Medical Neuroscience explores the functional organization and neurophysiology of the human central nervous system, while providing a neurobiological framework for understanding human behavior. In this course, you will discover the organization of the neural systems in the brain and spinal cord that mediate sensation, motivate bodily action, and integrate sensorimotor signals with memory, emotion and related faculties of cognition. The overall goal of this course is to provide the foundation for understanding the impairments of sensation, action and cognition that accompany injury, disease or dysfunction in the central nervous system. The course will build upon knowledge acquired through prior studies of cell and molecular biology, general physiology and human anatomy, as we focus primarily on the central nervous system. This online course is designed to include all of the core concepts in neurophysiology and clinical neuroanatomy that would be presented in most first-year neuroscience courses in schools of medicine. However, there are some topics (e.g., biological psychiatry) and several learning experiences (e.g., hands-on brain dissection) that we provide in the corresponding course offered in the Duke University School of Medicine on campus that we are not attempting to reproduce in Medical Neuroscience online. Nevertheless, our aim is to faithfully present in scope and rigor a medical school caliber course experience. This course comprises six units of content organized into 12 weeks, with an additional week for a comprehensive final exam: - Unit 1 Neuroanatomy (weeks 1-2). This unit covers the surface anatomy of the human brain, its internal structure, and the overall organization of sensory and motor systems in the brainstem and spinal cord. - Unit 2 Neural signaling (weeks 3-4). This unit addresses the fundamental mechanisms of neuronal excitability, signal generation and propagation, synaptic transmission, post synaptic mechanisms of signal integration, and neural plasticity. - Unit 3 Sensory systems (weeks 5-7). Here, you will learn the overall organization and function of the sensory systems that contribute to our sense of self relative to the world around us: somatic sensory systems, proprioception, vision, audition, and balance senses. - Unit 4 Motor systems (weeks 8-9). In this unit, we will examine the organization and function of the brain and spinal mechanisms that govern bodily movement. - Unit 5 Brain Development (week 10). Next, we turn our attention to the neurobiological mechanisms for building the nervous system in embryonic development and in early postnatal life; we will also consider how the brain changes across the lifespan. - Unit 6 Cognition (weeks 11-12). The course concludes with a survey of the association systems of the cerebral hemispheres, with an emphasis on cortical networks that integrate perception, memory and emotion in organizing behavior and planning for the future; we will also consider brain systems for maintaining homeostasis and regulating brain state.
Gustation, part 2
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Medical Neuroscience
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Sensory Systems: Audition, Vestibular Sensation and the Chemical Senses
Our survey of the sensory systems continues as we now turn our attention to the auditory system, the vestibular system, and the chemical sensory systems. As you study this content, notice the similarities and the differences that pertain to the general mechanisms of sensory transduction and the broad organization of the central pathways in each of these sensory systems. In particular, note the similarity in transduction mechanisms for audition and vestibular sensation; and note the “logic” of sensory coding in the chemical sensory systems.
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Leonard E. White, Ph.D.Associate Professor
Department of Neurology, Department of Neurobiology, Duke University School of Medicine; Department of Psychology & Neuroscience, Trinity College of Arts & Sciences; Director of Education, Duke Institute for Brain Sciences; Duke University
I like to say little bit more about what happens this information is passed from
the nucleus of the solitary tract through the thalamus.
Then from thalamus up to the cerebral cortex.
So with information is processed in our taste regions of the insular.
it's integrated with visceral sensory signals that are also processed in this
insular cortex. And that infromation then is passed onto
what we might consdier to be higher-order associational areas.
Especially in the orbital and medial parts of the prefrontal cortex.
And I would highlight one very interesting region of the orbital corext
of the brain, so here's the orbital surface.
It's that part of the brain that overlies the orbits of the eyes.
here again is our olfactory bulb and our lateral factory tract just for a little
bit of reference. I'd like to draw your attention to this
posterior part of the orbital cortex. This we think very well maybe the first
place in the cerebral cortex where flavors are represented.
This is a site anatomically that's receiving input from our Gustatory
pathway, as illustrated in the figure before us.
And if you'll recall our discussions about the olfactory pathways.
It's also a region that receives input from our primary olfactory cortex, such
as our piriform cortex. This is also an area, that is going to be
integrating signals derived from our trigeminal chemosensory pathways.
As well as, other higher order information from the somatic sensations
associated with feeding. So when we put all that together, what we
have is flavor, and a sense of the pleasure derived from that food.
be it of positive or, or negative, valence.
So we talk about the hedonic value of food, as being extremely important, in
reinforcing feeding behaviors. And this perspective on the value of food
seems to be represented in this part of the brain.
This posterior part of the orbital cortex.
Well information from, the cerebral cortex is also passed on, into structures
like the amygdala. As well as the hypothalamus, and so, we
can, appreciate from this anatomy, I think.
How information about the foods we ingest, can be distributed to a broad
cordical network. That's involved in representing the
signals derived from inside of our bodies and integrating that with our conscious
evaluations. And judgments about the significance of
these stimuli. Well we'll talk much more about these
topics when we get into the final unit of the course, when we talk about cognition.
The associational cortex in some of the truly fascinating, issues and modern
neuroscience, that revolve around our sense of judgement.
Our sense, of value, our sense of self. And how much of this is derived.
Not just from what we see and what we hear and what we touch in the world
that's beyond our fingertips. But also the world within these
sensations, including the chemical sensory signals.
And our visceral sensory signals that become integrated into our conscious
sense itself. So, I hope you'll enjoy that discussion.
Stay tuned for that. Okay, now, I think we're ready to turn
our attention to the problem of sensory transduction in the Gusatory system.
So, sensory transduction again, refers to the challenge of converting mechanical
energy, electromagnetic energy, or molecular energy into electrical signals
within a nerve cell. So we've seen how that happens in each of
the sensory systems. And let's consider now how it occurs in
the Gustatory system. So I'll just remind you that the
tastants, that is the actual molecules that will activate this system, obviously
are ingested. taken into the oropharynx where they can
interact with specializations of the epithelium.
We actually find these taste buds and specializations that we call papillae.
So, at the back of the tongue, we have a row of circumvallate papillae.
at the sides of the posterior tongue, we have.
They set a foliate papillae . And then in the interior part of the
tongue, we have what's called Fungiform Papillae.
And these names are just anatomical terms that describe a bit if the far,
morphology of these papillae. So don't, don't really be concerned with
the precise, configuration of this tissue that forms these papillae.
Unless this is of particular interest to you.
But otherwise I just want you to know that there are those papillae that are
distributed in different ways across the surfaces of the tongue.
And then the posterior part of the oral[UNKNOWN].
Now these posterior regions in the esophagus and in the soft palate are
going to be intervated probably by branches of the vegus nerve, nerve 10.
Here on the tongue, the posterior third of the tongue is going to be intervated
by branches of nerve 9, the glossopharyngeal nerve.
And the anterior two thirds of some of the tongue including these fungiform
papillae. These are going to be supplied By
branches of cranial nerve 7. Okay so this accounts for how nerves 7, 9
and 10 provide Gustatory signals into that nucleus of the solitary tract to the
brain stem. Well now let's look at the anatomy, the
microanatomy of one of these papillae. So, so what we see is that, is that
there's a bit of a trench so we can imagine that water soluble tastants, work
their way into these trenches. Where they can interact with these taste
buds, these specializations that we find in the lateral flanks of these trenches.
And it's within those specializations that we find our taste cells.
So here's a closer look at a taste bud. So, this is the, the tissue that we find
in the tongue. These taste buds include our sensory
receptor cells, our taste cells. And they also include a variety of basal
cells and supporting cells. Now the taste cells are the ones that, of
course, I would draw your attention to primarily.
these are sensory neurons that have an apical surface that comes together into
something that we call a taste pore. So it's specifically through this taste
pore that ingested molecules would interact with the ethical surface of our
taste cells. And its at that ethical surface, that
interact with the taste and molecules. Leading to the depolarization of those
taste cells. The release of neuro transmitter, and and
a synapse between the base of the taste cell.
And the axons of those three cranial nerves, nerves 7, 9 and 10.
Okay so, now let's focus in on this apical surface of taste cell.
And understand how sensory transduction happens.
Well, for taste cells sensory transductions happens through a variety
of different mechanisms. And as I'll tell you in just a moment.
The receptors that are expressed help to define the response of that taste cell.
And cells that are tuned for particular molecular forms.
Thereby giving rise to particular Gustatory sensations, are distributed.
And overlapping, but somewhat biased regions over the surface of the tongue
and in the posterior region of the oropharynx.
Well, I'll tell you that story in just a moment.
But first I want us to focus on the interaction between our tastant molecules
that are going to be presented to this taste pore.
The apical surface of the taste cells. And we know that there are lot of
different flavours up there right? Well so psychometrically we can
categorize them basically into 5 principle flavours.
There are tasted molecules will induce these flavour perceptions.
And those five basic categories are salt acid or sour, sweet, bitter, and just in
the last couple of decades. I think psychologists around the world
have agreed that there's a fifth category that we call Umami.
And Umami refers to the taste of monosodium glutamate and probably other
related amino acids. It's derived from a, a Japanese word that
means delicious. This is probably why some of us enjoy a
little bit of monosodium glutamate in our diet.
I hope you all appreciate now that too much monosodium glutamate can be a very
bad thing, it can induce [UNKNOWN] toxic injury.
but in small amounts it is sufficient to activate this taste pathway that gives
rise to this aparently for most of us very pleasurable sensation.
Okay, so the point here is that there are different categories of tastants that
interact with different taste receptors. And these receptors have quite distinct
mechanisms for transducing this information.
For the salt, and the acids or the sour tastants, they interact with Ion channels
that have receptors on the extracellular face of these receptors.
So, when the tastant interacts, an ion channel opens.
And positively charged cations can enter the cell, depolarise it.
And that leads to the activation of voltage gated channels that ultimately
allow for the entrance of calcium at the basal region of the cell.
And in a calcium dependent manner. There's the exocytosis of vesicles that
contain the neurotransmitter serotonin. There may be other neurotransmitters
involved. But, this is one that we know about.
And seratonin will interact with receptors expressed at the terminal of
that apherin fiber. With sufficient release of
neurotransmitter, there will be a depolarization that is sufficient to
generate an action potential, in that afferent fiber.
And then communicate the signal on into the central nervous system.
Well, other categories of tastants, specifically those tastants that we
perceive as being sweet, being bitter. And again this delicious amino acid taste
that seems to be distinct in various ways.
These tastants interact with G protein-coupled receptors.
Now, we've talked quite a bit about this category of receptors.
So, I hope it's now familiar to you. It's a seven transmembrane protein that
is associated on it's cytoplasmic surface with G proteins.
And these G proteins go on to activate second messenger systems.
And these second messenger systems can interact with a variety of other systems
within the cell. Some of which will lead to the
depolarization of the taste cell. Others will release calcium from
intercellular stores. So whether calcium is coming in through
voltage gated ion channel or being released from vesicular compartments.
the increase in Calcium in the basal region of the taste cell is potentially
sufficient to lead to the exocytosis of neurotrasmitter.
So, in this particular taste cell we're illustrating the presence of multiple
receptors. And, and that may be the case.
But for many taste cells there just may be one receptor that is expressed by that
taste cell. And this figure gives us more detail than
we need to know. But it just, gives us a closer look at
the transduction machinery associated with the receptors.
So again the salts and the acids, they interact with receptors that are on ion
channels. And as these receptors are gated then
sodium ions or protons can pass into the taste cell and lead to it's
depolarization. other tastants, such as the sweets, the
amino acids, the umami tastants, and bitter interact with G protein coupled
receptors. And we know a little bit more about the
detail of these G protein coupled receptors.
Some of them are model monomeric that is, there's one receptor that interacts with
G protein sum or dimeric. And even heteromeric meaning that there
are different types of receptors. In this case the tr2, the tr3 or the tr1
and the tr3 that come together to form the functional receptor that interacts
with the tastant. In some of these receptor systems, the
downstream effector is a trip channel. Specifically, a trip and five channel
that allows for the influx of calcium into the post synaptic cell being gated
by a second messenger system. in the case of preceptors for amino
acids, and for bitter we think that second messanger is an IP3 message.
So, you can see that in various means, there are a diversity of mechanisms that
allows for the depolarization of the taste cell.
And the release in a calcium dependent manner of the neurotransmitter at the
basal end of the taste cell.
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