Afrotherians overlap with both eulipotyphlans and glires. There is no significant correlation between olfactory bulb mass and cortical mass across primates Figure 1C. Despite the different absolute and relative sizes of the olfactory bulb across species and orders, the relationship between olfactory bulb mass and its number of non-neuronal cells is overlapping among eulipotyphlans, glires, afrotherians and primates Figure 2A. This indicates that the olfactory bulbs of eulipotyphlans, glires, afrotherians, primates, and scandentia share non-neuronal scaling rules, which agrees with previous observations for the cerebral cortex, cerebellum and remaining brain areas across mammalian orders Herculano-Houzel, Figure 2.
Neuronal and non-neuronal scaling rules for the olfactory bulb. Graphs show the relationship between average mass of both olfactory bulbs in each species and A the total number of other non-neuronal cells or B the total number of neurons in them. A Olfactory bulb mass varies similarly, and linearly, with its number of other cells across the ensemble of eulipotyphlans black , glires green , primates, and scandentia red and orange , but B as a power function of its number of neurons with an exponent of 0.
Notice that eulipotyphlans have more neurons packed into olfactory bulbs of a similar mass in glires or afrotherians. In contrast, different sets of neuronal scaling rules apply to the olfactory bulb of primates, glires and eulipotyphlans. Across eulipotyphlans, however, olfactory bulb mass is best described as a power law of the number of neurons with a smaller exponent of 0. Afrotherians overlap with the relationship for glires, not eulipotyphlans Figure 2B , blue.
Combined with the shared, near-linear scaling of olfactory bulb mass with numbers of non-neuronal cells, these exponents suggest that average neuronal size in the olfactory bulb remains constant with bulb size across primate and glires species, but decreases as olfactory bulb mass increases in eulipotyphlans Sarko et al. The direct comparison across species shows that olfactory bulbs of a similar mass above about 0. In contrast, eulipotyphlans exhibit much larger neuronal densities in the olfactory bulb than afrotherians, most glires, or primates Figure 3B.
In eulipotyphlans, as described before Sarko et al. However, the star-nosed mole is an outlier in the group, with a much smaller neuronal density than would be expected based on the other four insectivore species examined Sarko et al. As mentioned above, these data suggest that average neuronal size in the olfactory bulb does not vary systematically with increasing bulb size in primates of glires, but decreases in eulipotyphlans.
Figure 3. Neuronal and non-neuronal cell densities in the olfactory bulb. Graphs show the relationship between olfactory bulb mass and A average non-neuronal other cell density or B average neuronal density in the olfactory bulb across species.
B Neuronal cell density does not correlate with olfactory bulb mass in primates and scandentia or glires, but it does increase with larger olfactory bulb mass in eulipotyphlans when the outlier, the star-nosed mole, is excluded. Notice that neuronal densities in the olfactory bulb in eulipotyphlan species are higher than in most other species. Consistently with the larger neuronal densities in insectivores than in the other two groups, eulipotyphlans also differ from the other orders in the percentage of olfactory bulb cells that are neurons.
This percentage is higher in eulipotyphlans In both eulipotyphlans and glires, the rate of addition of neurons to the olfactory bulb as a function of number of neurons in the whole brain is indistinguishable from linearity power exponents, 1. Figure 4. Scaling of numbers of neurons in the olfactory bulb as a function of numbers of neurons in the brain and in the cerebral cortex. A The number of neurons in the olfactory bulb increases as power functions of the number of neurons in the whole brain without the olfactory bulb of exponent 1.
Notice that eulipotyphlans have more neurons in the olfactory bulb than glires and afrotherians for similar numbers of brain neurons. B The number of neurons in the olfactory bulb of eulipotyphlans increases at a faster rate than in the cerebral cortex, as a power function of exponent 2. Notice that eulipotyphlans have more neurons in the olfactory bulb than glires and afrotherians for similar numbers of cortical neurons.
There is no significant relationship between numbers of olfactory bulb and cerebral cortical neurons across primate species. Eulipotyphlans, however, have more neurons in the olfactory bulb than glires and afrotherians for a similar number of brain neurons Figure 4A. Indeed, the eulipotyphlan olfactory bulb has on average Thus, numbers of neurons increase concertedly between the olfactory bulb and the brain in eulipotyphlans and glires while the former have larger absolute and relative numbers of neurons in the olfactory bulb than the latter for a same number of brain neurons , but are independent of one another in primates.
Because the relative mass of the olfactory bulb is also negatively correlated with brain mass across all orders Figure 1B , the relationship between relative number of neurons and relative mass of the olfactory bulb both expressed as percentages of whole brain can be described as a single power function across primates, glires and scandentia, with an exponent that approaches linearity 0.
Figure 5. Scaling of relative numbers of neurons in the olfactory bulb compared to the brain and cerebral cortex. A The relative number of neurons in the olfactory bulb expressed as a percentage of the number of neurons in the brain excluding the olfactory bulb is not significantly correlated with brain mass within any of the mammalian orders examined, although it decreases with increasing brain mass across all species combined.
B The relative number of neurons in the olfactory bulb increases with increasing relative mass of the olfactory bulb both compared to the whole brain across primates, glires and afrotherians combined, as a power function of exponent 0. C The relative number of neurons in the olfactory bulb expressed as a percentage of the number of neurons in the cerebral cortex is not significantly correlated with brain mass within any of the mammalian orders examined.
Although the eulipotyphlan olfactory bulb gains neurons linearly with the brain as a whole, it gains neurons at a faster rate than the cerebral cortex, as a power function of the number of cortical neurons of exponent 2. In eulipotyphlans, the number of neurons in the olfactory bulb represents on average In constrast, the olfactory bulb in glires gains neurons at the same pace as the cerebral cortex power exponent, 1.
Similarly, the afrotherians examined have a number of neurons in the olfactory bulb that represents Eulipotyphlans, in comparison, have more neurons in the olfactory bulb than glires and afrotherians for a similar number of neurons in the cerebral cortex Figure 4B.
In contrast, in primates and scandentia, the number of neurons in the olfactory bulb represents only The olfactory bulb also has more neurons in eulipotyphlans than in glires, afrotherians or primates with a similar number of neurons in the rest of brain Figure 6A.
In eulipotyphlans, the cerebral cortex gains neurons more slowly than the rest of brain power exponent, 0. Despite the statistical uncertainty, the most likely scenario that best accounts for the scaling of numbers of neurons in the olfactory bulb in eulipotyphlans is that it gains neurons faster than the rest of the brain, while both structures gain neurons faster than the cerebral cortex, such that the olfactory bulb gains neurons even faster compared to the cortex than to the rest of brain.
In glires, the olfactory bulb gains neurons at the same pace as the rest of brain power exponent, 1. In contrast, the cerebral cortex gains neurons faster than the rest of brain in both glires and primates exponents of 1.
The scaling relationships that apply across structures are summarized in Figure 7. Figure 6. Scaling of numbers of neurons in the olfactory bulb and in the cerebral cortex as a function of numbers of neurons in the rest of brain.
A The number of neurons in the olfactory bulb increases as power functions of the number of neurons in the rest of brain of exponent 1. Notice that eulipotyphlans have more neurons in the olfactory bulb than glires and afrotherians for similar numbers of rest of brain neurons. B The number of neurons in the cerebral cortex of eulipotyphlans increases more slowly than the number of neurons in the rest of brain, as a power function of exponent 0.
Figure 7. Scaling relationships for neurons and mass across structures in each mammalian order. Lines indicate linear scaling in numbers of neurons top or mass bottom across structures; polygons represent reciprocal power laws in the scaling of one structure against another, where the power law with the exponent larger than 1 is the larger side of the polygon and has its exponent indicated in the figure.
Notice that the faster scaling of number of neurons in the eulipotyphlan olfactory bulb than in the cerebral cortex would not be predicted from the mass relationships across the two structures.
Here we find that the eulipotyphlan olfactory bulb gains neurons faster than the cerebral cortex across species, an unexpected finding in face of the decrease in the relative mass of the olfactory bulb and relative expansion of the cerebral cortex with increasing brain size.
Moreover, in some eulipotyphlan species the olfactory bulb has as many or more neurons than the cerebral cortex, despite the larger size of the latter. These findings have several implications for mammalian brain evolution. First, the large and growing absolute numbers of neurons in the olfactory bulb of eulipotyphlans compared to the cerebral cortex dispute the notion that, while olfaction was emphasized in early mammals, other sensory systems necessarily became more important as cortex expanded in evolution Rowe et al.
The fact that eulipotyphlans are now considered a later branch than afrotherians in mammalian evolution argues for a renewed importance of olfaction in the behavior of this group. Primates of all brain sizes do have many more neurons in the cerebral cortex than in the olfactory bulb, which is consistent with the expansion of other sensory systems, and eulipotyphlans have as many or more neurons in the olfactory bulb than in the cerebral cortex, which is consistent with a heavy reliance on olfaction.
However, both groups have a similar range of numbers of neurons in the olfactory bulb. We note that the distribution of projection and intrinsic neurons, both excitatory and inhibitory, is different across structures such as the cerebral cortex, the olfactory bulb and the cerebellum, with presumably decreasing proportions of projection neurons across these three structures, respectively.
However, given that both projection and intrinsic neurons are the basic units of information processing, combining the activity of thousands of synapses, we assume that total numbers of neurons are a valid proxy for total information processing capacity in each structure.
Thus, if total numbers of neurons in the olfactory bulb can be considered an indication of the amount of olfactory processing that can be performed, then it must be conceded that the increasing reliance on other sensory systems such as vision in primates did not occur at the expense of olfaction, but in addition to it.
Second, our finding that the neuronal scaling rules that apply to the olfactory bulb appear to be shared by afrotherians and glires, but not eulipotyphlans or primates, concurs with the phylogenetic analysis that shows that modern eulipotyphlans are a more recent group than afrotherians and actually do not reflect the ancestral mammalian state.
Predictably, the more overlap there was between two types of mixtures, the harder they were to tell apart. After calculating how many of the mixtures the majority of people could tell apart, the researchers were able to predict how people would fare if presented with every possible mixture that could be created from the different odor molecules. They used this data to estimate that the average person can detect at least one trillion different smells, a far cry from the previous estimate of 10, The one trillion is probably an underestimation of the true number of smells we can detect, said Vosshall, because there are far more than different types of odor molecules in the world.
No longer should humans be considered poor smellers. In fact, new research suggests that your nose can outperform your eyes and ears, which can discriminate between several million colors and about half a million tones. A beginner's guide to the brain and nervous system. See how discoveries in the lab have improved human health.
Read More. For Educators Log in. This image may look like a carnival mask, but it actually shows the key structures mammals use every time they smell. The Nose Knows Smell begins at the back of nose, where millions of sensory neurons lie in a strip of tissue called the olfactory epithelium.
A Better Smeller Although scientists used to think that the human nose could identify about 10, different smells, Vosshall and her colleagues have recently shown that people can identify far more scents. About the Author. The molecular receptive range of an odorant receptor. Those molecules interact with olfactory receptors, which are part of a family of G-protein coupled receptors. Stimulation of these receptors causes the production of second messengers like cyclic AMP cAMP , which leads to the opening of ion channels and the generation of action potentials in olfactory receptor cells.
The axons of these olfactory receptor cells terminate in the olfactory bulb, where they converge on the dendrites of olfactory bulb neurons in small clusters called glomeruli plural for glomerulus, which is a term sometimes used in anatomy to refer to a small cluster of structures. This does not mean that each glomerulus is only capable of detecting one odor, as each type of odorant receptor is capable of detecting multiple odorants.
The olfactory bulb is patterned in such a way, however, that similar odorants often stimulate glomeruli found close to one another in the olfactory bulb. This creates an organization in the olfactory bulb that seems to be related to odorant structure. There are several types of neurons in the olfactory bulb. These include mitral cells , tufted relay neurons, granule cells , and periglomerular neurons.
The mitral cells and tufted relay neurons form connections with olfactory receptor neurons in the glomeruli. A single receptor protein, however, appears to bind or recognize many different odors. Thus, rather than having neurons that respond selectively to coffee or vanilla or Bordeaux, most individual cells via their receptors respond to submolecular features of the volatile chemicals coming from those objects.
For example, an olfactory sensory receptor neuron may respond to a hydrocarbon chain of a particular length or a specific functional group like an alcohol or aldehyde. This means that any given sensory neuron will respond to many different odors as long as they share a common feature.
The brain specifically, the olfactory bulb and olfactory cortex then looks at the combination of sensory neurons activated at any given time and interprets that pattern in the context of previous patterns that have been experienced and other kinds of available information. The interpreted pattern is what you perceive as smell.
Olfactory sensory neurons, which sit in the mucus in the back of the nose and relay data into the brain via axons fingerlike projections that transmit information out from the cell body , do not live forever. In fact, they are one of the increasingly large number of neuron types that are known to die and be replaced throughout life. Fortunately, they do not all die at the same time.
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