Final answer:
Upper motoneurons, originating in the primary motor cortex, are essential in controlling voluntary movements and reflexes by synapsing on lower motor neurons in the ventral horn of the spinal cord.
Step-by-step explanation:
Upper motoneurons indeed play a crucial role in the descending control of reflexes. These neurons are primarily responsible for initiating and precisely controlling voluntary movements of skeletal muscles. Each upper motor neuron located in the primary motor cortex of the frontal lobe sends its axons down the corticospinal tract to synapse on lower motor neurons within the ventral horn of the spinal cord. The upper motor neurons facilitate or inhibit the activity of these lower motor neurons, thereby providing an additional layer of control over muscle contractions and reflexes.