The Olfactory Nerve
The olfactory nerve is the first of the twelve cranial nerves. It consists of both afferent and efferent sensory fibers. The word afferent means toward the centre, as in from a peripheral area of a limb to the central nervous system. The word efferent is the opposite of afferent, meaning away from the centre and toward the periphery which is when the stimulus is carried back to the brain from a peripheral area.
The pathway for the efferent fibers is the following (from the Initiation in the brain to the termination in the periphery):
- The primary efferent nerve fibre stems from the anterior commissure of the corpus callosum.
- It passes through the olfactory tract.
- It synapses in the contralateral olfactory bulb on an Inner nuclear cell. These cells are excited by mitral cells and inhibited by tuft cells, upon which the inner nuclear cell’s fibers attach.
- The mitral and tuft cell’s fibers then continue on to synapse with olfactory nerve fibers in the olfactory glomerulus.
- The olfactory nerve fibers converge into bundles and treponate first the dura mater and then the cribriform plate (The olfactory nerve exits the skull through the cribriform plate).
- They surpass the submucosa and terminate on the olfactory mucosa (stratified squamous non-keratinizing epithelium) as olfactory cells, in the roof of the nasal cavity.
- There is also a secondary efferent ganglion that stems from the olfactory trigone and continues along the same path as the primary efferent ganglion.
The pathway for the afferent fibers is the following (from the stimulatory cells in the periphery to the final synapse in the brain):
- The olfactory cells contain sensory nerve fibers that are alerted to the sense of smell. There is a refractory period for each cell so certain areas of the olfactory mucosa are stimulated each time a new smell enters the nasal cavity.
- Once this phenomenon occurs, which is hundreds of times per day, the olfactory nerve fibers (primary afferent nerve fibers) carry the stimulus information in the form of action potentials through the olfactory mucosa (stratified squamous non-keratinizing epithelium), the submucosa, the cribriform plate (entrance to the skull) and the dura mater and synapse with the periglomerular cells, the fibers from the mitral and tuft Cells and the collateral afferent fibers.
- Some of these fibers continue to the anterior olfactory nucleus, situated in the olfactory tract and then cross over to the contralateral side of the brain, while other fibers simply continue through the olfactory tract and only cross over.
- The ones that have synapsed on the anterior olfactory nucleus continue to the olfactory trigone, in which they synapse on three different nuclei, the main one being the nucleus of the lateral olfactory tract. This is where the routes of the afferent fibers diverge.
- The lateral olfactory tract fibers continue laterally to the amygdaloid body and terminally synapse there.
- The rest of the fibers that synapse within the olfactory trigone and olfactory tuberculum turn cranially and pass through the medial olfactory striae. They terminally synapse in the anterior commissure.
Injuries to the olfactory mucosa and/or the olfactory nerve fibers can lead to localized loss of sense of smell. This isn’t so apparent except when the working areas are in a refractory period and there are no free olfactory cells to process the stimuli. This can occur naturally with old age and is reversible during a bad cold.
Total loss of smell occurs when afferent fibers or structures such as the olfactory tract are damaged.