Chemical Senses PDF

Body and Brain Chemical senses Alex Galazyuk, NEOMED, 2024 Major Areas to be Covered ❑ Olfactory System ❑ Taste ❑ Trigeminal chemosensory system  Chemical senses Olfactory system Overview. ◼...

Body and Brain Chemical senses Alex Galazyuk, NEOMED, 2024 Major Areas to be Covered ❑ Olfactory System ❑ Taste ❑ Trigeminal chemosensory system  Chemical senses Olfactory system Overview. ◼ The only one sensory system in which the first synapse is in the telencephalon (in the olfactory bulb). ◼ Each of about 100 million receptor cells are replaced every two months.  Chemical senses Olfactory system Olfactory epithelium and receptor cells. ◼ The olfactory epithelium in the nasal cavity consists of the olfactory receptor neurons (ORNs). ◼ ORNs have a peripheral and a central process. ◼ The central process make up the olfactory nerve fascicles that penetrate the cribriform plate to terminate at the olfactory bulb. ◼ The peripheral process of an ORN is very short extending from the cell body to the surface of the epithelium consisting microvilli.  Chemical senses Olfactory system Olfactory epithelium and receptor cells. ◼ The microvilli are bathed in mucus where odor molecules are dissolved. ◼ Hydrophilic molecules can be dissolved and diffuse to the receptors on the microvilli. ◼ The hydrophobic molecules are transported to the microvilli by odorant- binding proteins. ◼ The odorant molecules bind with ORN receptors, produce electric potentials  Chemical senses Olfactory system Olfactory epithelium and receptor cells. ◼ Each ORN express one or very few of about 400 genes. ◼ Each olfactory receptor can be activated to varying degree by multiple odorant molecules within one or very few families of odorants. If so, there is a unique response pattern of a population of ORNs to an individual odorant. ◼ The potentials generated by G protein-coupled receptors propagate passively to ORN soma where action potentials are generated. ◼ ORNs send their axons to the olfactory bulb neurons through the cribriform plate.  Chemical senses Olfactory system Central olfactory pathways. ◼ Axons of ORNs constitute the olfactory nerve (cranial nerve I) which projects to the olfactory bulb ◼ Each olfactory bulb principal neurons (Mitral cells) receive inputs from thousands of ORNs that express the same single olfactory receptor gene. ◼ Mitral cells of olfactory bulb project to other brain regions via olfactory tract.  Chemical senses Olfactory system Central olfactory pathways. ◼ Olfactory tract projects to the olfactory cortex which is a broadly defined area on the ventral surface of the telencephalon near the uncus. It includes the olfactory tubercle, pyriform cortex (main target), entorhinal cortex, and the amygdala. ◼ One of the main pyriform cortex outputs is the orbitofrontal cortex where conscious discrimination of odors occurs.  Chemical senses Taste Introduction ◼ The taste system detects and assesses the chemical composition of ingested substances. ◼ Tastants are dissolved in saliva and the perceived intensity of taste is proportional to concentration of tastant. ◼ Detection threshold vary over a large range depending on the value or toxicity of the tastant. Chemical senses Taste Taste cells ◼ Taste cells are found in specialized structures called taste buds, ~4000 of which are distributed on the dorsal surface of the tongue, soft palate, pharynx, and upper esophagus. ◼ Each taste bud contains 30 to 100 taste cells. Chemical senses Taste Taste cells ◼ Taste buds are located in structures called papillae. ◼ There are three types of papillae: fungiform (25% of all taste buds), circumvallate (50%), and foliate (25%). ◼ Fungiform papillae are only on the anterior two-thirds of the tongue with the highest density at the tip. ◼ There are 9 circumvallate papillae at the rear of the tongue. ◼ Two foliate papillae are present on the posterolateral tongue. ◼ Chemicals first stimulate receptors in the fungiform papillae and then in the foliate and circumvallate papillae.  Chemical senses Taste Peripheral taste pathways ◼ Taste cells in individual taste buds synapse with axons from branches of three cranial nerves (VII, IX, and X). ◼ The taste cells in fungiform and foliate papillae on the anterior two- thirds of the tongue are innervated by the branch of facial nerve (linked to salts, umami, and sugar). ◼ The circumvallate papillae at the rear of the tongue are innervated by the glossopharyngeal nerve (linked to bitter). ◼ Taste buds of the epiglottis and esophagus are innervated by the vagus nerve.  Chemical senses Taste Central taste pathways ◼ The rostral part of solitary nucleus (nucleus solitarius) often called the gustatory nucleus where taste fibers (CN X, IX, VII) terminate. ◼ The gustatory nucleus projects through the central tegmental tract to the thalamus (ventral posterior medial (VPM) nucleus). There are also direct projections from the gustatory nucleus to the amygdala and hypothalamus that influence appetite and other behavioral responses.  Chemical senses Taste Central taste pathways ◼ The VPM projects to several neocortical areas collectively referred to as gustatory cortex including the anterior insula (insular taste cortex). ◼ There are projections from gustatory cortex to the amygdala that influence emotional responses to food. ◼ Orbitofrontal cortex also receive taste information from gustatory cortex and integrate it with olfactory, visual and somatosensory inputs to give rise to the appreciation of flavor and food reward.  Chemical senses Trigeminal chemosensory system ◼The trigeminal system includes nociceptive receptor neurons activated by high concentrations of irritating chemicals that are in direct contact with mucous membranes of head – mouth, nose, eyes. ◼Some important activators of this system are ammonia (smelling salts), ethanol (liquor), acetic acid (vinegar), CO2 (in soft drinks), and capsaicin (chili peppers). ◼This “pain” information is relayed to cerebral cortex along with other trigeminal pain and temperature information (i.e., via spinal V tract and nucleus, ventral trigeminothalamic tract, etc.). Thank you!