ABSTRACT
Jurgens (2002) reviewed the neural pathways that underlie vocal control in normal speech production. A simplified and partial list of the central nervous system (CNS) structures that he proposed were involved in voluntary vocal reactions, as opposed to involuntary ones, as when individuals make shrieks of pain, are listed, and these are indicated in diagrams after a brief description. (1) The pyramidal pathways (primary motor cortex). The pyramidal or corticospinal pathways are a collection of axons that travel between motor cortex and the spinal cord. These pathways are concerned specifically with discrete voluntary skilled movements. (2) The extrapyramidal pathways (parts of the basal ganglia structures). The extrapyrimidal pathways are motor paths that lie outside the pyramidal tract and are not under voluntary control. Their main function is to support voluntary movement and coordination. Nos. 1 and 2 operate together to activate muscles in the face, neck, and torso, which are involved in oral motor functions, including speech. (3) The previous structures need input from the cerebellum, which, according to Jurgens, allows for smooth transitions between speech segments. Other authors emphasize the role of the cerebellum in sensory-motor integration (see Chapters 6 and 11). The primary motor cortex receives additional inputs from (4) the ventral premotor and the prefrontal cortex, including Broca’s area, assumed
to be associated with motor planning. The primary motor cortex also receives inputs from the two other cortical motor areas usually mentioned in the literature: (5) the premotor area (PMA) and (6) the supplementary motor area (SMA). Nos. 5 and 6 together are assumed to give rise to the motor commands executed by the motor cortex. (7) Somatosensory cortex is the processing center that serves the several sensory modalities. The auditory modality is of particular interest for authors who consider that auditory feedback is important is speech control (see Chapter 8). Most of these structures are bilaterally represented. However, it has been known for several years that the left brain plays a major role in speech control in individuals with no pathology and there has been examination of lateral pathways in people who stutter. The main structures left out of this list that Jurgens included are proprioceptive input from the respiratory, phonatory, and articulatory organs via nucleus ventralis posterior medialis thalami and inferior parietal cortex, all of which supply input to motor cortex.
