that again warms the air as it pass through
the nasal cavity. Let us now have a look at
the olfactory mucosa. The olfactory mucosa
contains the epithelium that detects odiferous
substances that are dissolved in the watery
secretions that make the surface of the cells
and we will look at these in more detail in
a moment. But their information is then carried
back to the brain, through the olfactory nerve.
The olfactory nerve consists of a number of
bundles or fascicles of nerves rather one
single nerve and those bundles penetrate through
the base of the skull, through the cribriform
plate of the ethmoid bone and those nerve
fibres, nerve bundles are very very delicate.
Sometimes if there is a brain injury or a
cranial injury, the skull injuries, then when
the brain moves as a result of say the momentum
of the injury, then sometimes those little nerve
bundles can get torn off from the brain, from
the olfactory bulb and therefore the sense
of smell disappears because of that.
Thankfully, these neurons, these special neuroepithelial
cells are replaced by basal cells in the epithelium.
It is about the only place in the nervous
system where neurons can be replaced naturally
in the adult. Well, these glands, the olfactory
glands are also very important. As I mentioned
the odiferous substances are dissolved in
the fluid secreted by these glands and that
fluid sits on the surface of the olfactory
epithelium. Let us now have a look at the
olfactory receptor in more detail. On the
left-hand side, is a colorful diagram explaining
all the cell types including this receptor in
the olfactory epithelium. On the right-hand
side, is the histological section of the epithelium,
you can see the olfactory epithelium labeled
here. You can see the olfactory nerve that
I mentioned a moment ago and you can see now
the olfactory glands. All have important functions.
But now let us look at the diagram and understand
the type of epithelial cells that dominate
this particular epithelium. Let us focus on
the receptor cell. It is really a neuron and
it is labeled there the olfactory cell.
It has a dendritic, one dendritic process and
one axonal process. The axonal process as
we've explained forms the components of the olfactory
nerve and sends information all the way back
to the central nervous system, while the single
dendritic process has a swelling called the
olfactory vesicle. And from that vesicle, from
that dendritic process, a series of cilia
extend along the length of the epithelium
for at least 200 microns. They are very long
cilia and they align parallel to the surface
of the epithelium. Those cilia are going to
contain receptor proteins for their different
substances. There are also another group of
cells there, it's the supporting cells that wrapped
themselves around these receptor cells.
These supporting cells are really neuroglial
cells because they are supporting neurons.
They have a very important function. If you look
at the diagram, you can make out the representation
of microvilli on their surface. One of the
jobs is that the sustentacular cells or supporting cells
do is not only be a mechanical and a metabolic
supporting cell for the receptor cell, but
they secrete the odiferous binding proteins.
And these binding proteins move out into the
watery fluid on the surface of the epithelium
and bind to the dissolved odiferous substances
and then transport those of the receptors
on the cilia that I mentioned previously.
So they have a very very important function. And
then you have the basal cells at the bottom.
They are the ones that can divide and differentiate
into either the supporting cells or those
special olfactory receptor cells that I've just
described. You know there is one cell not
present there that is called a brush
cell. These brush cells, you cannot see them
in sections, they are very hard to find, but
they are present in the nasal cavity epithelium
and the olfactory epithelium and they are
present all the way down the conducting portions
of the respiratory systems. They have a little
tiny process that extends through their basal
laminas or basement membranes and communicate with
nerves. In the case of the nasal cavity,
they communicate with processes of the trigeminal
nerve, the fifth cranial nerve. And these brush
cells are probably cells that can give some
sort of sensory information about the mucosa
back to the central nervous system. But as
I mentioned before, you cannot identify these
cells normally in normal histological sections.
But they exist and they are important.