Reticulospinal and Corticoreticular Pathways

Pontine reticulospinal fibers project to the spinal cord via the medial reticulospinal tract only. This tract traverses the anterior funiculus ipsilaterally and extends along the entire length of the spinal cord, sending terminal branches to innervate the gray matter of the anterior horn (see Plate 2-12). It excites large numbers of spinal motor neurons of all types, especially flexor motor neurons and motor neurons controlling proximal (trunk and axial) muscles. The pontine system also has a strong, indirect influence on lumbar motor neurons, relayed by spinal interneurons (see bottom of illustration).

The medullary reticulospinal system has a more complex pattern of projection. Most fibers originating from the gigantocellular nucleus project to the spinal cord via the ipsilateral lateral reticulospinal tract (RST), which is part of the lateral anterolateral funiculus. In addition, some medullary reticular neurons project via the contralateral lateral reticulospinal tract, and some join the medial reticulospinal tract. Within the spinal gray matter, terminations of medullary reticulospinal fibers cover an exceptionally wide area, encompassing most of the anterior horn and the basal portion of the posterior horn (see Plate 2-12).

The physiologic action of the medullary reticulospinal system on spinal motor neurons is twofold and quite complex. Stimulation of the rostral part of the gigantocellular nucleus produces excitation of motor neurons, whereas stimulation of its caudal-anterior part produces inhibition. Actions on axial motor neurons are mediated by direct connections, whereas those on limb motor neurons are relayed by spinal interneurons. Stimulation in the caudal-anterior area also produces inhibition of spinal interneurons and inhibition of afferent transmission to the spinal cord. The exact pathways mediating these various effects are unknown, but they appear to involve reticulospinal fibers descending in the posterolateral funiculus (not shown in illustration).

Reticulospinal fibers seem to be involved in the control of certain voluntary and reflex movements, the integration of sensory input to guide motor output, and the coordination of bilateral movements. They can influence both diffuse motor activity and more focused goal-directed movements. The reticulospinal tracts include descending autonomic fibers that terminate on sympathetic and parasympathetic preganglionic neurons and allow the hypothalamus to influence the autonomic outflow.

Input to the medial brainstem reticular formation originates from many sources. Most major sensory systems send collateral branches to one of its regions, and the most pronounced sensory input to the source of reticulospinal fibers comes from the cutaneous and high-threshold muscle receptors of the body. As indicated by the relative width of the particular lines in the illustration, physiologic studies demonstrate that the great majority of both pontine and medullary reticulospinal neurons receive strong excitatory input from structures involved in motor control, including the motor or premotor cerebral cortex, themselves a part of an extensive corticoreticular system originating from all parts of the cortex, the cerebellar fastigial nucleus, and the superior colliculus. The resulting corticoreticulospinal connections constitute an extrapyramidal pathway by which motor regions of the cortex can act on the spinal motor apparatus. The physiologic role of the projections from sensory regions of the cerebral cortex to reticulospinal neurons is less certain, but such pathways may be involved in the regulation of sensory input to the spinal cord through reticular-evoked presynaptic inhibition of spinal afferent fibers or through postsynaptic inhibition of spinal sensory interneurons.

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