- any substance capable of stimulating the sense of smell by binding to an olfactory receptor
- substance must be volatile (able to vaporize)
Olfactory epithelium (aka: olfactory neuroepithelium)
- a sheet of cells that contains the olfactory receptors and that lines the upper part of the nasal passages
- the epithelium is covered by a mucous layer through which odorants must be absorbed before activating the olfactory receptors
Olfactory receptors (aka: odorant receptors)
- are expressed in the dendrites of the olfactory receptor neurons and are responsible for the detection of odorant molecules.
- Rather than binding only one specific odorant, olfactory receptors can bind to a range of odorant molecules with different degrees of activation, and, conversely, a single odorant molecule may bind to a number of olfactory receptors with varying affinities.
Olfactory receptor neurons (aka: olfactory receptor cells or olfactory sensory neurons)
- bipolar neurons with dendrites facing the nasal cavity (in the olfactory epithelium) and axons that pass through the openings in the cribriform plate (bone) to synapse in the olfactory bulb.
- Olfactory receptors are located along the dendrites and lie across the olfactory epithelium within the mucus layer.
- These neurons make up the ‘olfactory nerve’ – the first cranial nerve, and are one of only ~3 structures in the brain that have been found to undergo continuing neurogenesis in adult mammals.
- the first cranial nerve (CN I) is actually the many small nerve fascicles of the olfactory receptor neurons.
- The olfactory nerve is unique among cranial nerves, because it is capable of some regeneration if damaged
- a section of the bone that separates the nasal cavity from the brain.
- The cribriform plate contains many small holes through which the olfactory receptor neurons project axons.
- Clinical significance:
- (i) a fractured cribriform plate can result in leaking of CSF into the nose and loss of sense of smell.
- (ii) The tiny holes of the cribriform plate can become the entry point for a pathogenic amoeba (Naegleria fowleri).
- This amoeba destroys the olfactory bulb and the adjacent inferior surface of the frontal lobe of the brain before fatally spreading to the rest of the brain and CSF.
sensation of touch, pressure, pain, temperature (hot chili peppers!) in mouth, eye, nasal cavity that is carried by the trigeminal nerve (the fifth cranial nerve, CN V). ~70% of odorants co-activate both the olfactory nerve and the trigeminal nerve.
- is a multi-layered structure located on the ventral surface of the brain that receives inputs from olfactory receptor neurons and sends output to cortex via mitral cell axons.
- The olfactory-receptor-neuron axons that form synapses in olfactory bulb glomeruli are also capable of regeneration following regrowth of an olfactory receptor neuron in the olfactory epithelium.
- The neural circuitry here may play a role in identification of odor type and concentration.
- spherical structures located in the olfactory bulb where synapses form between the axon terminals of the olfactory nerve and the dendrites of mitral cells.
- Each glomerulus receives input from olfactory receptor neurons expressing only one type of olfactory receptor.
- The glomerular activation patterns within the olfactory bulb are thought to represent the odor being detected.
- Specifically, the glomeruli layer represents a spatial odor map organized by chemical structure of odorants.
- neurons located in the olfactory bulb that receive inputs from the olfactory receptor neurons within the glomeruli, along with inputs from modulatory cells, and then project axons to several cortical areas including the olfactory tubercle and piriform cortex and regions in the limbic system (the part of cortex involved in emotions and memory).
- May encode odor concentration in timing of firing.
the bundle of axons including those from the mitral cells that connects the olfactory bulb to several target regions in the brain (olfactory tubercle and piriform cortex and regions in the limbic system)
Olfactory tubercle and piriform cortex
- ventral regions that together are considered ‘primary’ olfactory cortex.
- The definition of ‘primary’ olfactory cortex is not as clear as for other senses – note that olfactory information does not pass through the thalamus before reaching these cortical regions, unlike all other senses. Both regions are involved in identification of odor type and concentration.
Orbitofrontal cortex for olfaction
- ventral region of the frontal lobes that is located just above the orbits of the eyes.
- Contains secondary olfactory cortex (among other functions), which is involved in odor identification and determining the reward value (e.g., pleasantness) of an odor.
- includes several interconnected regions in the medial temporal lobe (e.g., amygdala, entorhinal cortex) that play important roles in processing emotion and memory.
- One function is higher-order olfactory processing – tying odors to emotion and memory.
key for olfactory perception: odor localization, odor identification and olfactory attention
- lack of ability to smell; associated with trauma to olfactory neurons projecting through the cribriform plate bone.
- Damage to one olfactory bulb can cause unilateral (one-sided) anosmia, which is only noticeable if nostrils are tested separately.
decreased ability to smell; often associated with clogged nasal sinuses (e.g., stuffiness from a head cold can block odorants from reaching olfactory receptors), or from nerve damage from trauma/infection
things smell differently than they should; often associated with hormonal changes like pregnancy, with clogged sinuses (e.g., stuffiness from a head cold can block odorants from reaching olfactory receptors) as seen in colds, or with nerve damage from trauma, infection
an abnormally acute sense of smell; often associated with hormonal changes like pregnancy
- "hallucinated smell"
- often unpleasant
- commonly seen as an aura (i.e., early symptom) of a seizure in temporal lobe epilepsy