6 2 Chemoreception Chemoreception is the ability to detect chemicals in the internal and external environment Chemoreceptor cells have membrane receptor proteins that transduce the chemical stimulus into an electrical signal they generate graded potentials in response to particular chemicals in their environment The graded potentials open voltage gated Ca2 channels which cause vesicles of neurotransmitter to be released onto a neuron Eventually this signal is transmitted to the CNS where the animal perceives the chemical stimulus This lesson will focus on the chemoreceptive processes of olfaction sense of smell and gustation sense of taste These processes detect external stimuli There are also internal chemoreceptors such as those that detect changes in O2 levels CO2 levels and pH Internal chemoreception is crucial for regulating blood pressure and respiratory rate In terrestrial animals the distinction between olfaction and gustation is clear Olfaction is the detection of air borne chemicals this process allows animals to sense chemicals that are some distance from their bodies Thus olfaction allows animals to orient themselves towards favorable stimuli mates food and away from noxious stimuli predators toxic compounds Gustation is the detection of dissolved chemicals in ingested food which is why your sense of taste is impaired when your tongue is dry This process requires the animal to come into contact with the source of the chemicals This distinction is less clear in aquatic animals as all of the chemicals in their environment are suspended in an aqueous environment that is somewhat continuous with their bodily fluids Nevertheless aquatic olfaction is used to detect a wide variety of environmental chemicals food predators mates locations while gustation involves detecting food chemicals We can also compare these senses between vertebrates and invertebrates Vertebrates have separate organs signal transduction mechanisms and integrating centers for each sense Some of the chemoreceptive sensory cells are neurons and others are modified epithelial cells Either way a stronger chemical stimulus will cause a greater graded potential which will induce a higher frequency of APs to be fired by the sensory afferent encoding a stronger stimulus to the CNS In invertebrates olfactory and gustatory receptors may co occur instead of being isolated to separate organs It can be difficult to determine whether the animal is smelling or tasting something without closer inspection but there are indeed different cells that correspond to each sense As in other animals chemicals that are smelled are volatile and air borne chemicals that are tasted are picked up from direct contact with the source Vertebrate Olfaction Olfaction in vertebrates and invertebrates particularly insects occurs via the same cellular transduction mechanism but the genes involved have very little sequence similarity This suggests that the olfactory system in vertebrates and invertebrates evolved independently Vertebrates can distinguish between a huge number of odorants humans can tell tens of thousands of different smells apart using the chemoreceptors in their olfactory system which is located in the roof of the nasal cavity As the animal inspires air and volatile chemicals are drawn into the nasal cavity and swept over the olfactory epithelium The olfactory epithelium is covered in a layer of mucous which helps lipophilic odorants dissolve Within the olfactory epithelium are bipolar neurons called olfactory receptor cells They have highly ciliated membranes that project into the mucous layer where odorant binding proteins embedded in their membranes can contact dissolved chemicals The axons of the olfactory receptor cells connect directly to a portion of the brain called the olfactory bulb The proteins that bind odorants are encoded by genes All vertebrate genomes that we know of have huge families of odorant receptor genes genes that each encode for a different odorant receptor but are all similar in DNA sequence and protein structure For example the mouse has over 1 000 potential odorant receptor genes Each receptor is specific to a different chemical but they all respond in a similar fashion Odorant receptors are G protein coupled receptors so when a receptor s specific odorant binds it activates a second messenger mediated cascade inside the cell In vertebrates the membrane protein adenylate cyclase is activated which increases the concentration of the second messenger is cyclic AMP cAMP which is structurally related to ATP This cAMP mediated cascade ultimately activates ion channels that result in the olfactory receptor cell depolarizing creating a generator potential If sufficient this will cause the olfactory receptor cell to fire action potentials opening voltage gated Ca2 channels in the axon terminal and releasing neurotransmitter onto neurons of the olfactory bulb creating the sensation of smell Each olfactory receptor cell expresses only one odorant receptor gene Pheromones are a subset of volatile chemicals that are detected via olfaction These chemicals are conspecific signals they are used by members of the same species to communicate with each other Pheromonal signals are used to elicit or prime a stereotyped response or developmental process in a conspecific individual Pheromones are particularly important in mating and forming other social bonds For example when rabbits lactate their milk contains volatile compounds that bind pheromone receptors in the olfactory system of the pups which induces suckling Many vertebrates have a separate organ for pheromone detection called the vomeronasal organ It is an accessory olfactory organ that is structurally and molecularly distinct from the primary olfactory epithelium It is located at the base of the nasal cavity or just below it depending on the taxa Detection of pheromones is transduced by a different cellular mechanism although pheromone membrane receptors are also GPCRs they exert their effects on membrane permeability via the membrane protein phospholipase C and a second messenger called inositol triphosphate IP3 as shown in the figure below Pheromones are not restricted to terrestrial vertebrates invertebrates and aquatic animals both vertebrates and invertebrates also use pheromones as conspecific signals However invertebrates do not have a separate organ for pheromone detection nor do they seem to have a separate set of receptor cells for them Many
View Full Document
Unlocking...