As we review the major efferent (motor) pathways, we can gain an understanding of the etiology of the spasticity developed due to Sherrington's experiments.

The primary motor cortex in the above diagram is labeled with a "+" symbol as it is responsible for directly "exciting" alpha lower motoneurons for voluntary motor control. Damage to this area will create an inability to perform voluntary motor functions, but will not create spasticity.

The red nucleus is also designated with a "+" symbol as an excitatory center. The red nucleus "fires" in response to the need to assist in coordination of movement as a result of output from the corticospinal or pyramidal tracts. If damage occurs in this area, part of the "feedback" system is damaged and there will be loss of coordination, but not spasticity.

The reticular formation may be subdivided into lateral and medial components. The lateral component is quite large in comparison to the medial as is shown in the diagram above. It is also marked with "+" symbols as it is an excitatory center. Output from this region facilitates firing of lower motoneurons via the lateral reticulospinal tracts. This region is tonically active and will create excessive tone if not mediated in some way! To balance output from the lateral reticular formation and maintain normal tone, the medial reticular formation is an inhibitory center and, therefore, emits inhibitory signals to lower motoneurons to modulate lateral reticular formation output via the medial reticulospinal tracts. However, the medial reticular formation is not tonically active and is quite small in comparison to the lateral reticular formation. The medial reticular formation is "driven" by three other areas of the upper central nervous system to facilitate its firing. These areas are the premotor cortex, the basal ganglia, and some of the cerebellar nuclei. These are marked with a "-" symbol in the above diagram as the result of their actions on the medial reticular formation is inhibition. However, these areas are actually areas that facilitate the medial reticular formation to fire and should be marked with a "+" sign as they are actually facilitation centers. They are referred to as suppressor areas in order to better describe their functional effects! As these three suppressor areas facilitate output from the medial reticular forma-tion, there is enough to balance the facilitatory effects of the lateral reticular formation at lower levels to maintain normal tone. However, if one of these three suppressor areas is damaged, transient spasticity normally occurs as the other two suppressor areas "learn" to compensate for the loss. If two of the three suppressor areas are damaged, chronic spasticity normally occurs. In Sherrington's experiments, he took away the effects of two of the three suppressor areas by introducing an intercollicular transection. Therefore, he created a chronic spastic condition. This condition was alleviated by a dorsal rhizotomy at spinal levels. Therefore, we can conclude that the lateral and medial reticular formations primarily affect gamma lower motoneurons!

Another facilitation area noted above is the vestibular nucleus. Output from this area is constant as we live in a world with gravity which the vestibular apparatus is constantly reacting to! As a result, the vestibular nucleus constantly sends excitatory signals to lower motoneurons. This could result in spasticity if it weren't for the fact that the anterior cerebellum is an inhibitory center to the vestibular nucleus. Due to activity in the anterior cerebellum, vestibular out-put is inhibited to assist in maintaining normal tone at lower levels. If the anterior cerebellum is damaged (as indicated with the purple "X" in the above diagram), spasticity occurs at lower levels as out-put from the vestibular nuclei is increased. Since Sherrington demonstrated that this type of spasticity is not decreased by dorsal rhizotomies, vestibulospinal tracts must directly influence alpha lower motoneurons!

With this basic understanding of higher center influences on lower motoneurons, it is easier to understand the resultant changes in tone that often occurs as a result of damage to higher centers (e.g., stroke, head injury, or tumor).

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