Final answer:
The dual ability of hands to produce both fine and gross movements is explained by overlapping motor circuits that coordinate muscle control through both direct and indirect motor pathways.
Step-by-step explanation:
The ability of hands to produce both fine and gross movements can be explained by overlapping motor circuits in the nervous system. This convergence of pathways allows for both types of movements to be controlled in a coordinated manner. For instance, the descending pathways responsible for voluntary movements include the corticospinal and corticobulbar tracts, which send motor commands from the primary motor cortex to activate either upper motor neurons in the cranial motor nuclei or in the ventral horn of the spinal cord.
The corticobulbar tract's axons are ipsilateral, while the corticospinal tract's axons are largely contralateral. Additionally, the basal nuclei or basal ganglia include two pathways, the direct and indirect pathways, that modulate motor activity by affecting the degree of inhibition or disinhibition of the thalamus, which in turn influences the cerebral cortex. The combination of these motor control centers and pathways enables precise and varying degrees of muscle movement necessary for fine and gross motor activities.