What Is a Lower Motor Neuron Lesion?

In general terms, the medical term "lesion" refers to any type of damage or injury in the body. It may also refer to any physical changes that may occur as a result of some sort of illness that creates problems in the way the body or part of the body functions. A lower motor neuron lesion denotes an injury affecting the neurons associated with muscles.

Neurons are specialized cells responsible for conveying electrical and chemical signals to and from the brain and spinal cord. Motor neurons translate these communications into movement by directly or indirectly influencing the muscles. Neurons are generally broken down into two categories: upper motor neurons and lower motor neurons. Upper motor neurons send signals to the specific area whereas lower motor neurons innervate or run through muscles. A lower motor neuron lesion affects the body’s ability to move by interrupting the signals from the brain and spinal cord being sent to these muscles.

Though both types of nerve damage involve the muscles and affect their ability to move correctly, the difference between an upper motor neuron and a lower motor neuron lesion is in its presentation. Though an upper motor neuron also affects the muscles, it causes different symptoms as it disturbs the nerve pathways above the anterior horn of the spinal cord, a section in front of the gray matter in the spinal cord. The lower motor neurons, on the other hand, affect the nerves from the anterior horn directly into the muscles.

Depending on the exact location of the injury and which muscles it disturbs, common symptoms of a lower motor neuron lesion include such things as muscle fasciculation or fibrillation, involuntary twitching of the muscle fibers, and the quivering of individual fibers of the muscles, respectively. Another result could be flaccid paralysis, resulting in the inability of the muscle to move causing a reduction or loss of muscle tone and muscle reflexes. Long term flaccidity can cause a degeneration of the muscles, a condition known as atrophy or muscle wasting.

The causes of a lower motor neuron lesion can range from some type of injury to the nerves to health conditions such as amyotrophic lateral sclerosis, also known as Lou Gehrig’s disease or ALS, which causes a progressive loss of motor control. In general, if the decrease in motor function is due to an injury, regaining muscle control may be possible as the nerve pathways heal, a process that can take anywhere from several months to several years. However, if the dysfunction is due to a disease process, treatment can usually only provide a temporary decrease in symptoms.