The document summarizes key aspects of the olfactory system. It describes how odorant molecules in the air interact with receptor cells in the nasal epithelium, whose axons project to the olfactory bulb. The mitral cells of the bulb connect to structures like the pyriform cortex, amygdala and entorhinal cortex. It explains the sensory transduction process where odorants bind to G-protein coupled receptors, activating a signaling cascade that depolarizes the receptor cell. It also discusses concepts like olfactory adaptation, the combinatorial coding of odors, and the role of the glomerulus in receiving convergent input from receptor neurons expressing the same receptor.

1. Domina Petric, MD

2.  Olfaction begins with airborne molecules interacting
with receptors.
 Receptors are located in olfactory epithelium: dorsal
and medial aspect of nasal passage.
 Receptor cells grow axons that pass through the
perforations of the ethmoid bone: the cribriform
plate.
 Axons enter into the olfactory bulb and synapse with
neurons in the olfactory bulb.
Olfactory pathway

3.  Axons that grow from receptor cells are first cranial
nerve and olfactory bulb is actually part of the
brain.
 Olfactory bulb extends into the lateral olfactory
tract: it is an extension of the brain.
 Lateral olfactory tract is made of MITRAL cells.
 The mitral cells are the principal projection cells
that connect the olfactory bulb to the rest of the
brain.
Olfactory pathway

4. Olfactory
receptors
Primary axons are
first cranial nerve.
Olfactory
bulb
Axons of
the mitral
cells make
lateral
olfactory
tract.
Targets of the
olfactory
bulb (the
olfactory
cortex):
A) Pyriform
cortex
B) Olfactory
tubercle
C) Amygdala
D) Entorhinal
cortex
Pyriform cortex and entorhinal cortex
are cortex structures
(laminar cell organisation).
Entorhinal cortex is associated
with hippocampal formation.
Olfactory tubercle and amygdala
are corticoid structures.
Structures of olfactory cortex, both cortical and
corticoid structures are interconnected with
each other and connected with other structures
like thalamus, hypothalamus and
orbitofrontal cortex.

5. In the orbitofrontal cortex all the informations
from chemosensory systems (oflaction, gustation,
trigeminal chemoreception) is combined with
somatic sensation and visual sensation.
In this part of the brain the concept of flavour is
represented and sense of rewarding value of food.
Orbitofrontal cortex

6. http://serendip.brynmawr.edu
AnatomyOlfactory
tubercle
Olfactory
bulb
Lateral
olfactory
tract
Pyriform
cortex
Amygdala

7. There is no obligatory thalamic relay
between the olfactory bulb and olfactory
cortex.
There is no known map of the sensory
environment.
Olfaction

8.  Spindle shaped cells are mature olfactory receptor
neurons: function of sensory transduction.
 Among basal cell population is a set of neural stem
cells.
 Glandular cells produce thick mucus that covers the
upper part of the olfactory epithelium.
 Odorants pass the mucus and interact with olfactory
cilia: olfactory receptor proteins within the cilia
interact with odorant molecules.
Sensory transduction in the
olfactory epithelium

9.  The odorant receptor molecule is a G-protein coupled
receptor: interacts with odorant molecule.
 When odorant molecule binds on the receptor, G-olf
protein activates.
 Next target is adenyl cyclase III: activation causes a
production of cAMP.
 High levels of cAMP gate the opening of the cation
selective channel (for sodium and calcium ions).
 Sodium and calcium ions enter the cytoplasm of receptor
neuron cilium: DEPOLARISATION.
Sensory transduction

10.  High levels of calcium ions in the cytoplasm cause
interaction of calcium ions with CALMODULIN:
that causes gating of chloride channel.
 Opening of the chloride channel causes the eflux of
chloride ions outside the receptor neuron cell.
 This amplifies the depolarisation.
 Interaction of calcium ions with calmodulin has also
impact on cAMP gated cation channel (for sodium
and calcium ions).
Sensory transduction

11.  The interaction of calcium and calmodulin can
reduce the sensitivity of cation channels to the
binding of cAMP.
 This reduces the influx of sodium and calcium and
reduces the depolarisation causing the OLFACTORY
ADAPTATION.
 There is one more mechanism of adaptation:
sodium/calcium exchanger (sodium ions influx the
cell and calcium ions eflux out of the cell).
Sensory transduction

12. As calcium efluxes the cell, there is less
interaction of calcium with calmodulin.
This is also a mechanism of olfactory
adaptation to the persistent present of the
same odorant.
Sensory transduction

13.  Quality of the odorant can be sometimes modulated
by the concentration of the odorant.
 Low concentrations of the odorant INDOLE smell
like flowers, but high concentrations of INDOLE
smell putrid.
 Most odorants are complex molecules.
 Odorant can smell differently regarding the different
molecule rotation (right or left rotation).
Combinatorial olfactory code

14. Same odorant molecule can
interact with more than one
receptor depending upon the
geometrical configuration of
odorant molecule.
Combinatorial olfactory code

15.  Mitral cell recieves the synaptic input from an
afferent axon in the structure called GLOMERULUS.
 Periglomerulus neuron is a small interneuron.
 Granule cell is also a small interneuron.
 Interneurons mediate inhibitory interactions within
and among glomeruli.
 Tufted cell contributes postsynaptic targets for the
afferent input that is arriving in the olfactory bulb.
Glomerulus

16.  Glomeruli are the first site of synaptic connection
between the olfactory epithelium and the brain.
 Each glomerulus recieves input from about 25 000
olfactory receptor neurons.
 All of these 25 000 olfactory receptor neurons express a
receptor that will interact with the same set of odorants.
 All of the olfactory receptor neurons that express the
same olfactory receptor, grow their axons and converge
onto two bilaterally symmetrical glomeruli in the two
olfactory bulbs.
Glomerulus

17.  Pyriform cortex sends inputs to medial dorsal
thalamic nucleus.
 Both pyriform cortex and medial dorsal thalamic
nucleus are connected to the orbitofrontal cortex.
 Entorhinal cortex is associated with hippocampal
formation: declarative memory.
 The olfactory signals can be important triggers
for the recall of memory.
Olfactory cortex

18. Olfactory cortex sends inputs to
the hypothalamus.
In the hypothalamus odorants can
engage our visceral motor systems.
Olfactory cortex

19. Pheromones are detected with special part
of the olfactory epithelium and posterior
part of the olfactory bulb: effect on
motivated behavior in many mammals.
How does this work in humans?
Pheromones

20. Olfactory function declines with age.
Olfactory receptor neurons can regenerate after,
for example, head trauma.
Stem cells provide this ability.
Functional recovery is never 100%.

21. https://www.coursera.org/learn/medical-
neuroscience/lecture: Leonard E. White, PhD,
Duke University
http://serendip.brynmawr.edu
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