Divisions & Signs in Neuroanatomy

Divisions & Signs

Summary

Central nervous system (CNS)

Key components:

Peripheral nervous system (PNS)

Key components

upper & lower motor neurons and fibers

Upper Motor Neuron (UMN)

Lower Motor Neuron (LMN)

Three common clinical scenarios

I. Classic upper motor neuron injury: a stroke with injury to the cerebral cortex and underlying white matter tracts.

In an upper motor neuron injury, there is spastic muscle tone, hyperactive muscle stretch reflexes, and pathologic reflexes are present (ie, a Babinski sign IS present).

II. Classic lower motor neuron injury: peripheral polyneuropathy affecting a representative spinal nerve fiber.

In a lower motor neuron injury, there is flaccid muscle tone, hypoactive muscle stretch reflexes, and pathologic reflexes are absent (ie, NO Babinski sign is present).

III. Mixed pattern of injury: direct cervical spinal cord injury (aka myelopathy).

Above the level of lesion, the patient is normal, because neither the neurons or their tracts are affected.
Below the level of lesion, there is UMN injury, because the white matter tracts are disconnected from their neurons.
At the level of lesion, there is LMN injury, because at this level, the spinal motor neuron and emanating fibers are affected.