For the purpose of remembering the clinical manifestations of upper motor neuron lesion (UMNL) and lower motor neuron lesion (LMNL), a mnemonic has already been devised and discussed here.
Now, it’s time to understand the anatomical and physiological basis of these manifestations.
Upper Motor Neuron Lesion (UMNL) Syndrome
1. Spinal shock: Hypotonia and loss of all reflexes on contra-lateral side
- Gamma-motor neurons by stretching muscle spindle bodies, activate alpha-motor neurons leading to extrafusal muscle contraction.
- In upper motor neuron lesion, supraspinal excitatory input to gamma-neurons is lost.
2. Relative sparing of trunk muscles:
- Trunk muscles are bilaterally innervated by anterior corticospinal tract, so that a lesion of one side of the tract has minimal/imperceptible manifestations.
- Distal muscles, fingers, toes, fine articulations and flexors more than extensors are handled by lateral corticospinal tract and affected more.
1. Babinski sign:
- It is a primitive response present normally in newborns.
- The extensor response is modified to flexor response by developing corticospinal tract.
- Upper motor neuron lesion results in reappearance of primitive extensor response.
- Spasticity is increased muscle tone, hyperactive stretch reflexes and clonus.
- Due to removal of inhibitory influences exerted by cortex on postural centers of vestibular nuclei and reticular formation.
- The mechanism has already been discussed in detail in Clonus.
3. Hyporeflexia of superficial reflexes:
- Superficial reflexes are absent in infants and appear after about 6 months to 1 year.
- Their appearance may depend upon the myelination of the corticospinal tract.
- Hence, in upper motor neuron lesion, superficial reflexes may be lost.
- It may even be absent in normal individuals, hence, correlation with other corticospinal signs is necessary. 1
4. Contralateral or Ipsilateral Involvement:
- Pyramidal decussation occurs at the level of medulla-spinal cord junction.
- Lesion above pyramidal decussation leads to contralateral signs.
- Lesion below pyramidal decussation leads to ipsilateral signs.
5. Involvement below the lesion:
- Damage of UMN below the level of lesion.
6. Decorticate posture:
- Occurs in UMNL above the red nucleus – hence, rubrospinal tract still working.
- This leads to release of cortical inhibition of the rubro-, reticulo-, and vestibulospinal tracts. In this circumstance, the action of rubrospinal tract supercedes that of the reticulo- and vestibulospinal tracts, which results in arm flexion at the elbows and lower extremity extension, so-called decorticate posturing.
Rubro-spinal tract regulate flexor tone in upper limb.
Reticulo- and vestibulo-spinal tracts regulate extensor tone in the neck and both the upper and lower limbs.
7. Decerebrate posture:
- Occurs in UMNL below the red nucleus – hence, rubrospinal tract not working.
- This releases inhibition of the reticulo- and vestibulospinal tracts, which results in extension of the neck and all four limbs, so-called decerebrate posturing.