Gigantocellular reticular nucleus

The gigantocellular reticular nucleus (also magnocellular reticular nucleus) is the (efferent/motor) medial zone of the reticular formation of the caudal pons and rostral medulla oblongata. It consists of a substantial quality of giant neurons, but also contains small and medium sized neurons.

It gives rise to the lateral (medullary) reticulospinal tract which influences muscle tone of limb and trunk muscles, is involved in coordination of head-eye movements, promotes parasympathetic reduction of heart rate to decrease blood pressure, induces inspiration, and participates in the descending pain-inhibiting pathway.

Afferents
It receives connections from the periaqueductal gray, the paraventricular hypothalamic nucleus, central nucleus of the amygdala, lateral hypothalamic area, and parvocellular reticular nucleus.

It receives afferent corticoreticular fibers from the premotor cortex and supplementary motor area which modulate the activity of reticulospinal and reticulobulbar efferents of the its.

It receives vestibular, visual, and auditory afferents to mediate head-eye movement coordination.

It receives excitatory enkephalinergic afferents from the periaqueductal gray which influence its descending pain-inhibiting efferents.

Extrapyramidal motor functions
It gives rise to the lateral (medullary) reticulospinal tract (which excites flexors and inhibits extensors of the muscles of the axial and proximal limbs).

It is also involved in coordination of head-eye movements (receiving visual, vestibular, and auditory information to this end).

Blood pressure regulation
The GGRN forms part of the vasodepressor center which projects through the reticulobulbar tract to synapse upon pre-ganglionic parasympathetic neurons of the nucleus of vagus nerve. It acts to decrease blood pressure by decreasing heart chronotropy (rate) by increasing vagal parasympathetic outflow to the heart.

Respiration
GGRN induces inspiration (whereas the parvocellular nucleus causes expiration).

Descending pain-inhibiting pathway
The GGRN - together with the nucleus raphe magnus - gives rise to the descending serotonergic raphespinal tract which projects to the spinal cord to inhibit transmission of pain stimuli. The serotonergic analgesic component of the GGRN receives excitatory enkephalinergic afferents from the periaqueductal gray.