Olfactory receptor neurons location

What happens at the instant when an odor-active molecule comes in touch with our nasal cavity 205 The first interaction of odorant molecules takes place in the olfactory receptor neurons, which are embedded in the pseudostratified columnar epithelium (or simply, olfactory epithelium), which is located in the posterior nasal cavity in the case of mammals. Olfactory sensory neurons express receptor proteins on the surface membrane of the cilia, which gain access to the extracellular region covered with mucus. The airborne odorants are dissolved into the mucus, bind with the receptors, and then the receptor protein triggers a signal transduction cascade. This results in the opening of the cation channel that would depolarize the sensory neuron and eventually elicit a train of action potentials in the axon. The olfactory axon leads to the olfactory bulb through basal lamina and lamina propria. 

Olfactory receptor neurons are located in the nasal epithelium. The axons of these neurons project directly to the olfactory bulb, a part of the cerebral hemisphere. This anatomical arrangement provides a direct link between the central nervous system and the organism s external environment including the possible entry of pathogens and neurotoxic chemicals into the brain (McLean et al. 1989). 

Each aesthetasc is innervated by bipolar olfactory neurons (seen in cross section in Fig. 11.2c) whose cell bodies are located within the dorsolateral flagellum of the antennule (Mead and Weatherby 2002). In G. falcatus, there are 12-20 olfactory receptor neurons (ORNs) per aesthetasc. Each of these cells gives rise to multiple dendrites, as in some crabs, mysids, and other crustaceans (see Hallberg and Skog,... 

In vertebrates the neurons for olfaction are located in the nose mucosa and consist of short neurons with a peripheral ending endowed with odorant receptors for a large number of molecules in the environment. Each receptor neuron only contains one odorant receptor and is connected directly with the olfactory lobe of the brain. The vertebrate olfactory system must cope with a staggering developmental problem how to connect millions of olfactory neurons expressing different odorant receptors to appropriate targets in the brain. 

Although pheromones can be considered as a special form of odorants (scents), their actions, effects and functions have similarities to those of hormones. They bind to a specific receptor which then activates an effector system, which initiates an action potential. They bind to specific sensory cells, the neurones, in the olfactory epithelium, which is located on the roof of the nasal cavities. The epithelium consists of three types of cells, basal, supporting and sensory cells (neurones). The neurones are bipolar, that is they possess a single dendrite, which extends from the cell body to the surface of the olfactory epithelium, and an axon that forms a synapse with a nerve that transfers information to the olfactory centre in the brain. The epithelium is covered with a thick layer of mucus, in which the pheromones dissolve. The mucus contains proteins that bind the pheromone(s) for delivery to the olfactory receptors and then to remove them once they have been detected. 

Figure 14.1 Schematic of olfactory sensillum and a generalized biochemical pathway of odor reception. A An olfactory sensillum includes 2-3 neurons surrounded by 3 support cells olfactory dendrites/cilia project up the fluid filled lumen of a cuticular hair. The sensillum lumen is isolated from hemolymph by a cellular barrier. Modified from Steinbrecht (1969) see Steinbrecht (1999) for more details. B Hydrophobic odor molecules enter the aqueous sensillum lumen via pores penetrating the cuticular hair wall. Hydrophilic OBPs are proposed to bind and transport odors to receptor proteins located in the neuronal membranes. ODEs (pathway I) in the sensellum lumen are proposed to degrade these odor molecules. Cytoplasm of support cells contain xenobiotic inactivating enzymes, such as glutathione-S-transferase (GST) (pathway I la) which may also serve to inactivate odor molecules (pathway lib). Interactions between OBPs and ORs and the function of SNMP are unclear. Modified from Rogers et al. (1999).

One of the lesser functions of the nose in man is that of olfaction. The olfactory region of the nose, a small patch of tissue containing the smell receptors, is located towards the roof of the nasal cavity and is lined with non-ciliated neuro-epithelium. Approximately 20% of the air flowing through the nasal cavity is directed upwards to the olfactory region. Here, bipolar neurones react to inspired air and initiate impulses in the olfactory nerves. 

In the NAc core and in the olfactory tubercle, similarly to the CPu, D2 receptors are located in GABAergic neurons coexpressing enkephalin, whereas in the NAc shell D2 receptors are expressed in neurotensin-containing neurons (Le Moine et al., 1990b Diaz et al., 1994 Le Moine and Bloch, 1995 Delle Donne et al., 1996). In addition, in both the NAc and the olfactory tubercle an overall co-distribution of D2 receptor and TH immunoreactivity was found, consistent with the presence of D2 autoreceptors. In the shell portion of the NAc, D2 immunoreactivity was detected with similar frequency in terminals and dendritic spines (Delle Donne et al., 1997). 

Figure 32.8. Converging Olfactory Neurons. This section of the nasal cavity is stained to reveal processes from sensory neurons expressing the same olfactory receptor. The processes converge to a single location in the olfactory bulb. [From P. Mombaerts, F. Wang, C. Dulac, S. K. Chao, A. Nemes, M. Mendelsohn, J. Edmondson, and R. Axel. Cell 87(1996) 675-689.]...

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