Damage to Motoneurones results in Paralysis, Loss of Reflex Activity and Atrophy in the Denervated Muscles
Axons are susceptible to damage by trauma which either causes transection or compression of the nerve.
Carpal Tunnel Syndrome is very common: the median nerve becomes compressed as it passes into the hand between bones and ligaments. Median nerve compression gives rise to muscle weakness and atrophy, resulting in an inability to undo bottle tops and grasp objects.. In addition sensory symptoms such as tinglings and numbness are present in the distribution of the median nerve as a result in compression of sensory axons.
The effects of loss of motoneurones supplying a skeletal muscle is characterised by
loss of electrical activity or movement on electrical stimulation of a motor nerve
reduction in reflex activity
atrophy of the muscle
In addition denervated muscle exhibits abnormal electrical activity called fibrillation potentials. If the muscle becomes reinnervated the Motor Unit size is likely to increase. These changes are investigated methods described in the Clinical Neurophysiology section.
Bell's Palsy is a lower motoneurone lesion of the facial nerve. The nerve appears to become inflammed in a narrow bony canal, which compresses the nerve; bony fractures that imping on the facial nerve also cause this lesion. One side of the face droops, and there is no control of the muscles of the forehead or eyelid (unlike in stroke).
Polio is a viral infection that selectively kills motoneurones. The paralysis and wasted muscles of polio victims are characteristic of a lower motoneurone lesion.
Motoneurone Disease is a disease of unknown aetiology, in which the motoneurones progressively degenerate. As in Polio, the respiratory muscles can be affected and cause respiratory failure.
Further consideration of the pathology of peripheral neurones is discussed in the chapter on Neuropathology