Parabrachial area

The parabrachial nucleus, also known as the parabrachial complex, is an area in the dorsolateral pons that surrounds the superior cerebellar peduncle as it enters the brainstem from the cerebellum. It gets is name from the Latin term for the superior cerebellar peduncle, the brachium conjunctivum. In the human brain, the massive expansion of the superior cerebellar peduncle expands the parabrachial nucleus, which forms a thin strip of gray matter over most of the peduncle. The parabrachial complex is typically divided along the lines suggested by Baxter and Olszewski in humans, into a medial and lateral subnucleus.^[1] These have in turn been subdivided into a dozen subnuclei: the superior, dorsal, ventral, internal, external and extreme lateral subnuclei; the lateral crescent and Kolliker-Fuse nucleus along the ventrolateral margin of the lateral parabrachial complex; and the medial and external medial subnuclei^[2][3]

The parabrachial nucleus receives visceral afferent information from a variety of sources in the brainstem, including a massive input from the nucleus of the solitary tract, which brings information about taste to the ventral, medial, and external medial subnuclei, and information about the remainder of the body to the lateral subnuclei and Kolliker-Fuse nucleus.^[4] The external, dorsal, internal and superior lateral subnuclei also receive input from the spinal and trigeminal dorsal horn, mainly concerned with pain and other visceral sensations.^[5] Outputs from the parabrachial nucleus originate from specific subnuclei and target forebrain sites involved in autonomic regulation, including the lateral hypothalamic area, ventromedial, dorsomedial, and arcuate hypothalamic nuclei, the median and lateral preoptic nuclei, the substantia innominate, the ventroposterior parvicellular and intralaminar thalamic nuclei, the central nucleus of the amygdala, and the insular and infralimbic cortex.^[6] The Kolliker-Fuse nucleus and lateral crescent send efferents to the nucleus of the solitary tract, ventrolateral medulla, and spinal cord, where they target many respiratory and autonomic cell groups.^[7] Many of these same brainstem and forebrain areas send efferents back to the parabrachial nucleus as well.^[8][9]

Function

Many subsets of neurons in the parabrachial complex that target specific forebrain or brainstem cell groups contain specific neuropeptides.^[10] and appear to carry out distinct functions. For example, a population of neurons in the external lateral parabrachial subnucleus that contain the neurotransmitter calcitonin gene-related peptide (CGRP) appears to be critical for relaying information about hypoxia or hypercapnia (e.g., if one is being suffocated during sleep, such as by sleep apnea) to forebrain sites to wake up the brain, and prevent asphyxia.^[11] Other neurons in the superior lateral parabrachial subnucleus that contain cholecystokinin have been found to prevent hypoglycemia ^[12] and other neurons in the dorsal lateral parabrachial subnucleus that contain dynorphin sense skin warmth from spinal afferents, and send that information to preoptic neurons involved in thermoregulation.^[13] Parabrachial neurons in rodents that relay taste information to the ventroposterior parvicellular (taste) nucleus of the thalamus are mainly CGRP neurons in the external medial parabrachial nucleus and they project predominantly contralaterally, as well as a smaller number in the ventral lateral nucleus, which project mainly ipsilaterally.^[14]

Recent data indicate that glutamatergic neurons in the medial and lateral parabrachial complex, along with glutamatergic neurons in the pedunculopontine tegmental nucleus, provide a critical node in the brainstem for producing a waking state.^[15][16] Lesions of these neurons cause irreversible coma, while lesions of other components of the ascending arousal system, such as the locus coeruleus, histaminergic tuberomammillary nucleus, or lateral hypothalamic orexin neurons, have very little effect on the total amounts of wake-sleep.

References