Sensory Systems/Olfactory System/Sensory Organ Components

Similar to other sensory modalities, olfactory information must be transmitted from peripheral olfactory structures, like the olfactory epithelium, to more central structures, meaning the olfactory bulb and cortex. The specific stimuli have to be integrated, detected and transmitted to the brain in order to reach sensory consciousness. However the olfactory system is different from other sensory systems in three fundamental ways :

1. Olfactory receptor neurons are continuously replaced by mitotic division of the basal cells of the olfactory epithelium. This is necessary due to the high vulnerability of the neurons, which are directly exposed to the environment.
2. Due to phylogeny, olfactory sensory activity is transferred directly from the olfactory bulb to the olfactory cortex, without a thalamic relay.
3. Neural integration and analysis of olfactory stimuli may not involve topographic organization beyond the olfactory bulb, meaning that spatial or frequency axis are not needed to project the signal.

1: Olfactory bulb 2: Mitral cells 3: Bone 4: Nasal Epithelium 5: Glomerulus 6: Olfactory receptor cells

Human skull showing the Cribriform Plate in green and Olfactory nerve in yellow.

Olfactory Mucous Membrane

The olfactory mucous membrane contains the olfactory receptor cells and in humans it covers an area about 3 – 5 cm^2 in the roof of the nasal cavity near the septum. Because the receptors are continuously regenerated it contains both the supporting cells and progenitors cells of the olfactory receptors. Interspersed between these cells are 10 – 20 millions receptor cells.

Olfactory receptors are neurons with short and thick dendrites. Their extended end is called an olfactory rod, from which cilia project to the surface of the mucus. These neurons have a length of 2 micrometers and have between 10 and 20 cilia of diameter about 0.1 micrometers.

The axons of the olfactory receptor neurons go through the cribriform plate of the ethmoid bone and enter the olfactory bulb. This passage is in absolute the most sensitive of the olfactory system; the damage of the cribriform plate (e.g. breaking the nasal septum) can imply the destruction of the axons compromising the sense of smell.

A further particularity of the mucous membrane is that with a period of a few weeks it is completely renewed.

Olfactory Bulbs

In humans, the olfactory bulb is located anteriorly with respect to the cerebral hemisphere and remain connected to it only by a long olfactory stalk. Furthermore in mammals it is separated into layers and consists of a concentric lamina structure with well-defined neuronal somata and synaptic neuropil.

After passing the cribriform plate the olfactory nerve fibers ramify in the most superficial layer (olfactory nerve layer). When these axons reach the olfactory bulb the layer gets thicker and they terminate in the primary dendrites of the mitral cells and tufted cells. Both these cells send other axons to the olfactory cortex and appear to have the same functionality but in fact tufted cells are smaller and consequently have also smaller axons.

The axons from several thousand receptor neurons converge on one or two glomeruli in a corresponding zone of the olfactory bulb; this suggests that the glomeruli are the unit structures for the olfactory discrimination.

In order to avoid threshold problems in addition to mitral and tufted cells, the olfactory bulb contains also two types of cells with inhibitory properties: periglomerular cells and granule cells. The first will connect two different glomeruli, the second, without using any axons, build a reciprocal synapse with the lateral dendrites of the mitral and tufted cells. By releasing GABA the granule cell on the one side of these synapse are able to inhibits the mitral (or tufted) cells, while on the other side of the synapses the mitral (or tufted) cells are able to excite the granule cells by releasing glutamate. Nowadays about 8.000 glomeruli and 40.000 mitral cells have been counted in young adults. Unfortunately this huge number of cells decrease progressively with the age compromising the structural integrity of the different layers.

Olfactory Cortex

The axons of the mitral and tufted cells pass through the granule layer, the intermediate olfactory stria and the lateral olfactory stria to the olfactory cortex. This tract forms in humans the bulk of the olfactory peduncle. The primary olfactory cortical areas can be easily described by a simple structure composed of three layers: a broad plexiform layer (first layer); a compact pyramidal cell somata layer (second layer) and a deeper layer composed by both pyramidal and nonpyramidal cells (third layer). Furthermore, in contrast to the olfactory bulb, only a little spatial encoding can be observed; “that is, small areas of the olfactory bulb virtually project the entire olfactory cortex, and small areas of the cortex receive fibers from virtually the entire olfactory bulb” .

In general the olfactory tract can be divided in five major regions of the cerebrum: The anterior olfactory nucleus, the olfactory tubercle, the piriform cortex, the anterior cortical nucleus of the amygdala and the entorhinal cortex. Olfactory information is transmitted from primary olfactory cortex to several other parts of the forebrain, including orbital cortex, amygdala, hippocampus, central striatum, hypothalamus and mediodorsal thalamus.

Interesting is also to note that in humans, the piriform cortex can be activated by sniffing, whereas to activate the lateral and the anterior orbitofrontal gyri of the frontal lobe only the smell is required. This is possible because in general the orbitofrontal activation is greater on the right side than on the left side, this directly implies an asymmetry in the cortical representation of olfaction.