Upper Motor Neuron


What is upper motor neuron?

The central nervous system provides the activity of skeletal muscles through two sets of neurons- the upper and the lower motor neuron.

Upper motor neuron is a neuron that starts at the cerebral cortex or brainstem and creates a synapse (a junction between two nerve cells) with the lower motor neuron, which is usually located in the spinal cord. Upper motor neuron controls the lower motor neurons.

Lower motor neuron is a nerve cells that extends from the spinal cord to the skeletal muscles and provides nerve impulses to move the muscles [1,2].

upper motor neuron

Functions of upper motor neuron

The upper motor neuron controls the lower motor neuron through two different types of nerve fiber pathways – the pyramidal and the extrapyramidal tracts. These tracts are responsible for ensuring movements of different muscle groups.

Pyramidal tracts include:

  • Corticospinal tract

Extrapyramidal tracts include:

  • Reticulospinal
  • Vestibulospinal
  • Tectospinal
  • Rubrospinal
  • Olivospinal
  • Corticobulbar
  • Corticorubral [3]

Upper motor neuron damage

The upper motor neuron extends from the cerebral cortex to the spinal cord. Damage to the upper motor neuron pathway results in group of symptoms called the upper motor neuron syndrome.

The initial damage to the cerebral cortex causes flaccidity of the muscles of the contralateral side to the damage. It is commonly seen that in the acute stage of the damage, muscles are flaccid in the arms and legs, however the trunk muscle tonus is usually preserved. This occurs due to the remaining pathways from the brain to the spinal cord or due to bilateral projection of the nerve pathways. After the initial damage, the spinal cord pathways regain some of their function for unknown reasons.

Signs

There are several remaining signs that describe the upper motor neuron syndrome [4].

Babinski sign

One of the most often signs of the upper motor neuron syndrome is the Babinski sign. Normally, in response to stroking of the sole of the foot, the big toe (often also other toes) will flex. If the descending pathways are damaged, the stroke of the foot will make the big toe extend and the other toes to flex. The response is similar to the one seen in infants, which is normal at that age, since the corticospinal pathway is not completely matured.

babinski sign

Spasticity

Increased muscle tone- spasticity, is also a common sign of the upper motor neuron syndrome. The stretching reflexes and muscle response to stretching is also present. If the upper motor neuron lesion is extensive, muscle rigidity in the leg extensors and arm flexor muscles can also be seen. This symptom is caused by the removal of inhibitory influences by the cortex. Due to the loss of descending inhibition, there is a gain of spinal cord reflex.

Loss of reflexes

Upper motor neuron syndrome is also characterized by hyporeflexia of superficial reflexes. This includes loss or diminishing of:

  • Corneal reflex-
  • Superficial abdominal reflex- tensing of abdominal muscles when the overlaying skin is stroked
  • Cremasteric reflex in males- elevation of the scrotum when the inner aspect of the thigh is stroked

Loss of function

When the lesion involves descending pathways of the upper motor neuron that control the lower motor neurons of the libs, there is a loss of motor function. This means losing the ability to execute fine movements, like the movement of the fingers [5].

Symptoms

Motor neuron diseases are rare, affecting around 2 out of 100,000 people per year. the risk factors of this disorder include genetic predisposition, age over 50 years and male gender. The symptoms differ according to stage. In the initial stage, besides the previously mentioned signs, there is also:

  • Clumsiness
  • Fatigue
  • Pain in the muscles
  • Cramps
  • Twitching
  • Slurred speech
  • Weak grip

In advanced stages of disease, there is:

  • Muscle weakness
  • Pain in muscles and joints
  • Worsening of muscle spasms
  • Excessive yawning
  • Changes in personality and mood, like uncontrolled laughing or crying
  • Lowering of cognitive function
  • Breathing problems due to a continuous muscle weakness
  • Paralysis (read about sleep paralysis here) [6]

lou gehrig's disease

Treatment

The treatment of upper motor neuron syndrome depends on the underlying condition. In most cases, the condition gradually gets worse and the treatment is based on minimizing the symptoms and slowing the progression of the condition [6].

References:

  1. Upper motor neuron definition: http://www.neuroscientificallychallenged.com/glossary/upper-motor-neuron/
  2. Lower motor neuron definition: http://www.medicinenet.com/script/main/art.asp?articlekey=33870
  3. Functions: https://www.slideshare.net/saim_18/upper-and-lower-motor-neuron
  4. Upper motor neuron syndrome: https://www.ncbi.nlm.nih.gov/books/NBK10898/
  5. Signs: http://neuroscience.uth.tmc.edu/s3/chapter06.html
  6. Symptoms: http://www.newhealthadvisor.com/Upper-Motor-Neuron-Disease.html
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