  • First-order neurons

 

            • are found in dorsal root ganglia at all levels.

 

  • project axons via the dorsolateral tract of Lissauer to second-order neurons in the dorsal horn.

 

            • synapse with second-order neurons in the dorsal horn.

 

  • Second-order neurons

 

            • are found in the dorsal horn.
            • give rise to axons that decussate in the ventral white commissure and ascend in the ventral half of the lateral funiculus.
            • project collaterals to the reticular formation.

 

  • terminate contralaterally in the VPL nucleus and bilaterally in the intralaminar nuclei of the thalamus.
  • Third-order neurons

 

            • are found in the VPL nucleus and in the intralaminar nuclei.

 

  • VPL neurons
  • project via the posterior limb of the internal capsule to the somatesthetic cortex of the postcentral gyrus (areas 3, 1, and 2).
  • Intralaminar neurons

 

              • project to the caudatoputamen and to the frontal and parietal cortex.
        • Dorsal spinocerebellar tract
          • transmits unconscious proprioceptive information to the cerebellum.

 

  • receives input from muscle spindles, GTOs, and pressure receptors.

 

          • is involved in fine coordination of posture and the movement of individual muscles of the lower extremity.
          • is an uncrossed tract.

 

  • First-order neurons

 

            • are found in dorsal root ganglia from C8 to S3.
            • provide the afferent limb for muscle stretch reflexes (MSRs) (e.g., the patellar reflex)
            • project via the medial root entry zone to synapse in the nucleus dorsalis of Clarke.
          • Second-order neurons
            • are found in the nucleus dorsalis of Clarke (C8–L3).
            • give rise to axons that ascend in the lateral funiculus
            • reach the cerebellum via the inferior cerebellar peduncle.

 

  • contain axons that terminate ipsilaterally as mossy fibers in the cortex of the rostral
  • and caudal cerebellar vermis.

 

        • Ventral spinocerebellar tract
        • transmits unconscious proprioceptive information to the cerebellum.
        • is concerned with coordinated movement and posture of the entire lower extremity.
        • receives input from muscle spindles, GTOs, and pressure receptors.
        • is a crossed tract.

 

  • First-order neurons

 

          • are found in the dorsal root ganglia from L1 to S2.

 

  • provide the afferent limb for MSRs (e.g., the patellar reflex)

 

          • synapse on spinal border cells.

 

  • Second-order neurons
  • are spinal border cells found in the ventral horns (L1–S2).

 

          • give rise to axons that decussate in the ventral white commissure
          • ascend lateral to the lateral spinothalamic tract in the lateral funiculus.

 

  • give rise to axons that enter the cerebellum via the superior cerebellar peduncle

 

          • terminate contralaterally as mossy fibers in the cortex of the rostral cerebellar vermis.
        • Cuneocerebellar tract
          • is the upper-extremity equivalent of the dorsal spinocerebellar tract.

 

  • First-order neurons

 

            • are found in the dorsal root ganglia from C2 to T7.
            • project their axons via the fasciculus cuneatus to the caudal medulla, where they

 

  • synapse in the accessory cuneate nucleus, a homolog of the nucleus dorsalis of Clarke.
  • Second-order neurons

 

            • are located in the accessory cuneate nucleus of the medulla.

 

  • give rise to axons that project to the cerebellum via the inferior cerebellar pedun-
  • cle.
  • These axons terminate ipsilaterally in the arm region of the anterior lobe of the cerebellum.
  • Descending Spinal Tracts
  • are concerned with somatic and visceral motor activities.

 

      • have their cells of origin in the cerebral cortex or in the brainstem.
      • Lateral corticospinal (pyramidal) tract
        • is not fully myelinated until the end of the second year.

 

  • Functions
  • is concerned with volitional skilled motor activity, primarily of the digits of the upper limb.
  • modulates the transmission of sensory input via the ascending sensory pathways.

 

          • receives input from the paracentral lobule, a medial continuation of the motor and sensory cortices
            • subserves the muscles of the contralateral leg and foot.

 

  • Origin and termination: the lateral corticospinal tract

 

          • arises from lamina V of the cerebral cortex from three cortical areas, in equal proportions
            • the premotor cortex (area 6)
            • the precentral motor cortex (area 4)
            • postcentral sensory cortex (areas 3, 1, and 2).

 

  • terminates via interneurons on ventral horn motor neurons and sensory neurons of the dorsal horn.
  • Fibers of the lateral corticospinal tract

 

          • number approximately 1 million.
          • are usually (90%) between 1 and 4 μm in diameter
          • are usually (67%) myelinated
          • represent 4% of the fibers of the tract, with diameters greater than 20 μm.
            • These are the axons of the giant cells of Betz.

 

  • Betz cells are found in the precentral gyrus and in the anterior paracentral lobule.
  • Course: the lateral corticospinal tract

 

          • passes through the posterior limb of the internal capsule.
          • passes through the middle three-fifths of the crus cerebri (basis pedunculi) of the midbrain.
          • passes through the base of the pons.
          • constitutes the pyramid of the medulla.
          • undergoes a 90% decussation in the caudal medulla.
          • lies in the dorsal quadrant of the lateral funiculus of the spinal cord.

 

  • Transection

 

        • results in spastic hemiparesis with the Babinski sign.
    • Ventral corticospinal tract
      • is a small uncrossed tract
      • decussates at spinal cord levels in the ventral white commissure.
      • is concerned with the control of axial muscles.
    • Rubrospinal tract
        • arises in the contralateral red nucleus of the midbrain.
        • plays a role in the control of flexor tone.
              • is ventral to the lateral corticospinal tract.
            • Vestibulospinal tract
              • arises from the giant cells of Deiters in the ipsilateral lateral vestibular nucleus
              • plays a role in the control of extensor tone.
              • is located in the ventral funiculus.
            • Descending autonomic tracts
              • project to sympathetic (T1–L3) and parasympathetic (S2–S4) centers in the spinal cord.

    • innervate the ciliospinal center (T1–T2) of Budge
              • a pupillary center
              • interruption of this hypothalamospinal tract (found in the dorsal quadrant of the lateral funiculus) results in Horner syndrome.