Final answer:
The primary motor cortex serves as the main departure point for motor signals sent to lower levels of the sensorimotor hierarchy. It targets lower motor neurons, initiating movement that is executed by skeletal muscles. Located in the precentral gyrus of the frontal lobe, it comprises descending pathways essential for voluntary movements.
Step-by-step explanation:
The primary motor cortex is the main point of departure of motor signals from the cerebral cortex to lower levels of the sensorimotor hierarchy. Thus, the correct answer to the fill-in-the-blank question is 'c) Primary Motor'. Regarding the multiple-choice questions, the target of an upper motor neuron is 'b) lower motor neuron'. Upper motor neurons send signals to lower motor neurons, which in turn directly innervate skeletal muscle fibers and cause them to contract.
The primary motor cortex is located in the precentral gyrus of the frontal lobe and is responsible for initiating movement. This area of the brain contains cells known as Betz cells whose axons descend through the corticospinal and corticobulbar tracts to synapse on lower motor neurons in the spinal cord or cranial motor nuclei. The descending pathways from the primary motor cortex are thus critical for conscious or voluntary skeletal muscle movements.