Motor Neuron Diseases

 Diseases of the upper motor neurons


Diseases of the upper motor neurons affect neurons of the motor cortex - nerve cells in the brain. Upper motor neurons direct the lower motor neurons to produce movements such as walking or chewing. 

Diseases include Primary lateral sclerosis PLS and Pseudobulbar palsy both described below.

"Involvement of upper motor neurons results in stiffness of muscles (spasticity) and slowed movements, sometimes the muscles will spasm or spontaneously jerk." MND Fact Sheets Australia 

Primary lateral sclerosis PLS affects only upper motor neurons and is nearly twice as common in men as in women.  Onset generally occurs after age 50. 

The cause of PLS is unknown.  It occurs when specific nerve cells in the cerebral cortex (the thin layer of cells covering the brain which is responsible for most higher level mental functions) that control voluntary movement gradually degenerate, causing the muscles under their control to weaken.  The syndrome -- which scientists believe is only rarely hereditary -- progresses gradually over years or decades, leading to stiffness and clumsiness of the affected muscles.  The disorder usually affects the legs first, followed by the body trunk, arms and hands, and, finally, the bulbar muscles.  Symptoms may include difficulty with balance, weakness and stiffness in the legs, clumsiness, spasticity in the legs which produces slowness and stiffness of movement, dragging of the feet (leading to an inability to walk), and facial involvement resulting in dysarthria (poorly articulated speech). 

Major differences between ALS and PLS (considered a variant of ALS) are the motor neurons involved and the rate of disease progression.  PLS may be mistaken for spastic paraplegia, a hereditary disorder of the upper motor neurons that causes spasticity in the legs and usually starts in adolescence.  Most neurologists follow the affected individual's clinical course for at least 3 years before making a diagnosis of PLS.  The disorder is not fatal but may affect quality of life.  PLS often develops into ALS

Pseudobulbar palsy, which shares many symptoms of progressive bulbar palsy, is characterized by upper motor neuron degeneration and progressive loss of the ability to speak, chew, and swallow.  Progressive weakness in facial muscles leads to an expressionless face.  Patients may develop a gravelly voice and an increased gag reflex.  The tongue may become immobile and unable to protrude from the mouth.  Patients may also experience emotional lability



Primary Upper Motor Neuron Disorders Chart  Displays differences between Hereditary Spastic Paraplegia and Primary Lateral Sclerosis 

upper motor neuron

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