Motor Tracts

Summary

• Overview
• The descending (motor) tracts:
  1. Pyramidal:
     • Corticospinal
       • Lateral corticospinal tract
       • Anterior corticospinal tract
     • Corticobulbar
  2. Extra-pyramidal:
     • Tectospinal
     • Vestibulospinal
     • Rubrospinal
     • Reticulospinal
     • Olivospinal

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Summary Table

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Overview of the Descending Tracts

• Required for control of:
  • Skeletal muscle movement
  • Muscle tone
  • Spinal reflexes
  • Modulation of sensory transmission
  • Motor autonomic functions

• There are two types of tracts:
  • Pyramidal – these are associated with voluntary movement
    • Corticospinal – discrete voluntary movement of body
      • Lateral corticospinal tract – limb muscles
      • Anterior corticospinal tract – axial muscles
    • Corticobulbar – voluntary movement of muscles of the face and neck (via cranial nerves)
  • Extra-pyramidal – generally associated with  postural tone
    • Tectospinal – reflexes in response to visual stimuli
    • Vestibulospinal – promotes antigravity action (stimulates extensors/inhibits flexors)
    • Rubrospinal – promotes flexor activity of the upper limb
    • Reticulospinal – modulates reflex activity
    • Olivospinal

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Corticospinal Tract

Structure

• Made up of 2 parts
  1. Lateral CT 
  2. Ventral/Anterior CT

• Functions
  • Control of discrete voluntary movements:
    • Lateral CT – distal limb (especially flexors)
    • Ventral/Anterior CT – Proximal/axial muscles
      Innervates a-motor neurones only (unlike other tracts)\
  • Some role in modulation of local reflexes
    • Dampens down local reflexes

• Pathway:
  • UMN originates in 1o motor cortex
    • Some fibres from 2o sensory cortex
  • Fibres pass through the internal capsule
  • Decussation
    • 85% of fibres decussate at medulla – These become the lateral corticospinal tract
    • Remaining 15% continue ipsilaterally as the medial/ventral corticospinal tract
    • Medial tract decussates at level of synapse
  • Both synapse with their LMN at the ventral horn of the spinal level where they exit
    • 55% cervical
    • 20% thoracic
    • 25% lumbar
    • (Most of the ventral corticospinal tract in particular terminates in thoracic spinal cord)

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Corticobulbar tract

• Functions
  • Controls muscles of face and neck
    • Innervates bilaterally (except for lower facial nuclei)
  • Directly controls CNV, CNVII, CNXII
  • Indirectly controls CNIII, CNIV, CNVI (via interneurons)

• Structure – see diagram
  • Varies depending on each nerve

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Tectospinal tract

• Functions:
  1. Contraction of neck muscles on contralateral side (toward stimulus)
     • E.g. right TST contracts left neck muscles to turn head to the left
  2. Orientates head and neck to auditory/visual stimuli
     • E.g. keeps head straight when bending over laterally

• Pathway:
  1. Originates from superior colliculus
     • Rostral aspect of midbrain
     • Receives visual and auditory afferents
  2. Decussates in dorsal tegmentum (also in midbrain)
  3. Descends ventromedially alongside medial vestibulospinal tract
  4. Predominantly terminates in cervical spinal cord
     • Control muscles of head/neck

 

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Vestibulospinal tracts

• 2 tracts:
  1. Lateral vestibulospinal tract (supplies lower limb)
  2. Medial vestibulospinal tract (supplies head and neck)

• Functions
  • Antigravity action: stimulate extensors and inhibit flexors (of the legs/neck in particular) – remember as vestibular system being responsible for balance
    1. Lateral – Lower limb extension
    2. Medial – Neck extension

• Pathway
  1. Originate in vestibular nuclei in pons/medulla
     • Lateral – Deiter’s nucleus (lateral vestibular nucleus)
     • Medial – medial vestibular nucleus
     • Receive afferent input from labrynthine system via CNVIII/cerebellum
  2. Do not decussate:
     • Therefore remain ipsilateral
  3. Terminate in:
     1. Lumbar spine (lateral tract) => leg extensors
     2. Cervical spine (medial tract) => neck extensors

 

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Rubrospinal tract

• Functions:
  • Maintains tone of limb flexors – inhibits anti-gravity extensors
    • Particularly arm
    • Essentially opposite in function to the vestibulospinal tracts

• Pathway
  • Originates in red nucleus (in midbrain tegmentum)
  • Descends ventromedially
  • Decussates in ventral tegmentum
    • Becomes ventrolateral after decussation
  • Terminates in cervical and lumbar spine
    • Only innervates limb LMNs

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Reticulospinal

• 2 parts:
  1. Medullary RST => excites antigravity extensors  (like vestibulospinal)
  2. Pontine RST  => inhibits antigravity extensors (like rubrospinal)

• Other functions
  • Modulates voluntary movement and muscle tone
  • Modulates CVS
  • Modulates pain impulses
  • Mediates autonomic functions

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Decorticate + Decerebrate lesions

• A key phenomenon is decortication
  • This is where the upper limbs become flexed, and lower limbs extended
  • Normally, arm extension is facilitated by:
    1. The vestibulospinal tracts
    2. The medullary reticulospinal tract
  • Arm flexion is facilitated by
    1. The rubrospinal tract
    2. The pontine reticulospinal tract
       • Normally suppressed by the corticospinal tract
  • With an injury above the brainstem, the the tracts responsible for arm flexion are disinhibited (due to loss of the corticospinal tract) and overcome those causing arm extension
    • With decerebrate lesions (i.e. the brainstem is also involved), the rubrospinal tract is also lost. Therefore, an extension posture can develop