NEUROSCIENCE: Motor Systems
Motoneuron Lesions

MOTONEURON LESIONS

A. Lower Motoneuron Lesion (e.g. traumatic peripheral injury, polio -- poliomyelitis)

1. Muscle paralysis or paresis (depending upon severity of injury)

2. Complete loss of muscle tone or reduced tone (hypotonia)

3. Complete loss of reflex response or weak response from the muscles innervated (hyporeflexia)

4. Immediately following motoneuron axon lesion, muscle shows small, random contraction of motor units (fasciculation) due to peripheral axon segment injury-associated APs (injury potentials) and development of new ACh receptors on the muscle fiber membrane; these involuntary contractions eventually disappear

Note: the paralysis characterized by hypotonia, hyporeflexia, and (initially) fasciculation is termed flaccid paralysis

Note: section of a peripheral mixed nerve also causes sensory anesthesia of the innervated region

5. Muscle atrophy

6. When the lesion involves the lower motoneuron axon but not the soma, axon regeneration is possible under favorable conditions; when the lesion involves a CNS tract, axon regeneration is not possible (at present)

B. Upper Motoneuron Pathway Lesion (e.g. spinal cord section; stroke -- sudden interuption of brain blood supply, hemorrhagic or ischemic -- involving motor pathways)

1. Muscle paralysis or paresis

2. Increased muscle tone (hypertonia, particularly in flexors), due to loss of descending inhibition of alpha and gamma motoneurons

3. Increased myotatic reflex response (hyperreflexia, “brisk” stretch reflex)

4. Often an oscillating response to brief muscle stretch (clonus)

e.g. response to a tendon tap: brief muscle stretch to elicit synchronous discharge of dynamic muscle spindle endings; results in brief contraction (normal response) or clonus (upper motoneuron lesion

EMG (electromyogram): the summed electrical activity from a whole muscle recoreded with extracellular (usually superficial) electrodes (usually an asynchronous pattern)

Clonus: an oscillating sequence of muscle contractions resulting from a brief stimulus

Note: the paralysis characterized by hypertonia, hyperreflexia, and (perhaps) clonus is termed spastic paralysis

5. Babinski sign: fanning and dorsiflexion of the toes upon stroking the plantar surface of the foot

Note: The Babinski sign is normal in newborns during the first 12-18 months of life until the myelinization of the corticospinal tract is complete

6. No muscle atrophy