The Anterior Horn and Motor Neuron Diseases

Understanding the Spinal Cord and Diseases Like ALS

Cerebrospinal fluid, artwork
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The spinal cord is made up of grey matter and white matter. If you were to cut it cross-sectionally, you would see grey matter in the shape of a butterfly surrounded by white matter. The grey matter is made up predominantly of neurons (specialized nerve cells which transfer messages to other nerve cells) and glial cells (which surround and insulate the neuron cells).

The grey matter forms the core of spinal cord and consists of four projections called "horns." The horn is further divided into segments (or columns) with to the dorsal horn situated to the back, the lateral horns placed to the sides, and the anterior horn located up front.

The anterior horn of the spinal cord (also known as the anterior cornu) contains the cell bodies of motor neurons that affect the skeletal muscles.

Understanding Motor Neurons

When you move, the brain will send a message to the cells in the spinal cord. These cells then relay the message to the peripheral nervous system, the part of the nervous system situated outside of the brain and spinal cord.

Skeletal muscle movement is just one of the functions regulated by the peripheral nervous system. The nerve cells responsible for relaying these message are called motor neurons.

The nerves that send messages between the brain and the spine are called upper motor neurons, and those that relay messages from the spine to the muscles are called lower motor neurons.

Understanding Motor Neuron Diseases​

Diseases that selectively attack these neurons are called motor neuron diseases. As the name suggests, motor neuron diseases reduce a person's ability to move.

The best-known example of this is amyotrophic lateral sclerosis (ALS). Other include polio and Kennedy's disease.

Neurologists will use a physical exam to determine where in the nervous system the disease is located. The features of the diseases can vary significantly. For example:

  • Diseases affecting the upper motor neurons exclusively will typically exhibit rigidity or spasticity (continuous muscle contraction affecting normal movement).
  • Diseases affecting the lower motor neurons are characterized more by muscular atrophy (wasting) and fasciculations (brief, spontaneous contractions of muscles).
  • In some forms of motor neuron disease, such as ALS, both upper and lower motor neuron functions are affected.

Types of Motor Neuron Disease

Motor neuron diseases are rare conditions that progressively damage parts of the nervous system which regulate movement. Motor neuron disease can appear at any age is most often seen in people over 40. It affects men more than women.

There are several types of motor neuron disease:

  • ALS (also known Lou Gehrig's disease) is the most common form of the disease. It affects muscles of the arms, legs, mouth, and respiratory system. While the cause is not known in most cases, around 10 percent are directly linked to family genetics.
  • Progressive bulbar palsy (PBP) involves the brain stem and causes frequent choking spells, difficulty speaking, eating, and swallowing. The cause of PBP is unknown but is believed linked to genetics.
  • Progressive muscular atrophy (PMA) is a variation of ALS that slowly but progressively causes muscle wasting (atrophy).
  • Primary lateral sclerosis (PLS) is a rare form of the disease which is slowly progressive. While PLS is not fatal, PLS can interfere with normal activity and a person’s quality of life.
  • Spinal muscular atrophy (SMA) is an inherited form of motor neuron disease that affects children. It is associated progressive muscle wasting, poor development, and the loss of strength of respiratory muscles.

Source:

Tiryaki, E. and Holli, H. "ALS and Other Motor Neurons Diseases." Continuum: Lifelong Learning in Neurology. 2014; 20(5):1185-1207.

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