Our sense of smell is one of our most primitive senses, and is a special sense like taste, sight and hearing. The olfactory nerve is the first cranial nerve, anosmia is a symptom of a wide range of diseases. The nerve enters the cranial cavity through the cribriform plate of the ethmoid bone, and synapses with the olfactory bulb.
The olfactory nerve is also known as cranial nerve 1. It is a purely special sensory nerve that gives us our sense of smell. It arises from the olfactory epithelium of the nasal mucosa that lines the nasal cavity. Many small unmyelinated first order olfactory neurons will run superiorly to pass through the cribriform plate 9separated in the midline by the crista galli, a bony ridge) of the ethmoid bone, where they synapse with the second order neurons of the ipsilateral olfactory bulb.
The olfactory bulb consists of 5 layers. The olfactory nerves synapse with the mitral cells of the olfactory bulb (the middle or 3rd layer), which then transmits second order neurons to the olfactory tract. The tract runs beneath the inferior surface of the frontal lobe (that rests against the floor of the anterior cranial fossa). The nerve reaches the olfactory trigone, where it divides into the lateral and medial stria. The lateral stria will travel to the primary olfactory cortex (pyriform lobe and entorhinal cortex) directly, and the medial stria travel to the secondary sites of the anterior olfactory nucleus and the septal area, and some of the fibers will cross the anterior commissure and join the contralateral olfactory bulb. From these secondary sites, third order neurons will be sent to the thalamus; the conscious element of odour perception and the basal forebrain and limbic system (amygdala), which contribute to aversive subjective perception of a small.
The amygdala (the danger detector of the brain) that lies in the medial part of the temporal lobe (anterior to the hippocampus), receives the olfactory sensory information. The human amygdala has descending cortical input, and we are therefore able to moderate our response to a perceived danger. A more primitive brain such as that in most animals, will immediately respond to a smell deemed to be a threat.
The pyriform lobe consists of the uncus, olfactory tract and parahippocampal gyrus. The primary olfactory cortex consists of the prepyriform (anterior and periamygdaloid (cortex around the amygdala in the anterior part of the temporal lobe) and pre. The entorhinal cortex is known as the secondary olfactory cortex, and is the only cranial nerve sensation that does not pass through the thalamus. The entorhinal cortex sends projections to the hippocampal formation, frontal cortex and anterior insula.
Kallmann Syndrome- This is a congenital syndrome defined by hypogonadotrophic hypogonadism causing a primary failure to begin puberty. The disease is also characterised by anosmia. It is caused by failure of migration of the Gonadotrophin releasing hormone (GnRH) producing neurons to migrate into the hypothalamus.
Parkinson’s Disease- One of the earliest symptoms of Parkinson’s disease in some patients is anosmia. Dopamine is required for the sense of smell, so a depletion in the level of dopamine, is one of the earliest signs of the disease. The depletion occurs from the substantia nigra region, which is a darkly pigmented area of the brainstem due to neuromelanin (a by product of dopamine synthesis).
Cribriform plate fracture- A fracture of the base of the skull across the cribriform plate is likely to tear the olfactory nerve fibers and result n partial or complete anosmia.
Developmental precursor- Otic placode, Telencephalon (secondary vesicle of the developing nervous system)
Special sensory- Smell
Think of the word olfactory as ‘old’ and ‘factory’ mixed together. The idea of an old ‘smelly’ factory should remind you of the nerve’s function.
Remember the nerve is the fibers that pass through the cribriform plate to synapse with the olfactory bulb. The bulb, tract and trigone are all part of the second and third order pathway of the nerve within the brain.
The olfactory nerve is the first cranial nerve. It gives us our sense of smell, and enters the cranial cavity through the cribriform plate of the ethmoid bone.