Upper Motor Neurons (UMN)
motoneurons that originate in the cerebral cortex (motor strip), terminating in the brainstem or spinal cord
UMNs in the pyramidal tract directly synapse with LMNs
UMNs in the extrapyramidal tract synapse with neurons in the red nucleus or the reticular formation
UMNs that provide contralateral innervation to LMNs are on the opposite side of the brain as the muscle, UMNs that provide bilateral innervation to LMNs "talk" to muscles on both sides of the body
UMNs course through the corona radiata, internal capsule, cerebral peduncle, and finally synapse in the midbrain/pons/medulla/spinal cord
Lower Motor Neurons (LMN)
motoneurons that originate in the brainstem (cranial nerves) or spinal cord (spinal nerves), terminating at the neuromuscular junction (muscles)
all LMNs innervate ipsilateral muscles (the muscle and the LMN that innervates it are always on the same side)
LMNs receive input from upper motor neurons from the opposite side of the brain (contralateral UMN innervation) or from both sides of the brain (bilateral UMN innervation)
Cranial Nerves with bilateral UMN innervation: V, VII (upper face), IX, X, XI
Cranial Nerves with primarily contralateral UMN innervation: VII (lower face), XII
LMN Symptoms include pharyngeal muscle weakness
(involved with swallowing
), weak jaw and facial muscles, progressive loss of speech, and tongue muscle atrophy. Limb weakness with both lower and upper motor neuron signs is almost always evident but less prominent.
UMN- Primary lateral sclerosis is a disease of the upper motor neurons, while progressive muscular atrophy affects only lower motor neurons in the spinal cord. In progressive bulbar palsy, the lowest motor neurons of the brain stem are most affected, causing slurred speech and difficulty chewing and swallowing.