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.
Olfaction, part 3
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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
So what we know are that humans are sensitive to an enormous range of
odorants. And odorants come in all kinds of
molecular shapes and sizes. And these shapes and sizes interact with
a large number of olfactory receptors. In fact the genes that encode olfactory
receptors, are the among largest gene families that we know about.
Within vertebrate and even invertebrate systems.
So, there is tremendous conservation of these genes, but also tremendous
diversity. In our case, we've got probably about
1200 olfactory receptor genes most of them are pseudo genes, that is, they
don't express a competent protein, that functions as an olfactory receptor.
The actual number of true genes, for olfactory receptors in us, is about 400
or so. Which is maybe about a 3rd the number
that might be present in a typical canine.
And perhaps this has a great deal to do with the relative sensitivity of
olfactory discrimination and detection in canines versus humans.
So humans can be sensitive to odorants down to a concentration range of
nanomolar. So that's, really a remarkable amount of
sensitivity, even though we don't typically think as humans as being the
champions in the animal kingdom, in olfactory sense.
yet, we're definitely holding our own. Relative to our mammalian kindred.
And it's really fascinating to consider how these odorants interact with these
receptors. So there can be very small and subtle
changes in the molecular structure of an odorant that can give rise to very
different percepts. So we want to understand some of the
logic as to how that happens. For example, the d isoform of the odorant
carvone for many of us smells like rye. Whereas the l isoform of carvone smells
like spearmint. Very different perceptual qualities with
a subtle rotation. Of a carbon bond in this molecule.
another interesting aspect of olfaction is that the quality of the odor is
sometimes modulated by the concentration of the odorant.
So for example, low concentrations of the odorant indole smells like flowers, a
floral bouquet. Whereas high concentrations of the very
same odorant molecule can smell quite putrid.
what we might think of as exactly the opposite end of this perceptual spectrum.
And lastly I, I would make the point that most natural odors, they're really not a
single odorant but they're quite complex. Much in the way we talked about human
speech being spectrally complex, being composed of many fundamental frequencies.
In an analogous way, most odorants are, are quite complex.
There are many different odorant molecules that are present within that
odor, and these molecules will interact with odorant receptors in various ways.
And it's, to those interactions that I'd like to turn our attention now, using the
slide that's before you. So what we have is an example of a
combinatorial code. That relates odorants to olfactory
receptors. And this code is based on the molecular
shape of the odorant. Now, this figure provided by my colleague
here at Duke, Hiro Matsunami, illustrates for us the relationship between molecular
shape. And odorant receptor.
For example, this odorant here at the top, it has sort of a triangular shape on
the left hand side of this molecule, and a rectangular shape to the right.
Well the rectangular part would appear to fit nicely into a receptor that has
rectangular shaped receptor motif. Whereas the triangular piece would appear
to fit quite nicely into a molecule that is configured in a way to interact with
this triangular shape. Well, of course the molecules themselves
aren't really rectangles or, or triangles, but rather there is a
molecular shape that is interacting with a particular receptive motif that's
present in the receptor protein itself. And the point here is that the same
odorant molecule can interact with more than one receptor depending upon the
geometrical configuration of odorant molecule.
The receptor. In the case of this first odorant that is
illustrated in this schematic here, we see that this one odorant can interact
with at least two different olfactory receptors.
For example, we see the complex shape of the odorant to the bottom has molecular
form and structure to it that allows it to interact with three different odorant
receptors. So, what we mean by combinatorial code,
is a code based on the number and, as it turns out, the location, of odor
receptors that interact with the same odorant molecule.
So in this scheme, the first odorant would be indicated by the activation of
these two odorant receptors. Here in the two left hand columns,
whereas the odorant to the bottom would be indicated by activation of the three
odorant receptors that are off to the right.
Now this combinatorial code that we find in the relationship of odorants to
olfactory receptors. Is converted into a spatial code that
allows for the receptor neurons that express a given olfactory receptor allele
to grow their axons and converge on the same pair of glomeruli in the olfactory
bulb. So, let me explain this anatomy.
And how this remarkable degree of convergence works.
So, here's an illustration that shows us our olfactory epithelium, and we see a
population of olfactory receptor neurons that grow their axons through the
cribriform plate, and they reach the olfactory bulb.
What we find in the olfactory bulb is a really particular compartment that is
rich with receptors and synaptic connections between our sensory axons
from our olfactory receptor neurons, and a variety of postsynaptic targets,
including the principal neuron of the olfactory bulb called the mitral cell.
So here we see a mitral cell that grows a dendrite into a spherical structure that
we call the glomerulus. And it's within that glomerulus that the
mitral cell receives synaptic input from an afferent axon.
While there are a variety of other cell types that are present in the olfactory
bulb, namely a small inter neuron called the periglomerulus neuron, there is also
a small inter neuron called the granule cell.
both of these neurons seem to mediate inhibitory interactions within and among
glomeruli. There is also a cell called the tufted
cell that contributes post synaptic targets for the afferent input that's
arriving in the olfactory bulb. So, these spherical structures called
glomeruli, are the first site of synaptic connection between the olfactory
epithelium and the brain. Here's the challenge in setting up this
circuitry between the olfactory epithelium, and the, olfactory bulb.
Each glomerulus receives input from about 25,000 olfactory receptor neurons.
But what's absolutely astounding about this convergence of information into a
single glomerulus is that nearly all the olfactory receptor neurons.
That provide input to that same glomerulus, will express the very same,
alelle, for a particular olfactory receptor.
That is, all of these 25,000 neurons, are expressing a receptor that will interact
with the same set of odorants. This allows for convergence, of both
anatomy, and function. Into each and every glomerulus that we
find in the olfactory bulb. If we look across the entire olfactory
epithelium, that's present on both sides of the midline, what we find is that, 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. So this is an example of really a, a
remarkable achievement in path finding, whereby axons from a broad epithelial
surface, will grow and target, and ultimately establish sona, synaptic
connectivity with a very limited spatial target.
In this case, a spatial target of probably less than 100 microns or so, of
typical dimension. That would be about the diameter of a
typical olfactory glomerulus. Now, as I mention there are other cells
that contribute to the circuitry of the glomerulus.
I won't really say anything more about that except to suggest that the granule
cells are a particularly interesting population of neurons.
They seem to set up inhibitory networks across the olfactory bulb.
They appear to have a role to play in. Synaptic plasticity that occurs within
circuits of the olfactory bulb, and for many mammals, although it's not so clear
that it happens in humans, the granule cell population is one set of neurons
that is repopulated in life. That is, there is a set of stem cells,
that produce new neurons that migrate along what's called the rostral migratory
stream to enter the olfactory bulb, and contribute new neurons to the granule
cell layer of the olfactory bulb. Again we're not so sure whether that
really exists in our brains, as well as many mammals.
But it is curious that of all the populations of neurons in the brain to be
supplied throughout adult life, it's the granule cell population of the olfactory
bulb in many mammals that, benefits from this neurogenesis that occurs even into
adult life. We'll say more about development when we
come to unit five, but just hang onto that thought now.
That the olfactory bulb is a recipient of the production of new neurons in many
mammals. One last thing about the olfactory bulb,
before we move on to talk about the olfactory cortex in the brain, is that
the olfactory bulb seems to solve some of the challenge of having complex odors
that contain many different odorants. if we go back for just a moment to the
previous slide, where we saw something about the logic of the relationship
between odorants to receptors, and we discuss this notion of a combinatorial
code. I think its perhaps apparent to you that
the same complex odor might activate quite a large number of olfactory
receptor neurons that express, each their own allele.
but the protein that's produced by that allele, the olfactory receptor,
potentially can interact with more than one odor molecule.
And so as a result of this system of combinatorial coding in the olfactory
epithelium and the convergence of the sensory neurons to individual glomeruli
in the bulb, you might imagine that a complex odor ought to give rise to the
activation of, of tens perhaps even hundreds of glomeruli.
In the olfactory bulb, but that often does not seem to be the case.
It seems to be that a much more sparse coding mechanism exists in the olfactory
bulb for complex odors. Which is to say that there seem to be
certain glomeruli that might dominate the representation of a complex odor.
Not all glomeruli that may be receiving afferent information may be equally
activated. And perhaps this is where these
additional neurons come into play. With lateral inhibitory networks.
much remains to be discovered about how representation in the bulb was actually
achieved. But I do think it's notable that there is
a sparse coding of complex odors in the activation of only a limited subset of
glomeruli. And now the next challenge is to
understand how does that information get transferred back into the rest of the
ventromedial fore brain. And then how is that information coded
and processed and then ultimately how do these olfactory signals impact behavior.
So let's turn now from the bulb. We'll travel down the lateral factory
tract and consider some of the central processing mechanisms that we find in the
olfactory cortex.
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