Motor Neurons

Posted by Nurse Linda in Life After Paralysis on January 13, 2021 # Health

Frequently, there are questions about upper motor neuron injuries and lower motor neuron injuries. Motor neurons work with sensory neurons, which we will discuss next week. Researchers are studying how these neurons work to make repairs for improvement in function. doctor and patient

How Motor Neurons Work

Motor neurons are cells in the central nervous system with axons or arms that stretch through the central nervous system and into the human body. The cell body of the motor neurons is in the central nervous system, which is the brain and spinal cord. Their axons can be far-reaching outside of the central nervous system into the body. Nerves in the body are called peripheral nerves.

Motor neurons are nerve cells that control sending messages from the brain to move the body. Alternatively, sensory neurons send messages to the brain. There are two types of motor neurons, upper motor neurons (UMN) and lower motor neurons (LMN). Although they share the same name, motor neurons, there are more differences than similarities.

Characteristics of Motor Neurons:

1.The cell bodies of UMNs are located within the cerebral cortex or brain. They are the source of voluntary movement. The cell bodies of LMNs are in either the brain stem or spinal cord. They are the connectors between UMNs and the target muscle for movement.

2.UMNs originate in the motor strip of the brain. They utilize the chemical neurotransmitter glutamate to send messages to the LMNs in the brain stem and spinal cord. The LMNs then send messages to the body using the chemical neurotransmitter acetylcholine.

3.The target or the endpoint of the individual motor neuron of the UMN is within the central nervous system, which is the brain and spinal cord. The target or endpoint of the LMN is anywhere else in the body.

4.Injury results to a motor neuron are spasticity for UMN injuries and flaccidity in LMN injuries.

LMNs are arranged in three categories.

-Branchial motor neurons innervate the face and neck through five of the cranial nerves (CN). Cranial nerves are numbered using roman numerals: (CN V) trigeminal, (CN VII) facial, (CN IX) glossopharyngeal, (CN X) vagus, and (XI) accessory nerves.

-Visceral motor neurons control the smooth muscles of the internal organs and glands. The visceral motor neurons are under the control of the autonomic nervous system or ANS. That is the part of the nervous system that works automatically or the part of your body that is working outside of your control.

-Somatic motor neurons innervate skeletal muscles for movement. There are three types, one for each of your muscle types: alpha, which is cardiac muscle, beta or smooth muscle found in body organs, and gamma or skeletal muscle.

LMN can result from spinal cord injury or diseases such as:

Amyotrophic lateral sclerosis (ALS)

Primary lateral sclerosis (PLS)

Hereditary spastic paraparesis (HSP)

Progressive bulbar palsy (PBP), including hereditary forms.

Spinal muscular atrophy (SMA)

X-linked spinobulbar muscular atrophy (SBMA; Kennedy disease)

Post-polio syndrome (PPS)


Only a neurological examination can establish if an injury is UMN, LMN or mixed. The medical evaluation will include a neurological examination, may include an EMG (electromyogram) or NCS (nerve conduction study), CT Scan or MRI. After an injury to the nervous system from trauma or a medical condition, there are often disruptions to the motor neurons and sensory neurons.

Because the nervous system is so complicated, injuries are not always easily identified as UMN or LMN. An injury to the brain that affects the motor function of the face can result in an UMN injury. However, if the trauma is in the motor nerves of the brain stem or spinal cord area or to the peripheral nerves, the injury can be classified as an LMN to the face.

In spinal cord injury, a complete assessment of every nerve dermatome of the body is assessed. This is called the ASIA, or an updated name is the AIS exam. The ASIA examination flow sheet can be seen here:

Most typically, injuries to the cervical or thoracic levels of the spinal cord will result in changes to UMN functioning. Injuries at the lumbar or sacral areas result in LMN function changes. This is often the case, but not always depending on how the motor neurons are affected. Some will have injuries to the UMNs, others to LMNs but there can also be mixed effects. Mixed-effects result is a combination of symptoms of UMN and LMN injury. Because UMNs affect the function of LMNs, injury to a UMN will affect the function of the LMN.

Symptoms and Treatments

The result of a UMN injury is tone or spasticity in the body. Individuals might see tone or spasms by looking at their body and noticing the rhythmic movement of body limbs or tightness in body parts. They will not be able to see tone, which is also occurring within the body. The results of internal tone/spasticity might be reflected through symptoms such as difficulty breathing, slow digestion, incontinence of urine with or without complete emptying or incontinence of stool with or without some stool retention.

LMN injury results in flaccid muscles. You will see flaccid muscles lose tone quickly, becoming very small, especially in the legs. The bladder and bowel will fill without expulsion of urine or stool. Sometimes, there might be an overflow in the form of incontinence, but the bladder and bowel do not spontaneously empty.

Even though these two situations respond to similarly named treatments, they are uniquely different. For example, in both UMN and LMN, injury bladder programs are established to keep the bladder empty and the kidneys safe. A bowel program is used to regulate the bowel to avoid incontinence. A difference is in UMN injury, and the bladder could possibly use spasticity to empty, resulting in the use of an external catheter for men. However, pressures can still cause renal complications. In LMN injury, reflexes may be used to empty the bladder spontaneously. A person with a UMN bowel probably uses gentle digital stimulation to relax spasticity so the bowel can empty. A person with an LMN bowel will perform manual removal of the stool.


Research has enhanced care for individuals with neurological injury. If muscles are stimulated with electrical medical devices, these are used to enhance function and recovery. This treatment is used for individuals with UMN injury. For individuals with LMN, functional electrical stimulation can be used at the zone of transition to strengthen muscles, resulting in increased function and peripheral nerve stimulation.

Implants of functional electrical devices are available in research settings but soon to be more available in general. Opportunities for this type of treatment will become more common as progress is occurring rapidly. This will create a huge opportunity to reduce secondary complications of spinal cord injury and enhance function and even recovery. Nerve stimulating implants can be used to enhance bladder, bowel, and sexual functions that are attached to peripheral nerves. This includes both UMN and LMN injuries. Significantly, nerve rerouting procedures to peripheral nerves to improve function and avoid complications are available.

Many research studies are being conducted to better understand neurotransmitters, especially glutamate and acetylcholine, the scaffolding that allows transplanted nervous tissue or stem cells to remain in the area of spinal cord injury and disease-specific studies. This is a time of major leaps in spinal cord injury research and treatment. Not a quickly as we would all be desired, but in scientific time, a huge leap. Nurse Linda

Pediatric Consideration:

Children’s bodies are developing. This includes the nervous system. As infants, we know the nervous system is not yet developed enough to manage urinary continence. Use of the potty is not presented until age 2 or 3 because the nervous system and child development are not yet in place to accomplish this goal. As a child grows, so does the nervous system. This can be a great advantage for children as the nervous system is more rapidly developing throughout childhood. This nervous system development can be manipulated for improvement of function and recovery.

Researchers are especially keen to understand fetal and early development to be able to harness the rapid growth of the nervous system to treatment options. This is another way science is seeking to improve the lives of individuals with neurological issues. Nurse Linda

Linda Schultz, Ph.D., CRRN, a leader and provider of rehabilitation nursing for over 30 years, and a friend of the Christopher & Dana Reeve Foundation for close to two decades. Within our online community, she writes about and answers your SCI-related healthcare questions in our Heath & Wellness discussion.

The National Paralysis Resource Center website is supported by the Administration for Community Living (ACL), U.S. Department of Health and Human Services (HHS) as part of a financial assistance award totaling $8,700,000 with 100 percent funding by ACL/HHS. The contents are those of the author(s) and do not necessarily represent the official views of, nor an endorsement, by ACL/HHS, or the U.S. Government.