Unit 5A - Olfactory (Smell) Disorders

Flashcards on Olfactory Disorders

Odorant molecule

Any substance capable of stimulating the sense of smell by binding to an olfactory receptor; substance must be volatile (able to vaporize).

Olfactory epithelium (also known as 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 (also known as odorant receptors)

Expressed in the dendrites of the olfactory receptor neurons and are responsible for the detection of odorant molecules. They can bind to a range of odorant molecules and a single odorant molecule may bind to a number of olfactory receptors.

Olfactory receptor neurons (also known as olfactory receptor cells or olfactory sensory neurons)

Bipolar neurons with dendrites facing the nasal cavity and axons that pass through the openings in the cribriform plate to synapse in the olfactory bulb. They 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.

Olfactory nerve

The first cranial nerve (CN I) is actually the many small nerve fascicles of the olfactory receptor neurons. It is unique among cranial nerves, because it is capable of some regeneration if damaged.

Cribriform plate

A section of the bone that separates the nasal cavity from the brain. It contains many small holes through which the olfactory receptor neurons project axons. Fractures can result in leaking of CSF into the nose and loss of sense of smell. The tiny holes can become the entry point for a pathogenic amoeba (Naegleria fowleri).

Trigeminal sense

Sensation of touch, pressure, pain, temperature 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.

Olfactory bulb

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.

Olfactory glomerulus

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.

Mitral cells

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.

Olfactory tract

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. 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; see below for taste), which is involved in odor identification and determining the reward value (e.g., pleasantness) of an odor.

Limbic system

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.

Sniffing

Key for olfactory perception: odor localization, odor identification and olfactory attention

Anosmia

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.

Hyposmia

Decreased ability to smell; often associated with clogged nasal sinuses or from nerve damage from trauma/infection

Dysosmia

Things smell differently than they should; often associated with hormonal changes like pregnancy, with clogged sinuses, or with nerve damage from trauma, infection

Hyperosmia

An abnormally acute sense of smell; often associated with hormonal changes like pregnancy

Phantosmia

"Hallucinated smell", often unpleasant; commonly seen as an aura (i.e., early symptom) of a seizure in temporal lobe epilepsy