Smell

While the human brain is dominated by a large visual cortex, the dog brain is dominated by an olfactory cortex.[109] The olfactory bulb in dogs is roughly forty times bigger than the olfactory bulb in humans, relative to total brain size, with 125 to 220 million smell-sensitive receptors.[109] The bloodhound exceeds this standard with nearly 300 million receptors.[109] Consequently, it has been estimated that dogs, in general, have an olfactory sense ranging from one hundred thousand to one million times more sensitive than a human's. In some dog breeds, such as bloodhounds, the olfactory sense may be up to 100 million times greater than a human's.[122] The wet nose, or rhinarium, is essential for determining the direction of the air current containing the smell. Cold receptors in the skin are sensitive to the cooling of the skin by evaporation of the moisture by air currents. The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain. The term visual cortex refers to the primary visual cortex (also known as striate cortex or V1) and extrastriate visual cortical areas such as V2, V3, V4, and V5. The primary visual cortex is anatomically equivalent to Brodmann area 17, or BA17. The extrastriate cortical areas consist of Brodmann area 18 and Brodmann area 19. There is a visual cortex in each hemisphere of the brain. The left hemisphere visual cortex receives signals from the right visual field and the right visual cortex from the left visual field. The body of this article describes the primate (especially, human) visual cortex. The olfactory system is the sensory system used for olfaction, or the sense of smell. Most mammals and reptiles have two distinct parts to their olfactory system: a main olfactory system and an accessory olfactory system. The main olfactory system detects volatile, airborne substances, while the accessory olfactory system senses fluid-phase stimuli. Behavioral evidence indicates that most often, the stimuli detected by the accessory olfactory system are pheromones. The olfactory system is often spoken of along with the gustatory system as the chemosensory senses because both transduce chemical signals into perception. Main olfactory bulb The main olfactory bulb has a multi-layered cellular architecture. In order from surface to the center the layers are Glomerular layer External plexiform layer Mitral cell layer Internal plexiform layer Granule cell layer The olfactory bulb transmits smell information from the nose to the brain, and is thus necessary for a proper sense of smell. As a neural circuit, the glomerular layer receives direct input from olfactory nerves, made up of the axons from approximately ten million olfactory receptor neurons in the olfactory mucosa, a region of the nasal cavity. The ends of the axons cluster in spherical structures known as glomeruli

such that each glomerulus receives input primarily from olfactory receptor neurons that express the same olfactory receptor. Glomeruli are also permeated by dendrites from neurons called mitral cells, which in turn output to the olfactory cortex. Numerous interneuron types exist in the olfactory bulb including periglomerular cells which synapse within and between glomeruli, and granule cells which synapse with mitral cells. As a neural circuit, the olfactory bulb has one source of sensory input (axons from olfactory receptor neurons of the olfactory epithelium), and one output (mitral cell axons). As a result, it is generally assumed that it functions as a filter, as opposed to an associative circuit that has many inputs and many outputs. However, the olfactory bulb also receives "top-down" information from such brain areas as the amygdala, neocortex, hippocampus, locus coeruleus, and substantia nigra. With this in mind, its potential functions can be placed into four non-exclusive categories: discriminating among odors enhancing sensitivity of odor detection filtering out many background odors to enhance the transmission of a few select odors permitting higher brain areas involved in arousal and attention to modify the detection or the discrimination of odors While all of these functions could theoretically arise from the olfactory bulb's circuit layout, it is unclear which, if any, of these functions are performed exclusively by the olfactory bulb. By analogy to similar parts of the brain such as the retina, many researchers have focused on how the olfactory bulb filters incoming information from receptor neurons in space, or how it filters incoming information in time. At the core of these proposed filters are the two classes of interneurons; the periglomerular cells, and the granule cells. The basal dendrites of mitral cells are connected to interneurons known as granule cells, which by some theories produce lateral inhibition between mitral cells. It is not clear what the functional role of lateral inhibition would be, though it may be involved in boosting the signal-to-noise ratio of odor signals by silencing the basal firing rate of surrounding non-activated neurons. The synapse between mitral and granule cells is of a rare class of synapses that are "dendro-dendritic" which means that both sides of the synapse are dendrites that release neurotransmitter. In this specific case, mitral cells release the excitatory neurotransmitter glutamate, and granule cells release the inhibitory neurotransmitter Gamma-aminobutyric acid (GABA). As a result of its bi-directionality, the dendro-dendritic synapse can cause mitral cells to inhibit themselves (auto-inhibition), as well as neighboring mitral cells (lateral inhibition). Olfaction is distinct from the other sensory systems where peripheral sensory receptors have a relay in the diencephalon. Therefore the olfactory bulb plays this role for the olfactory system.