Olfactory System
Olfaction (smell) is an animal’s primary special sense, and their sense of smell is far more sensitive than that of humans. Olfaction is essential for the localisation of food, reflex stimulation, secretion of digestive enzymes, detection of danger, finding of direction, seeking of prey or avoiding of predators, avoiding of poisonous and spoiled feeds, social recognition of kin and sexual attraction to a mate.
For example, mice sniff for potential mates while avoiding those that are infected with parasites, and hermit crabs find buried snail shells by smelling calcium leaching from them. The sense of taste and smell is important to discriminate between desirable and undesirable feed, and by acting together, they contribute to the palatability of foods by detecting flavours that influence appetite.12.3.1 Components of Olfactory System
Olfaction in animals is mediated by two important sensory systems—the main olfactory system with receptors in the dorso-caudal part of the nasal cavity and the accessory olfactory system with receptors in the vomeronasal or Jacobson’s organ located near the external layers. The olfactory mucosa, present in the ceiling of the nasal cavity, occupies a relatively larger area in dogs (100 cm2) in comparison with that in humans (about 5 cm2). The mucosa has three cell types: basal cells, supporting cells and olfactory receptor cells. The basal cells being the precursor cells constantly replace the olfactory receptor cells. The supporting cells secrete mucus that forms a protective coat over the nasal passages. The olfactory receptors are bipolar cells with a single dendrite at one end that terminates in the olfactory mucosal surface as an expanded olfactory knob. Approximately, 10-20 cilia arising from each olfactory knob spread across the olfactory mucosal surface. The cilia have sensory receptors that act as transducers for the olfactory stimulus.
The membrane of cilia is covered with many G protein-coupled olfactory receptors.Fig. 12.3 Mechanism of taste. [The odorant molecules combine with olfactory binding proteins and concentrate to bind with odorant receptors (ciliary G protein-coupled receptor). Binding odorant molecule to the receptors activates G protein (Golf)-mediated signal transduction mechanism that leads to a series of electrical events]
12.3.2 MechanismofOlfaction
The volatile and water-soluble odorant molecules entering the nasal cavity are concentrated by the olfactory binding proteins secreted by the olfactory glands and are bound to the ciliary sensory G protein-coupled receptors (Fig. 12.3). Binding of an odorant molecule to the ciliary G protein-coupled receptor activates G protein (Golf) that unites with guanosine triphosphate (GTP). The GTP-Gof complex activates phospholipase C, generating inositol triphosphate (IP3) that opens Ca2+ channels and opens Na+ and Ca2+ channels through adenylyl cyclase-generated cAMP. Thus, the GTP- Golf complex leads to a series of electrical events (i.e. increase in intracellular Ca2+) that result in the generation of excitatory postsynaptic graded potentials (EPSPs) in the cilia, which ultimately results in the propagation of action potential along the axons of the olfactory cells to the olfactory bulb that is dispersed to wide areas of the cortex. The second set of neurons from the olfactory bulb divide into lateral and medial and project to the limbic system including hippocampus, frontal and temporal cortices, thalamus, amygdala, hypothalamus and reticular formation, thereby regulating feeding, sexual and emotional behaviours. Thus, olfactory signals, unlike other sensory systems, do not directly project to the thalamus. The projection of olfactory signals suggests that emotional reaction to olfaction is carried out by the entorhinal cortex, hippocampus, septal nuclei and amygdala of the limbic system.
Nonmyelinated axons arising from the olfactory receptor synapse with the mitral and tufted cells in the olfactory bulb in the olfactory lobe of the cerebral cortex. The specific affinity of olfactory receptor for each odorant and the differential electrical discharge rates of the mitral and tufted cells generate a unique activation pattern and subsequently a unique sense of odour. A dog, for example, has more than 220 million olfactory receptors in its nose, while humans have only five million.
A unique feature of olfactory transmission is its rapid adaptation to stimulus; that is, the initial discharge of axons in response to stimulation is followed by quick decline to a steady-state discharge of lower amplitude. Although the olfactory system is sensitive and highly discriminating, it is also quickly adaptive. Sensitivity to a new odour diminishes rapidly after a short period of exposure to it, in spite of the continued presence of odour source; this could be attributed to the presence of the odorant-clearing enzymes in the nasal mucosa. This mechanism might serve the dual purpose of clearing the olfactory mucosa of old odorants and transforming potentially harmful chemicals into harmless molecules. Such detoxification helps to avoid the entry of toxicant through the open passageway between the olfactory mucosa and the brain.
12.3.3 VomeronasalOrgan(VNO)
In addition to the olfactory mucosa, the vertebrate nose has accessory olfactory system with receptors in the Jacobson’s or vomeronasal organ (VNO) that mediates sex odours. The VNO plays a significant role in governing reproductive and social behaviours, such as identifying and attracting a mate. VNO is the major site for pheromone detection. The VNO is open at one end and forms a blind sac at the other end. The location of the opening is variable; in rodents, the opening is into the nasal cavity and in cows it opens into the oral cavity. The VNO is also involved in the perception of large, non-volatile molecules that could not reach the main olfactory system.
The receptors are similar to those of the main olfactory system, and they project to the olfactory bulb. The axons of the receptor from the olfactory nerve terminate in the olfactory bulb. The second set of neurons from the olfactory bulb divide into lateral and medial and project to limbic system including hippocampus, frontal and temporal cortices, thalamus, hypothalamus and reticular formation, thus regulating feeding, sexual and emotional behaviours. Hence, odour signals are not just for olfaction. Processing of odour signals by the limbic system is the basis for forming olfactory memories, and olfaction can evoke strong emotional reactions. In animals, the mother recognises its newborn young ones by odour. The olfactory system helps in the regulation of reproduction—animals are attracted by pheromones produced by opposite sex; individual recognition, mother-young interaction and establishment of dominance are some social behaviours affected by olfactory signals. Ungulates and rhesus monkeys detect the female in heat by the odour of vaginal secretions. Male ungulates, such as stallions, put their lips into the urine of an oestrus mare and curl their upper lip in the flehman position, which partially blocks the nostril opening. Specialised pumping mechanism aided with deep breathing serves to suck molecules (pheromones) into the organ and to carry the urine into the VNO, where the stallion determines sexual receptivity of the mare by the concentration of pheromone. Sex pheromones are also produced in the urine of female elephants about to ovulate, which initiates the flehman response in reproduc- tively active males. Males first identify the urine through the sense of smell and then detect the pheromone by placing urine from the tip of their trunk to the opening of ducts leading to the VNO. Androstenone, a steroid in the saliva of male pigs and human male sweat, is one of the few mammalian pheromones that have been characterised. Release of this potent pheromone in pigs causes sows to assume the lordosis (mating) position. Androstenone elicits this behaviour only in oestrus sows, suggesting that the response to a releaser pheromone is specific and contextual dependent. Animals differ in their ability to detect odours. Animals like dogs are very sensitive in the detection of odour and are called as macrosmatic; those that can detect odour but with less sensitivity like birds are termed microsmatic; those that lack olfactory apparatus like dolphins and whales are termed anosmic.12.4