Major Somatosensory and Motor Tracts Review

Overview

• Lecture reviews major long tracts connecting cortex and spinal cord.
• Focus on three main systems: dorsal column–medial lemniscus (DCML), anterolateral (spinothalamic), and corticospinal (descending motor).
• Also covers trigeminal pathways for head sensation.
• Emphasis on entry points, synapse locations, decussation sites, and clinical deficits.

Dorsal Column–Medial Lemniscus (DCML)

• Carries proprioception, fine touch, and vibration from body.
• First-order cell bodies: dorsal (posterior) root ganglia (pseudo-unipolar neurons).
• First-order axons enter spinal cord and ascend ipsilaterally in dorsal columns.
• Two major fasciculi:
  • Fasciculus gracilis: medial, carries input from lower limb and lower torso.
  • Fasciculus cuneatus: lateral, carries input from upper limb and upper torso.
• Synapse location: gracile and cuneate nuclei in the caudal (inferior) medulla.
• Decussation: internal arcuate fibers in the medulla; all DCML fibers cross at once.
• After crossing: axons form the medial lemniscus and ascend to thalamus.
• Thalamic relay: VPL (ventral posterolateral) nucleus.
• Third-order axons: VPL → primary somatosensory cortex (postcentral gyrus), somatotopically organized.
• Clinical notes:
  • Lesion in spinal cord dorsal columns: ipsilateral loss of vibration, proprioception, fine touch below lesion.
  • Lesion above medullary decussation: contralateral loss.
  • Lesion at decussation: bilateral loss of proprioception/vibration/fine touch.

Anterolateral System (Spinothalamic Tract)

• Carries pain and temperature (and crude touch) from the body.
• Receptors: free nerve endings and lightly myelinated fibers.
• First-order cell bodies: dorsal root ganglia; axons enter posterior horn.
• Synapse location: dorsal (posterior) horn (substantia gelatinosa/other dorsal horn neurons).
• Decussation: second-order axons cross near entry level via anterior white commissure (may ascend one or two segments before crossing).
• Ascending tract: spinothalamic tract (part of anterolateral system) in contralateral anterolateral spinal cord.
• Thalamic relay: VPL nucleus.
• Third-order axons: VPL → primary somatosensory cortex, somatotopically organized.
• Clinical notes:
  • Lesion of anterolateral tract in spinal cord: contralateral loss of pain and temperature below lesion.
  • Lesion above decussation: contralateral deficits persist.

Trigeminal Sensory Pathways (Face)

• Trigeminal nerve carries facial sensation via three branches: V1 (ophthalmic), V2 (maxillary), V3 (mandibular).
• Peripheral ganglion: trigeminal (semilunar) ganglion — analogous to dorsal root ganglia.
• Two main trigeminal sensory streams:
  • Trigeminal lemniscus: vibration, proprioception, fine touch from face.
    • Synapse in chief (principal) sensory nucleus (pons).
    • Second-order axons decussate and ascend to VPM.
  • Trigeminal spinal tract/tract of trigeminal lemniscus (spinal trigeminal nucleus and tract): pain and temperature from face.
    • Pain and temperature fibers descend in spinal trigeminal tract to synapse in spinal trigeminal nucleus (extends from pons to upper cervical spinal cord).
    • Second-order axons decussate and ascend as trigeminothalamic tract to VPM.
• Thalamic relay: VPM (ventral posteromedial) nucleus.
• Third-order axons: VPM → facial region of primary somatosensory cortex.
• Clinical notes:
  • Face somatosensory modalities are separated: lemniscus for fine touch/vibration; trigeminal spinal tract for pain/temperature.

Corticospinal Tract (Descending Motor)

• Main descending motor pathway for voluntary movement; especially fine distal control.
• Upper motor neurons: primary motor cortex (precentral gyrus), somatotopically organized.
• Pathway: internal capsule → corona radiata → brainstem (through pons) → medullary pyramids (anterior medulla).
• Decussation: majority (about 85–90%) of fibers decussate in lower medulla (pyramidal decussation), forming lateral corticospinal tract.
• Lateral corticospinal tract: descends contralaterally in lateral funiculus; controls distal limb muscles and fine motor control.
• Anterior (ventral) corticospinal tract: small proportion do not decussate at medulla; descend ipsilaterally and often decussate segmentally near spinal level; involved in proximal/trunk control.
• Synapse: lateral corticospinal fibers synapse on lower motor neurons in anterior (ventral) horn; lower motor neurons project via anterior root to peripheral nerves and muscles.
• Clinical distinctions:
  • Lower motor neuron lesion (peripheral nerve or anterior horn/LMN): flaccid paralysis, decreased tone, hyporeflexia, possible fasciculations, muscle atrophy.
  • Upper motor neuron lesion (corticospinal): spastic weakness, increased tone, hyperreflexia, clonus, Babinski sign; loss of coordinated fine movement.

Key Terms and Definitions

• Decussation: crossing of axons from one side of the CNS to the other.
• Fasciculus gracilis: dorsal column tract for lower limb/lower torso.
• Fasciculus cuneatus: dorsal column tract for upper limb/upper torso.
• Internal arcuate fibers: site where DCML second-order axons cross in medulla.
• Medial lemniscus: ascending tract after DCML decussation.
• VPL (ventral posterolateral nucleus): thalamic relay for body somatosensation.
• VPM (ventral posteromedial nucleus): thalamic relay for face somatosensation.
• Anterior white commissure: spinal decussation site for spinothalamic fibers.
• Spinothalamic tract: major anterolateral ascending tract for pain/temperature.
• Pyramidal tracts: corticospinal fibers forming medullary pyramids.
• Lateral corticospinal tract: crossed corticospinal fibers for limb/fine motor control.
• Anterior corticospinal tract: uncrossed/segmentally crossing fibers for trunk/proximal control.
• Trigeminal ganglion: sensory ganglion for trigeminal nerve (face).

Action Items / Clinical Relevance

• For spinal lesions:
  • Lesion of dorsal columns → ipsilateral loss of proprioception/vibration (below level).
  • Lesion of anterolateral tract → contralateral loss of pain/temperature (below level).
• For brainstem/above-medulla lesions:
  • DCML lesion above medullary decussation → contralateral loss of proprioception/vibration.
  • Corticospinal lesion above pyramidal decussation → contralateral motor deficits.
• To differentiate LMN vs UMN lesions clinically:
  • LMN: flaccid paralysis, hyporeflexia, fasciculations.
  • UMN: spastic paralysis, hyperreflexia, increased tone.
• Remember somatotopy: both sensory and motor cortical areas preserve body map (face, upper limb, trunk, lower limb).

Summary Points

• Three principal long tract functions: DCML (fine touch, vibration, proprioception), spinothalamic (pain/temperature), corticospinal (voluntary motor).
• Decussation timing determines ipsilateral vs contralateral deficits after lesions.
• Trigeminal pathways separate modalities similarly for the face, relaying to VPM.
• Clinical localization uses tract modality, laterality, and lesion level to predict deficits.