DESCENDING MOTOR PATHWAYS / DESCENDING TRACTS

Connect the neurons in,

cerebral cortex , Midbrain, Pons, Medulla & Cerebellum

with

motor neurons (LMN) at various levels.

Axons of UMN synapse on spinal : Alpha motor neuron,gamma motor neuron, Spinal interneuron, Sensory afferents

TRADITIONALLY TRACTS ARE CLASSIFIED AS:


RECENT CLASSIFICATION : Hans Kupyres – 1981,85.

CORTICOSPINAL TRACT / PYRAMIDAL TRACT

Most important and largest descending tract.

Axons of Cortical neurons descend, form Pyramids in Medulla –> to Spinal Cord (Corticospinal tract)

Some terminate in :

  • Brain Stem Reticular formation
  • Cranial N Nuclei (V, VII,XII, Corticobulbar Tract) except those supplying to External eye muscle

Phylogenetically newest, only in mammals. (No of fibers have progressively increased from monkeys –> Apes –> Man.

1 million fibers (approx.) in human, >90% – small diameter & unmyelinated (SLOW CONDUCTING PATHWAY)

Myelination starts in neonate on 10th – 14th day and completes in 2 years.

NEUROTRANSMITTERS : GLUTAMATE, ASPARTATE.

FUNCTION

  1. Fine, skillful voluntary movement of Distal limb parts (LCST)
  2. Postural stability of Axial & proximal muscle.
  3. influence on stress reflexes.
  4. Facilitatory (++) –> alpha & gamma where it ends directly. Inhibitory (–) –> via Interneurons
  5. Efferent limb of superficial reflexes (e.g, abdominal, plantar etc)
  6. Opposite side lower part of face and hypoglossal (Corticobulbar tract)
  7. useful in sensori-motor coordination.
  8. Path of transcortical load compensation mechanism (Increased load –> Increased muscle contraction mediated by Cortex)

LESION

MOTOR DEFICITS OCCUR DUE TO DESCENDING TRACTS. PURE Pyramidal tract lesions are rare.

ACUTE Lesion (Stage of shock)–> Paralysis of opposite half, No reflexes, Emotions are intact. Lasts for 2-3 weeks. (Symptoms include lesions of both PT+ EPT )

CHRONIC Lesion (Stage of recovery) :

EXTRAPYRAMIDAL TRACT

  • Outside Pyramidal tract
  • Multineuronal motor system with number of synapses involving many regions of brain.
  • All from cortex & subcortical (brainstem) areas other than those in Pyramidal tract.

FUNCTIONS OF EXTRAPYRAMIDAL TRACT

  • Regulation of Posture, Equillibrium, Muscle tone
  • Coordinated, synchronized movement of head, eye, neck etc
  • Provide platform for Pyramidal system to operate fine, skilled movement.
EXTRAPYRAMIDAL TRACTS

LESIONS IN EXTRAPYRAMIDAL TRACTS:

  • MUSCLE TONE – Increased (upper limb – Flexion, Lower limb – Extension)
  • ABNORMAL POSTURE
  • INVOLUNTARY MOVEMENT – tremor
  • LOSS OF RIGHTING/POSTURAL REFLEXES
  • DEEP REFLEXES – exaggerated, clonus may appear
  • CLASP KNIFE RIGIDITY (Lengthening reaction) present
  • PLANTAR REFLEX – Normal/extensor.

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