在本课程中，学生将学习认识和应用说明人体九个器官系统中整体人体机能（作为完整有机体）的基本概念。
化学感觉
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From the course by Duke University
人体生理学导论（中文版）
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From the lesson
感觉和躯体神经系统
在此模块中，我们探讨两种类型的细胞：一种将信息转送到中枢神经系统（大脑）以进行解读，第二种从中枢神经系统转送信息来控制随意运动，它们是运动神经元。 通向大脑的输入通路由叫做感觉细胞的特定细胞来介导。 感觉细胞将能量（如光或热）转化为大脑中的神经元可识别的能量形式（电位）。 接下来，大脑解读这些信息（解读为视觉和痛觉），然后通过躯体神经系统的运动神经元将运动反应发送至身体的效应细胞。运动神经元激活骨骼肌来控制呼吸和四肢的运动。
Meet the Instructors
Jennifer CarbreyAssistant Research Professor
Department of Cell Biology
Emma JakoiAssociate Research Professor
Department of Cell Biology
Welcome back.
We're going to finish the senses with the chemical senses.
So we're going to talk about the last of the special senses.
So we're going to be dealing with chemoreceptors,
molecules that are going to bind certain chemicals
and then eventually lead to a graded
potential which will lead to an action potential.
So we'll be, obviously, talking about taste and the sense of smell.
But then keep in mind as we've mentioned before, there are going to
be other chemicals that our body is also monitoring and responding to.
Such as detecting osmolarity in the brain, and diss, which we'll
talk about when we talk about the kidney, and then talking
about the amounts of oxygen and CO2 and the pH of
the blood which we'll be talking about when we get to the
respiratory system.
Let's consider taste first where you're going to have along the
surface and kind of in recesses of your tongue, you're going to have taste buds.
Which are going to be made up of taste cells which you can think
of kind of like segments in an orange that are forming this
sphere kind of around a central channel which is called the taste pore.
So you can see, that because this taste pore is kind of
recessed from the surface of the oral cavity, and
because it's going to be really small, that the substances,
the molecules that we taste are going to have to be
dissolved in liquid.
So that's fine because we pretty much constantly have saliva
in our mouth, and certainly we have it when we're eating.
But that does mean that in order to taste
something, we, it needs to be dissolved so that
it can enter this little taste p-, pore and
expose the tops of these taste cells to the molecule.
Then these
taste cells will form graded potentials that
if they're strong enough, will affect the afferent
neurons leading from the taste bud, and to
send action potentials into the central nervous system.
We can taste different flavors or different taste modalities.
We've already said how this, these substances
are going to have to be dissolved in saliva.
And no matter what we're tasting, the result
is that we're going to increase intracellular calcium.
That is going to cause a release of neurotransmitters by these tase cells that
are going to because graded potentials that
if they're great enough or strong enough will
because action potentials in that post-synaptic neuron, that
neuron that's leading away from the taste bud.
And we have a couple of major types of channels that are going to be activated.
So, when we're tasting something that's
sour it's because we are detecting protons.
We're detecting something that's acidic.
And then often when we're tasting something
that's salty what we're detecting is sodium.
Also it can be potassium as well but in either,
any case these are going to be ions, and so
they're going to act on ion channels that are then
going to because an increase in intracellular calcium in the cell.
Versus sweets, tasting substances which are going to be sugars.
Bitter which are compounds which are often going to be plant alkaloids.
And the a fifth taste modality is umami, which is a meaty flavor.
So it's, what makes meat taste like meat, but
also is what causes mushrooms to taste like meat.
Which is when we are
sensing amino acids like glutamates. So sugar's, bitter molecules, and amino
acids like glutemate, are going to be detected by G-protein coupled receptors.
So these tastants will bind the G-protein
coupled receptors and then the G-Protein coupled receptors
will because a signal transduction pathway to
initiate that will increase intracellular calcium and cause
the release of neurotransmitter. We're going to finish up with smell which
is going to be unique compared to most of the other systems that we've talked about.
Especially of the special senses because it is actually the neurons themselves,
the primary afferent neurons themselves that are going to be
expressing olfactory receptors and actually binding to the odorants.
So, in the
epithelium covering the nasal cavity is where we have
these neurons that are sending out cilia that are,
have receptors on them that can bind the
odorants and these are again going to be
G-protein coupled receptors. So each neuron will express a specific
type of G-protein coupled receptor that combine a certain type of molecule.
We have several hundred olfactory receptor types.
So we're going to have several hundred neuron
types that are expected, expressing a certain receptor.
But yet we can discriminate about ten thousand odors.
So when these odorants
bind, they're going to cause a graded potential, which, when great
enough, or big enough can lead to an action potential.
And it's going to be which neurons are activated and how much they are
activated by binding a certain molecule that lets us convert that to a smell.
So if we have a certain molecule.
Let's say that there are two or three different odorant
neurons that combine different portions of that
odorant molecule, and so, which neurons are
activated and how much they're activated is
what we translate into smelling that molecule.
And so again it's very similar to our ability to see color where we don't have
a different receptor type and a different neuron
type for each color or for each molecule
we can smell.
But instead it's the matter of which neurons are activated and how much in
combination that allows us to detect so
many colors, and so many different or, odors.
So we've talked about the two special senses that involve chemo
reception, taste where we're going to have taste receptors in the tongue
that are going to be associated with our five
basic taste modalities; umami, salt, sour, sweet and bitter.
And again when were sensing a taste, it's going to be combinations of taste receptor
activation that's going to give us certain
tastes and we're going to be interpreting those in
terms of preceding what we are tasting.
And then in smell we're going to
be activating olfactory receptors which themselves are going
to be the primary afferent neurons leading
in sending information into the central nervous system.
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