The Lower Motor Neuron (LMN) is the efferent neuron present in the peripheral nervous system (PNS), which connects the muscle to the Central Nervous System (CNS) for innervation. Furthermore, LMN is a multipolar neuron that connects the UMN to skeletal muscles. Cell bodies of the lower motor neurons are located in the ventral horn of the spinal cord (gray matter) and motor nuclei of the cranial nerves in the brainstem. It is responsible for transmitting signals from UMN to the effector’s muscle to generate a movement.
Any injury, trauma, or illness that affects the nerve fibers in the ventral horn of the spinal cord or motor nuclei of cranial nerves can result in a “Lower Motor Neuron Lesion.” Approximately 2-3 out of every 100,000 people are affected yearly by lower motor neuron lesions.
Types of Lower Motor Neurons and their lesions
There are two types of lower motor neurons. These are as follows:
- α-Motor Neurons: These neurons are essential for producing contraction in muscles and are also known as the “final common pathway.” Any damage to these refers to as “Lower Motor Neuron Syndrome.”
- γ-Motor Neurons: This type of LMN is responsible for maintaining muscle tone and nonconscious proprioception. They also help in signaling the length and velocity of the muscle.
What is the difference between α-motor neurons and γ-motor neurons?
The significant difference between both types of lower motor neurons is that α-motor neurons receive input from both muscle spindle sensory afferents and UMNs, while γ-motor neurons are exclusively under control from the UMNs.
Damage to any of these can result in LMN lesions. In addition, lower motor neuron lesions can affect the upper and lower extremities, with the lower extremities being more commonly affected (60-70%).
Causes of Lower Motor Neuron Lesion
Causes of Lower Motor Neuron Lesion
Various causes affect lower motor neurons functioning, such as diseases, infections, trauma, or injury. However, the most common cause of LMN lesions is an injury to peripheral nerves or viruses that attack the spinal cord’s ventral horn.
Trauma is the most common cause of lower motor neuron lesions, accounting for approximately 40% of all cases.
The condition is characterized by muscle atrophy and muscle shrinkage replaced by fibrous tissue. However, various diseases can also cause LMN lesions. These are as follows:
- Poliomyelitis
- Bells Palsy
- Spinal Muscular Atrophy
- West Nile fever
- Guillain–Barré syndrome
- Localized lesion near the spinal cord
- Cauda Equina Syndrome/ Conus medullaris Syndrome
- C. botulism
- Amyotrophic lateral sclerosis
Other causes of lower motor neuron lesions include metabolic disorders (15%), vascular disorders (15%), and infectious diseases (10%).
Signs and Symptoms of Lower Motor Neuron Lesion
Muscle atrophy is a significant symptom of LMN lesions. In addition, the other most common signs and symptoms of lower motor neuron lesions are as follows:
- Flaccid paralysis
- Fasciculations (Muscle Twitching)
- Fibrillations
- Muscle Weakness (limited to the affected region)
- Decreases reflexes (Hyporeflexia)
- Impaired muscle tone (Hypotonia or atonia)
- Plantar reflex (Negative-Babinski sign)
- Muscle Denervation
Approximately 70% of lower motor neuron lesions are unilateral, while the remaining 30% are bilateral.
To understand the difference, also Read About Upper Motor Neuron Lesion.
Diagnosis of Lower Motor Neuron Lesion
Diagnosis of Lower Motor Neuron Lesion
Medical history and Physical examination is the foremost step for diagnosing any condition. However, in the case of LMN, neuro-localization is the vital step to confirm the diagnosis.
Other tests and assessments include the following:
- Postural reaction and nociceptive testing
- Laboratory Tests such as blood or urine test
- Imaging Tests such as X-ray or CT Scan
- Magnetic Resonance Imaging (MRI)
- EMG/ NCS (Nerve Conduction Study)
Differential Diagnosis of Lower Motor Neuron Lesion
Various diseases have the same clinical features as the LMN lesion. Thus, some of them are as follows:
- Diffuse vascular disease
- Myasthenia gravis
- Amyotrophic lateral sclerosis
Treatment of Lower Motor Neuron Lesion
Treatment of Lower Motor Neuron Lesion
Treatment for LMN lesions depends on the patient’s condition and the severity of the symptoms. However, the common treatment options include the following:
- Medication – to manage the symptoms
- Physical Therapy
- Balanced Diet
- Muscle relaxers and Botulinum toxin (Botox)
- MS-modifying treatment (If MS is present)
- Occupational Therapy
- Social Support