The reticular formation receives much afferent input, both from the spinal cord and from other parts of the brain. The lateral column of the reticular formation is an important reception area for incoming sensory and other afferent inputs; inputs from collaterals arising from ascending spinal sensory tracts and from spinoretic-ular tracts are especially prominent. The lateral column is made up of nuclei consisting chiefly of small cells, including the parvicellular nucleus in the medulla and pons and the parabrachial nucleus in the pons and midbrain.
Ascending spinoreticular fibers travel in the ventral portion of the lateral funiculus with spinothalamic fibers (see p. 160), but they diverge in the caudal medulla. The fibers of the spinoreticular tracts terminate in the lateral column of the reticular formation, but give off long collaterals that travel further up to the thalamus. Sensory inputs to the lateral column are relayed to the medial column and the raphe nuclei. For example, the parvicellular nucleus relays inputs from the ascending auditory, trigeminothalam-ic, and spinothalamic pathways to the medial column and raphe nucleus. Functionally, these ascending pathway systems modulate sensory inputs to maintain alertness. In addition, the reticular formation may receive information about noci-ceptive and thermoceptive signals through collaterals given off by the spinothalamic tracts. The reticular formation also receives information about visual signals through inputs from the superior colliculus.
The reticular formation receives several other sensory inputs in addition to those from the ascending spinal tracts. For example, the lateral column receives visceral inputs in collaterals from ascending fibers of the tractus solitarius (solitary tract), which originates in the nucleus
Greenstein, Color Atlas of Neuroscience © 2000 Thieme of the tractus solitarius, which in turn receives inputs from the vagus and other sensory nerves. The lateral column receives auditory inputs from the vestibular nuclei.
The reticular formation receives descending inputs from the cerebral cortex through corticoreticular fibers that originate principally in cortical regions giving rise to the pyramidal tract. These fibers terminate mainly in areas of the reticular formation that project caudally to the spinal cord; this system is the corticoreticulo-spinal pathway, which is involved in the control both of automatic and voluntary motor activity. The reticular formation also receives inputs from the cerebellum. These originate principally in the fastigial nucleus, and the pathway is important in the mediation of cerebellar control over the activity of both a and y spinal moto-neurons.
The periaqueductal gray matter (PAG) surrounds the cerebral aqueduct. Many of its larger neurons form complex connections with the midbrain reticular formation. The PAG also has powerful reciprocal connections with the frontal cortex, hypothalamus, and with the 5-HT (serotonergic) neurons of the medulla. Together with the reticular formation, the PAG plays an important part in the integration of nociceptive, autonomic, and limbic activities.
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