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

Summary Table


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
Diagram showing the key descending (and ascending) tracts.

Corticospinal Tract

Structure
  • Made up of 2 parts
    1. Lateral CT 
    2. Ventral/Anterior CT
  • Functions
    • Control of discrete voluntary movements:
      • Lateral CTdistal limb (especially flexors)
      • Ventral/Anterior CTProximal/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 medullaThese 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)
The anatomy of the anterior and lateral corticospinal tracts

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
Anatomy of the corticobulbar tract. Note the differences in the pathways for different nerves.

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
Image showing the path of the tectospinal tract.

 


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. MedialNeck extension
  • Pathway
    1. Originate in vestibular nuclei in pons/medulla
      • LateralDeiter’s nucleus (lateral vestibular nucleus)
      • Medialmedial 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
Paths of the vestibulospinal tracts.

 


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


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

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