What is the Difference Between UMN and LMN?

The main difference between upper motor neuron (UMN) and lower motor neuron (LMN) lies in their functions and the clinical manifestations of their respective lesions.

Upper Motor Neurons (UMNs):

• UMNs are part of the central nervous system and are responsible for transmitting impulses from the brain to the spinal cord and brainstem, where they activate lower motor neurons.
• UMN lesions result in muscle weakness, hypertonia, and spastic paralysis.
• Clinical manifestations of UMN lesions include minimal disuse atrophy or contractures, increased tone (spasticity/rigidity), pyramidal pattern of weakness, and hyperreflexia.

Lower Motor Neurons (LMNs):

• LMNs are part of the peripheral nervous system and directly stimulate muscles to contract.
• LMN lesions result in muscle weakness, muscle atrophy, fasciculations (muscle twitching), decreased reflexes, decreased tone, negative Babinski sign, and flaccid paralysis.
• LMN lesions are usually associated with hypotonia and flaccid paralysis.

In summary, UMNs are involved in transmitting impulses from the brain to the spinal cord and brainstem, while LMNs directly stimulate muscles to contract. UMN lesions result in spastic paralysis, whereas LMN lesions lead to flaccid paralysis.

Comparative Table: UMN vs LMN

The main differences between upper motor neurons (UMNs) and lower motor neurons (LMNs) are their functions, locations, and the symptoms they cause when damaged. Here are the key differences between UMNs and LMNs:

In summary, UMNs are the motor components of the CNS that transmit impulses, while LMNs are the motor components that connect with muscles and control voluntary muscular movements. Damage to UMNs results in spastic hyperreflexia and muscle weakness, whereas damage to LMNs leads to muscular atrophy, flaccid muscle weakness, fasciculation, and hyporeflexia.