5A: Olfactory Disorders

Question 1

odorant molecule

Answer 1

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

Question 2

olfactory epithelium

Answer 2

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

Question 3

Olfactory receptors

Answer 3

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.

Question 4

Olfactory receptor neurons

Answer 4

• 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.

Question 5

Olfactory nerve

Answer 5

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 (see olfactory bulb below).

Question 6

Cribriform plate

Answer 6

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.

Question 7

Trigeminal sense

Answer 7

– 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.

Question 8

Olfactory bulb

Answer 8

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 circuity here may play a role in identification of odor type and concentration.

Question 9

Olfactory glomerulus

Answer 9

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.

Question 10

Mitral cells

Answer 10

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.

Question 11

Olfactory tract

Answer 11

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)

Question 12

Olfactory tubercle and piriform cortex

Answer 12

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.

Question 13

Orbitofrontal cortex for olfaction

Answer 13

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.

Question 14

Limbic system

Answer 14

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.

Question 15

Sniffing

Answer 15

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

Question 16

Anosmia

Answer 16

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.

Question 17

Hyposmia

Answer 17

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.

Question 18

Dysosmia

Answer 18

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

Question 19

Hyperosmia

Answer 19

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

Question 20

Phantosmia

Answer 20

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