Upper vs Lower Motor Neuron: Difference and Comparison

Neurons are a fundamental part of the basic human body. Upper motor neuron lesions are just another name given to UML. They are commonly known as upper motor neurons.

UML are neurons in the brain and spinal cord that have been damaged. Lower motor neuron lesions are another name for LMN. 

Key Takeaways

  1. Upper motor neurons originate in the brain and transmit signals to lower motor neurons, while lower motor neurons relay those signals to muscles for movement.
  2. Damage to upper motor neurons results in spasticity, hyperreflexia, and muscle weakness, while damage to lower motor neurons leads to muscle atrophy, fasciculations, and hyporeflexia.
  3. Different diseases affect upper and lower motor neurons, such as amyotrophic lateral sclerosis (ALS) impacting both types and spinal muscular atrophy affecting only lower motor neurons.

Upper Motor Neuron vs Lower Motor Neuron

Upper motor neurons is a type of neuron in the nervous system which originate in the brain and carry signals down to the lower motor neurons in the spinal cord. Lower motor neurons is a type of neuron in the nervous system which transmits signals to the muscles, causing them to contract and move.

Upper Motor Neuron vs Lower Motor Neuron

 There will be hypertonia, hyperreflexia, and spasticity in the upper motor neuron.

So if there is damage to UMN, there will be a loss of control to LMN because LMN or Lower motor neurons are controlled by UMN or upper motor neurons.

It causes an increase in Tonia, an increase in reflexes, and spasticity. 

In lower motor neurons, there will be hypotonia, hypo-relexia, and flaccidity. Here also, there will be atrophy, but it is due to denervation(loss of nerve supply) of motor nerve muscles.

Additionally, in Lower motor neuron lesions, there will be a fasciculation which is the spontaneous contraction of small muscle fibers because of stimulation of a few single muscle fibers. 

Comparison  Table

Parameters of ComparisonUpper Motor NeuronLower Motor Neuron
Muscle weaknessIn UMN it can be Quadriplegia, Hemiplegia, paraplegia and Diplegia.In LMN, it can be of two types: if it involves muscles 
Muscle toneDepending on muscle tone, it can be spasticity and rigidity. LMN having Hypotonia means an abnormally low level of muscle tone. 
FasciculationsIn UMN it is absent In LMN it is present
Tendon ReflexesIn UMN it is hyperreflexia In LMN there will be hypo/Areflexia 
EMG(electromyography) Analysis.Normal nerve conduction. Abnormal nerve conduction.            

What is Upper Motor Neuron?

The body part upper motor neuron was first given in 1886 by William Gowers. The upper motor neuron, or UML, is present in the cerebral cortex brain stem.

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The upper motor neurons, or UML, are the primary source of voluntary movement and are present in the cerebral cortex. UMN carries information down to inter-motor neurons and lower motor neurons.

Upper motor neurons control the lower motor neuron. The cerebral cortex contains pyramidal cells, including a kind of gigantic pyramidal cell known as Betz cells.

Betz cells are the biggest cell bodies in the brain, with a diameter of 0.1mm. Upper motor neuron refers to the pyramidal cells of the precentral gyrus. In the spinal cord root, the top motor neuron descends. 

In the neural passage which is present above the anterior horn of the spinal cord. Any UMN injury, known as pyramidal insufficiency, occurs.

Upper motor neurons pass through the central nervous system through numerous neural routes. If there is an injury to UMN or Upper motor neuron, then it could result in exaggerated and spasticity reflexes.

Upper motor neuron lesion affects a group of muscles and does not serve as muscle atrophy. Hyperactive deep reflex and Babinski sign become negative.

What is Lower Motor Neuron?

The higher motor neuron will connect with the LMN present at the interior side of the spinal cord once it reaches the right spinal level.

The lower motor neuron focuses on the muscles that control the movement of the bodily part in question.

The lower motor neuron is found at either the cranial nerve nuclei of the brainstem or the anterior grey column and motor function with cranial nerves. 

Lower motor neurons start to activate towards the bottom mean from the spinal cord. Lower motor neurons carry signals to muscles for the contract.

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The fibers which come from the cranial nerve are considered lower motor neurons. The lower motor neurons act into muscles.

The lower motor neuron is connected to the upper motor neuron, so if the upper motor neuron gets removed, it works freely, targets muscles, and increases its activity power more. 

But if the lower motor neuron gets removed, the Upper motor neuron muscles get flaccid paralysis. So ultimately, if lower motor neurons go, we will suffer a lot of loss.

Lower motor neuron works for only single muscles. In lower motor neurons, reflexes are lost, and the Babinski sign is positive.

Main Differences Between Upper Motor Neuron And Lower Motor Neuron

  1. The UMN or upper motor neurons are found in the area of the cerebral cortex of the brain, while LMN or lower motor neurons are there in the spinal cord.
  2. The Upper Motor Neuron carries information on the different parts of the body, while Lower Motor Neuron allows it to receive inputs from the higher portion of the neuron system.
  3. The Upper Motor Neuron is used to pass signals to the different muscles, while the Lower Motor Neuron receives that signal and passes it to other body parts.
  4. The Upper Motor Neuron is based on only one type of neuron, while Lower Motor Neuron is based on two different types of neurons.
  5. The Upper Motor Neuron is distributed in the pyramidal system, while the LMN is found in the cranial nerves and anterior horn cells.
References
  1. https://n.neurology.org/content/68/19/1571.short
  2. https://www.sciencedirect.com/science/article/pii/S1388245706001143

Last Updated : 11 June, 2023

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9 thoughts on “Upper vs Lower Motor Neuron: Difference and Comparison”

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