The Olfactory Pathway
The olfactory pathway is the pathway of smell. It is a sensory system of olfaction made up of structures that lie both outside and within the cranium, and are arranged in an anteroposterior fashion. The pathway involves structures that primarily constitute the pathway itself, and two groups of structures that constitute its projections, that is, into which fibres from the olfactory pathway terminate. One of these projections is referred to as the primary olfactory cortex. It receives direct fibres from the olfactory tract (precisely from the part called olfactory bulb). The second group of structures receive few fibres from the olfactory pathway directly and a large amount of fibres of olfaction from the primary olfactory cortex. They are referred to as the secondary olfactory cortex.
Parts of olfactory pathwayThose structures which lie outside the cranial cavity and form the anterior portions of the olfactory pathway are the:
Olfactory mucosa: which also is the peripheral end organ for smell, lining the upper and posterior parts of the nasal cavity and the olfactory nerve: made up of fibres that arise in the olfactory mucosa and collect to form up to 20 bundles. These 20 bundles (the olfactory nerve) course through the foramina in the cribriform plate of the ethmoid bone of the cranium to enter the cranial cavity.
The second half of the olfactory pathway is formed by structures which constitute its posterior portions and lie within the cranial cavity. They include:
Olfactory bulb: which is an elongated oval structure lying just above the cribriform plate. The olfactory nerve synapses with the second order neurons here and continues as another structure of the pathway called the olfactory tract . Through this the pathway it makes connections with the base of the cerebral hemisphere. Further posteriorly from the olfactory tract, the pathway is continuous as the medial and lateral olfactory striae (two posterior divisions of the olfactory tract). At the point of bifurcation of the olfactory tract into the medial and lateral olfactory striae, the olfactory tract also gives off a short posterior extension known as the intermediate stria and forms a trigone called the olfactory trigone. This trigone is intimately related to the a mass of grey matter called the anterior perforated substance, and forms the anteromedial, anterior and anterolateral boundaries of this grey matter. The intermediate stria slightly extends into the anterior perforated substance and its most posterior portion forms an elevated oval structure called the olfactory tubercle. The olfactory tubercle is one of the most terminal portions of the olfactory pathway proper.
From this posterior portion of the olfactory pathway, the pathway becomes continuous other structures (mainly grey matter) which constitute its projections. These structures (projections of the olfactory pathway) are related posterolaterally and posteromedially to the anterior perforated substance, and includes the uncus, the diagonal band (of Broca), the medial dorsal nucleus of the thalamus, as well as other areas of the cerebral cortex that constitute what is referred to as the olfactory cortex.
The olfactory nerve
The fibres of the olfactory nerves are processes of olfactory receptor cells lying in the epithelial lining of the olfactory mucosa of the nasal cavity. These olfactory receptor cells are highly sensitive to odor and are directly responsible for smell perception. They are first order sensory neurons consisting of a cell body and of two processes i.e., they are also regarded as bipolar cells. Their peripheral processes or dendrites reach the surface of the olfactory epithelium and end in a small swelling, to which, a number of cilia are attached. The central processes or axons of these olfactory receptor cells enter the submucosa of the nasal cavity, and form fibres of the olfactory nerve. The olfactory nerve fibres terminate in the olfactory bulb.
The olfactory bulbSeveral types of cells are present in the olfactory bulb, of which, the most important are the tufted and mitral cells that give origin to fibres of the olfactory tract, granule and periglomerular cells which have dendrites that are confined to the olfactory bulb. Furthermore, the arrangement of these and other cell layers of the olfactory bulb is into six main layers, which, from superficial to deep, are:
- The most superficial layer which houses fibres of olfactory nerves.
- Glomerular layer which contains the synaptic glomeruli in which terminals of olfactory nerve fibres divide and synapse with the dendrites of mitral cells, tufted cells and periglomerular cells.
- The external plexiform layer which is composed of a plexus of dendrites of mitral and tufted cells. Somata of the tufted cells are also present in this layer.
- A layer of mitral cells containing somata of mitral cells and few granule cell bodies. These cell bodies send dendrites to glomerulus and external plexiform layer and axons to olfactory tract.
- The internal plexiform layer composing of the axons of the mitral, tufted cells and granule cells.
- The granular layer contains clusters of granule cells, their superficial and deep processes.
Several neuropeptides are present in the olfactory bulb and include GABA, dopamine, glutamate, aspartate, enkephalin, luteinising hormone releasing hormone (LHRH) and substance P. These neuropeptides or neurotransmitters play a role in the modulation of smell perception. The olfactory bulb receives fibres of the olfactory nerves. These olfactory nerve fibres synapse with neurons within the bulb, and fibres arising from this bulb in turn, form the olfactory tract.
The olfactory tractThe olfactory tract makes connection with the base of the cerebral hemisphere runs along the olfactory sulcus on the orbital surface of frontal lobe. It is mainly made up of axons of mitral and tufted cells of the olfactory bulb and also contains centrifugal fibres travelling to the olfactory bulb from various centres in the brain. Some scattered cell bodies are also present within the tract, and these cell bodies together with the axons of the tufted and mitral cells, constitute the anterior olfactory nucleus. On reaching the anterior perforated substance the tract divides into lateral and medial striae. The lateral stria forms the main olfactory pathway which terminates in primary olfactory cortex. The medial stria together with the diagonal band of Broca terminate ipsilaterally in the septal and parolfactory gyri. Through this pathway olfactory impulses influence the autonomic centres.
Olfactory cortex gets direct input from the olfactory bulb through olfactory tract. The olfactory cortex includes anterior olfactory nucleus, olfactory tubercle, piriform cortex, entorhinal cortex, insular cortex and the amygdala. The largest is the piriform cortex; projections from here are widespread and include orbitofrontal cortex, thalamus, hypothalamus, amygdala and hippocampal formation.
Injury to the olfactory pathway leads to dysfunction of the olfactory pathway and loss of smell (anosmia). Anosmia is also associated with several neurodegenerative disorders especially those of the cerebellar degeneration such as schizophrenia and Friedreich's ataxia. Furthermore, some individuals are born with anosmia, that is, congenital anosmia. In the case of congenital anosmia, the olfactory bulb is poorly developed, and as such affects the normal functioning of the olfactory pathway seen in this condition. Anosmia is also implicated in some genetic disorders. It is one of the symptoms of Kallmann's syndrome.
There are usually two types of this distortion. Troposmia: distorted quality of an odorant stimulation and phantosmia: perceived odor when no odorant is present (hallucination). The pathophysiology of troposmia is decreased number of functioning olfactory primary neurons so that an incomplete characterization of the odorant is made. In phantosmia abnormal signal or inhibition from the primary olfactory neurons or peripheral olfactory or trigeminal signals that trigger's a central process. Cacosmia is also a term sometimes used. It is the perception of a bad smell without an odorant stimulus, or an unpleasant phantosmia.