What are motor neurons?

Jul 5, 2017 | Disease Models

You’re in a bakery full of delicious treats. After a lot of soul-searching, you set your heart on a pecan danish. You reach for your wallet. But wait – how did you go from deciding how wanted your wallet to having your wallet in your hand?

Thank your motor neurons.

What are motor neurons?

Motor neurons are the nerve cells that come from the brain, go through the body and eventually reach your muscles. When you want to move, they signal the appropriate muscles to contract.

There are two types of motor neurons: the upper motor neurons (UPNs) and the lower motor neurons (LMNs). The names are helpful: the uppers are in your central nervous system (CNS; your brain and spinal cord) and the lowers are in your peripheral nervous system (PNS; the nerves that connect the CNS to the organs, tissues and limbs).

How does the brain tell the muscles to contract?

First the short story:

The brain generates and sends  signals to the UMNs. The UMNs in turn send the signal to the LMNs. The LMNs in turn send the signal to the muscles and they contract.

UMNs also tell the LMNs when to stop signals the muscles.

Now some more detail:

Note: The explanation below is best understood through this image of the upper and lower motor neurons in action. The text will help to add breadth.

The LMN is composed of a cell body or “soma”. Projections, called “dendrites”, extend from the soma. These pick up signals from the UPN. The signals dock on the dendrites, like a boat docks at a port. The signal is then passed down into the soma and then along the body of the neuron, called the “axon”. When it reaches the muscle, the muscle is signaled to contract.

What happens if you have nerve damage?

If the signal reaches the LMN but there is a lesion/damage to the nerve cell, the signal may be partially or completely lost. This results in muscle weakness or even paralysis. The signal can also fail to reach its target as a result of myelin sheath damage. This fatty coating insulates the nerve cells and allows the signals to be amplified much faster than if the nerve cells were uncoated. Thus demyelination will result in the poor transmission of signals.

The distinguishing feature of LMN damage versus UMN damage is that while both will result in impaired signaling to the muscle to contract, the UMN is responsible for turning off the contracting signal, whereas damages to the UMN will result  in muscle spasticity.

Where do oligodendrocytes and Schwann cells fit in?

Motor neurons are myelinated by either oligodendrocytes or Schwann cells. Oligodendrocytes and Schwann cells are glia. However, they are very different!

What is the differences between oligodendrocytes and Schwann cells?

Location: Schwann cells: PNS

Oligodendrocytes: CNS

Manner of myelinating cells: When Schwann cells myelinate, their cell body’s actually wrap around the axon of MNs and they can only wrap around one axon.

Oligodendrocytes secrete the myelin sheaths around the axons of MNs and can form a segment of myelin for up 50 adjacent axons.

Types of cells: There may be subtypes of oligodendrocytes but their significance is as of yet unknown. They have also only been seen in vitro and in rats. However all suspected subtypes produce myelin, as shown in rodent species.
Schwann cells can be myelinating or non-myelinating, as shown in rodent species.
A single oligodendrocyte can extend its processes to 50 axons, wrapping approximately 1 μm of myelin sheath around each axon; Schwann cells, on the other hand, can wrap around only one axon. Each oligodendrocyte forms one segment of myelin for several adjacent axons.
Diseases associated with each cell type: Oligodendrocytes: multiple sclerosis, various leukodystrophies (like Pelizaeus-Merzbacher disease and Pelizaeus-Merzbacher-like diseases), spinal cord injury, and cerebral palsy.

Schwann cells: Charcot–Marie–Tooth disease (also known as hereditary motor and sensory neuropathy or peroneal muscular atrophy), Guillain–Barré syndrome, schwannomatosis, and chronic inflammatory demyelinating polyneuropathy and Hansen’s disease (also known as leprosy).

Why do researchers study motor neurons?

In order to understand how to prevent, treat or cure motor neuron disorders  and heal spinal cord injuries, researcher use both cell and animal models. A popular technology  is to use human induced pluripotent stem cell (iPSC; multipotent progenitor cell) lines that have been reprogrammed to create iPSC-derived motor neurons.

This has many applications including:

  • In disease research including those mentioned above but also other disease such as ALS (otherwise known as Lou Gehrig’s disease)
  • Toxicity testing
  • Phenotypic screening for drug discovery
  • In vitro assays
  • Developmental studies and co-culture (with myotube) studies

Do you know an interesting fact about motor neurons? Have you heard of a cool use for iPSC-derived motor neurons?

Tweet us @TempoBioscience or let us know in the comments below!

Article by Olwen Reina. Contact Olwen at olwen@tempobioscience.com.