Final answer:
The target of an upper motor neuron is the lower motor neuron. Upper motor neurons originate in the frontal lobe and project to lower motor neurons, which in turn innervate skeletal muscles to produce voluntary movements. Option D is correct.
Step-by-step explanation:
The query concerns the motor control hierarchy in humans and which level is considered the lowest. To provide context, upper motor neurons have specific target connections within the motor system. The target of an upper motor neuron is the lower motor neuron.
The upper motor neurons originate in the primary motor cortex located in the frontal lobe and are responsible for the initiation of voluntary movements. They project their axons to various locations, including the cranial motor nuclei or the ventral horn of the spinal cord, where the lower motor neurons reside.
The lower motor neurons then extend out to innervate skeletal muscle, causing it to contract upon stimulation. This system can be summarized as two main cells: the upper motor neuron (UMN) and the lower motor neuron (LMN). Movements require the sequential activation of these neurons, with the upper motor neuron acting first, followed by the lower motor neuron, which directly contacts the skeletal muscles to produce movement.