Week 6 Reading: The Apraxias Geschwind 1974 Preface The praxis have special importance in the history of callosal syndromes, since it was the study of these disorders which led Lepmann to his analyses of syndromes of disconnection · The praxis are disturbances which are characterized by "the incapacity for purposive movement of the limbs despite retained mobility" - This definition excludes from consideration all forms of weakness, disturbances of tone or posture, reflex abnormalities whether elementary or complex (ex .: the movement disorders resulting from the grasp reflect), disturbances of coordination or abnormal movements, as well as the failures of movement which result from complete deafferentiation of the limb · To show that a failure of movement is apraxic, we must show that a movement not made under one set of conditions can be performed in other conditions, or that movements of equal or greater complexity can be made under other circumstances - We must also show that the failure to make a movement is NOT the result of incomprehension of a command, the result of unwillingness of the patient to cooperate with the examiner, or non-recognition of objects shown to him · This paper suggests that the praxis are not complex motor or complex perceptual disturbances, nor are they the result of incomprehension or uncooperativeness · To understand the apraxias, they must be seen predominantly as associative disturbances, or, failures of connection between higher regions of the nervous system, in particular different parts of the cerebral cortex · To do this, we must understand the patterns of connection between cortical regions Connections (pg. 3 para #2) · The primary cortical motor and sensory regions (ex .: those which receive or send long subcortical projections, have no direct connections one to the other; they only have connections with the association areas (the immediately adjacent cortical regions) . The association areas of the brain are the only cortical regions that have long connections with other parts of the cortex
So how might one carry out a learned motor act to a learned auditory stimulus? · The auditory stimulus will travel along the auditory pathways and will eventually reach the primary auditory cortex (aka Heschl's Cyrus) on the superior surface of the temporal lobe - From here, it is sent to the auditory association cortex adjacent to Heschl's gyrus . From the auditory association cortex there are probably either no direct connections to the primary motor cortex or only a small number of such connections - BUT there are connections which probably arise in the auditory association cortex and run to the cortex lying anterior the to motor cortex (called the motor association cortex (MAC)); the MAC then has connections to the primary motor cortex - The pathway between auditory and motor association cortices is part of the arcuate fasciculus . See Figure 1 for these pathways! MC (area 4) MAC (area 6) AF W Fig. 1. Lateral view of left side of brain. AF = arcuate fasciculus, MAC = motor association