Question

Discuss the concept of an “upper motor neuron” vs a “lower motor neuron”. List all the neurons and their motor system pathways that might be considered upper motor neurons. Describe the consequences of a lesion to an upper motor neuron and a lower motor neuron. This answer should address issues of muscle tone, reflexes, and any other characteristics that help to differentiate between upper and lower motor neuron lesions. Give some common examples (i.e. diseases, dysfunctions, trauma) when these respective lesions would occur. What is the prognosis for recovery from UMN and LMN lesions. What are the differences between spasticity, rigidity, decerebrate rigidity and decorticate rigidity?

Answer 1

somatic motor pathways involve at least two motor neurons: an upper motor neuron and a lower motor neuron.

1.upper motor neuron Vs lower motor neuron:

• An upper motor neuron whose cell body lies in a central nervous system processing center, and a lower motor neuron, whose cell body lies in a nucleus of the brain stem as it relates to cranial nerves or in the spinal cord as it relates to peripheral nerves.
• The upper motor neuron synapses on the lower motor neuron and innervates a single motor unit in a skeletal muscle.
• Activity in the upper motor neuron may facilitate or inhibit the lower motor neuron. Activation of the lower motor neuron triggers a contraction in the innervated muscle. Only the axon of the lower motor neuron extends outside the CNS. Destruction or damage to a lower motor neuron will eliminate voluntary and reflex control over the innervated motor unit.

2.pathways:

Upper motor neurons travel in several neural pathways through the central nervous system (CNS):

1.Tract : corticospinal tract            

· Pathway: from the motor cortex to lower motor neurons in the ventral horn of the spinal cord    

· Function: The major function of this pathway is fine voluntary motor control of the limbs. The pathway also controls voluntary body posture adjustments.

2.Tract : corticobulbar tract           

· Pathway :from the motor cortex to several nuclei in the pons and medulla oblongata       

· Function:Involved in control of facial and jaw musculature, swallowing and tongue movements.

3.Tract : colliculospinal tract (tectospinal tract)  

· Pathway: from the superior colliculus to lower motor neurons

· Function : Involved in involuntary adjustment of head position in response to visual information.

4.Tract : rubrospinal tract  

· Pathway:from red nucleus to lower motor neurons

· Function:Involved in involuntary adjustment of arm position in response to balance information; support of the body.

5.Tract : vestibulospinal tract        

· Pathway:from vestibular nuclei, which processes stimuli from semicircular canals

· Function: It is responsible for adjusting posture to maintain balance.

6.Tract : reticulospinal tract

· Pathway:        from reticular formation

· Function: Regulates various involuntary motor activities and assists in balance.

3.MOTOR NEURON LESION

An upper motor neuron lesion is a lesion of the neural pathway above the anterior horn of the spinal cord or motor nuclei of the cranial nerves. A Lower motor neuron lesion is a lesion which affects nerve fibers traveling from the anterior horn of the spinal cord to the associated muscle

CAUSES

Some of the likely causes of lower motor neuron lesions are motor neuron disease, peripheral neuropathy, poliomyelitis, and spinal cord injury with nerve root compression. Lower motor neurons control movement in the arms, legs, chest, face, throat, and tongue. Mixed upper and lower motor neuron diseases include multiple sclerosis.

UPPER MOTOR NEURON LESION SIGNS:

• Weakness – the extensors are weaker than the flexors in the arms, but the reverse is true in the legs

• Muscle wasting is absent or slight

• Hyperreflexia with clonus

• Spasticity

• No fasciculation’s

LOWER MOTOR NEURON LESION FINDINGS:

· Weakness – limited to focal or root innervated pattern

· Muscle Wasting – prominent in a focal pattern

· Reflexes – absent or reduced in a lower motor neuron lesion

· Fasciculation’s present in the associated muscle group