The common terminal pathway of all centers involved in motor activity is the large anterior horn cell and its axon (alfa motor neuron), which innervates the voluntary skeletal muscles. Most of the tracts running to the anterior horn do not end directly on the anterior horn cells but terminate on interneurons. The latter influence the motor neurons either directly or act by inhibiting or activating the reflexes between muscle receptors and motor neurons. The anterior horn is therefore not simply a relay station as described earlier but a complex integration apparatus that regulates motor activity. The central regions that influence motor activity via descending pathways are interconnected in many ways.
The most important afferent pathways stem from the cerebellum, which receives the impulses of muscle receptors via the spinocerebellar tracts and the stimuli of the cortex via the corticopontine tracts. The cerebellar impulses are transmitted via the parvocellular part of the dentate nucleus and the ventral lateral nucleus of thalamus to the precentral cortex (area 4). The corticospinal (pyramidal) tract descends from area 4 to the anterior horn and gives off collaterals in the pons that return to the cerebellum. Additional cerebellar impulses are transmitted via the emboliform nucleus and the centromedian nucleus of the thalamus to the striatum and via the magnocellular part of the dentate nucleus to the red nucleus. From here fibers run in the central tegmental tract via the olive back to the cerebellum and in the rubroreticulospinal tract to the anterior horn. Fibers from the globose nucleus run to the interstitial nucleus of Cajal and from there in the interstitiospinal fasciculus to the anterior horn. Finally, cerebellofugal fibers are relayed in the vestibular nuclei and in the reticular formation to the vestibulospinal tract and the reticulospinal tract, respectively.
The descending pathways can be divided into two groups according to their effect on the muscles: one group stimulates the flexor muscles, and another group stimulates the extensor muscles. The corticospinal tract and the rubroreticulospinal tract activate mainly the neurons of the flexor muscles and inhibit the neurons of the extensor muscles. This corresponds to the functional importance of the corticospinal tract for delicate and precise movements, especially those of hand and finger muscles where flexor muscles play an important role. In contrast, the fibers of the vestibulospinal tract and the fibers from the pontine reticular formation inhibit the flexors and activate the extensors. They belong to a phylogenetically old motor system that is directed against the effect of gravity and, thus, is of special importance for body posture and balance.