The Anatomical Record | 2019
A Toast to the Cranial Nerves
Abstract
Letmemake a toast to the cranial nerves. To this purpose, I would use, for example, a Spanish redwine.Many different types of red wine would perfectly serve the purpose. Young wine, oak barrel-aged wine, Crianza, or Reserva wine. Grapes from the native varieties tempranillo, bobal, garnacha, or mencía. Alone, with olives, or accompanied by local cuisine. No matter the combination, all of these possibilities will bring you to distinguish the color of the wine, the smell but also the aroma, if its taste contains notes of wood, grass, red or green fruit, a thousand different possibilities come to a zenith when you try, besides, to get the best of a perfect marriage of the wine and the food. From your senses to the brain, all the complexity of wine savoring is made by means of highly specialized sensory organs and the incredible information-processingmachine that is our brain. But all of this information is never lost along its travel; its way from senses to brain and back is made by means of the most intricate and complex bulk of neural projections ever: the cranial nerves. In the Special Issue Cranial Nerves of The Anatomical Record, our Guest Editors Professor Alino Martínez-Marcos (UCLM, Ciudad Real) and Professor José Ramón Sañudo (UCM, Madrid) compiled an excellent list of papers divided in two volumes. In Volume 1 devoted to Phylogeny and Ontogeny, our Guest Editors grouped papers analyzing the development, the evolution, and the basic organization of cranial nerves. That work is now completed with Volume 2 devoted to the Morphology and Clinical Significance of the cranial nerves. The complexity of the organization and function of the cranial nerves is well represented in the papers grouped in these two volumes. My Commentary to Volume 1 (Trejo, 2019) drew a scheme of how andwhy cranial nerves show an extraordinarily intricate development serving the purpose of cephalization throughout species evolution. In Volume 2, my commentary focuses on describing one example of how this complex machinery carries the information up to the brain and to transfer the cerebral orders back to the skull, the jaws, the glands, the muscles, and the sensory organs to perform an apparently simple task. After testing the first sip of wine, we will let it breathe for a while to permit the plethora of visual, olfactory, and taste stimuli to arise. Meanwhile, I will expand upon what happened after that first contact with the wine. First, and according to the recommendations of the sommeliers, we have to see the wine. From violet blue to tile or brick red, through crimson or granate tones, the colors of wine talk about its age, health, and evolution. The intensity, luminosity (brightness), and clarity talk about other aspects of the wine’s quality. All of the visual properties of the wine are processed by the retina and sent to the thalamic lateral geniculate nucleus (diencephalus), through the second cranial pair, the optic nerve. Visual information ends its trip into the occipital visual cortex. If you wonder why the color of the wine is so relevant, let me cite here the legendary paper by Morrot (Morrot et al., 2001) demonstrating that students of a school of sommeliers confused a red wine with a white one during a wine testing after the experimenters tinted the white wine with a dye without flavor. The perception and processing of the color attribute of the wine is also very relevant for the next “sommelier” step: smelling. The reason is that the color of the wine (or food) influences our perception of its taste (Engen, 1972; for a revision, see Shepherd, 2012). The smell and aroma of wine, either orthonasal (through the nose -smell-) or retronasal (through the back part of the mouth -aroma-), are certainly core aspects of wine savoring. They inform about the age, the grape variety, and the procedure to make the wine. Expert sommeliers talk about coffee, flowers, or thyme scents among a hundred others, as well as about humidity or cork. The wine smell/aroma reports typical features of the wine strain, the fermentation process, and the bouquet or maturation. For all these reasons, smell is considered as the first proper pleasure of wine savoring. The odors stimulate receptor neurons in the olfactory mucosa. This information is sent to the olfactory bulb (first synapse) from where the processed information travels to the olfactory cortex, all this way constituting thefirst cranial pair, the olfactory nerve. The main output of the olfactory nerve is the orbitofrontal cortex, from which the processed information is sent to other brain regions related to the reception of information from other sensory organs. This way, the sense of smell influences our subjective perception of all the other senses. One of the best ways to understand the relevance of this path from the nose to the brain (LopezMascaraque and Trejo, 2013) is to take into account the data by Takagi’s group (reviewed by Takagi, 1986), demonstrating that the cells involved in the processing of olfactory information are increasingly more tuned along the neural pathway, with the most finely tuned being at the orbitofrontal cortex. Next, our sommelier will ask for us to “go to the mouth.” Here is when most properties of the wine appear, when a