Further, unlike spinal nerves, cranial motoneurons have no collaterals and thus exhibit no recurrent inhibition. These and many other unique features are paralleled by muscle tissue differences. Orofacial muscles, like muscles in primate distal extremities, appear to be monosynaptically activated by motor cortex pyramidal cells. These neurophysiological principles and the substantial variations among orofacial muscle groups offer insights into speech movement disorders. The most common speech movement disorder is dysarthria.
Dysarthria is a speech system manifestation of a more extensive movement disorder.
The manifestation of these disorders that is most obvious is the slowing and slurring of speech with characteristics that are somewhat characteristic for different forms of neurological impairment. For example, cerebellar disease typically yields speech that is similar to that of someone who is inebriated, while the speech of someone with ALS is characterized as breathy or weak. Most, importantly, due to the special and variegated neural substrate of the speech motor system, described previously, systemic or supra-nuclear neurological disorders yield muscle contraction and movement aberrations that are different from the classic patterns observed in the upper or lower limbs. Additionally, a given basal ganglia, cortical or cerebellar deficit does not appear to impair motor control of the different respiratory, laryngeal and orofacial muscle groups in the same way. These and other recent data on speech motor system pathophysiology indicate that the aberrations associated with systemic or general movement disorders are different in degree and kind among the parts of the speech system and different from motor aberrations classically described for the limbs.