UMN/LMN Study Guide

LMN/UMN Study Guide

Basic Concepts

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

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

Exercises

1. Draw a diagram depicting the flow of information from the motor cortex to the lip muscle to produce the /b/ sound in the word "baby."

show how both the left and right lip muscles receive information from the pyramidal system
label the identifiable structures, including the proper name of the LMN innervating these muscles
using your diagram, identify (& illustrate, if desired) two different sites of lesion that could result in left sided lip weakness
using your diagram, identify (& illustrate, if desired) two different sites of lesion that could result in bilateral lip weakness

2. For each of the following sites of lesion, determine the most likely impact on movement. The first one is completed as an example.

Site of Lesion	Impact on Movement	Rationale
Right Motor Cortex	Noticeable weakness of tongue and lower face on the left side. Upper face should be symmetrical	The right motor cortext contains UMNs. Only CN VII (lower face) and CN XII (tongue) have contralateral UMN innervation. The other CNs should not be notably impaired because they get innervation from both hemispheres.
Left Vagus Nerve (Recurrent Laryngeal branch only)
Left internal capsule
Right Facial Nerve (all branches)
Right Trigeminal Nerve

3. Consider evalution of motor speech disorders to predict site of lesion. From the descriptions below, identify the most likely site(s) of lesion. If you can only "narrow it down," identify the additional information needed to more confidently make a diagnosis

Behavioral Characteristics	Likely Site of Lesion	Additional Info Needed
weakness & hypertonicity in muscles of right lower face and tongue
weakness & hypotonicity of velum, pharynx, and larynx
left side of mouth fails to move upon command, but has appropriate movement during laughter

4. For each of the following muscle groups, identify the side of the body where the LMN and UMNs will be found

Muscle Group	LMN Location	UMN Location
Lips
Tongue
Jaw
Velum
Larynx