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.
Temporal Associational Cortex: Memory, part 1
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Medical Neuroscience
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Complex Brain Functions: Associational Cortex
It may surprise you to know that in all of our studies of the neural systems for sensation and action, we have yet to properly account for the organization and function of roughly 75% of the entire cerebral mantle. Thus, only 25% of the cerebral cortex is accounted for by the modal sensory and motor cortical areas. The majority of the human cerebral cortex is multi-modal cortex that associates signals derived from one or more modal systems. We now turn our attention to this “associational cortex” as we consider more complex aspects of brain function.
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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
Well let's switch gears now and move on to a consideration of memory function.
As we talk about memory very briefly here in this tutorial I think it's important
to be clear really what kind of memory are we talking about.
And it's possible to differentiate different categories of memory.
in fact it's quite helpful to do so because as it turns out, different forms
of memory are supported by different networks in the brain.
So one can imagine that there is a kind of memory that is available to conscious
cognition. So what I have in mind here is the kind
of memory for people, for places, for the episodes of our lives, for the things
that can access our language faculty and allow us to describe verbally the content
of that memory. And so this kind of memory we call
declarative memory. It's available to concious processing.
It reflects the daily episodes of our lives.
the semantic symbols that give meaning to our thoughts and we can access those
symbols and apply them in the form of language.
So with declarative memory we have access to our lexicon that allows us to report
the content of that memory. It provides a, a personal history of our
life events. So declarative memory, sometimes called
episodic memory is that memory for which we can declare its content.
It reflects the daily episodes of our lives.
Now this is to be contrasted with another category of memory really multiple
subcategories of memory that in our textbook we simply choose to call
nondeclarative memory but it can be subdivided in various ways.
Sometimes this is called implicit memory, Some subdivisions of this form of memory
can be called motor memory so the nomenclature is actually not entirely
consistent across various sources. So for those of you that are in this
field and prefer a particular nomenclature that might be different from
this one I hope you'll pardon my use of the nomenclature that we present in our
book. But I think it's helpful to just make
this broad point that it's important to differentiate from declaritive memory,
these other forms of memory. So these are forms of memory that
generally speaking are not available to consciousness.
That is, it's much more difficult to acess our lexicon to put our semantic
labels upon the content of this memory. So in this category would be motor skills
for example, learning to ride a bicycle. Learning to ice skate or learning to do
some other motor skill that once acquired seems to be with us,
Perhaps for life, of course, depending upon how often we practice and resurrect
that scale. So it's quite challenging is it not to
put into words the nature of the skill that allows us to ride a bicycle?
We can describe riding a bicycle but we cannot access the content of that skill
through our semantic facilities. This an important clue that this kind of
memory might actually be aquired and consolidated by a different kind of brain
system than is operating our declarative memories.
Well in addition to motor skills there are various associations that are
established. either explicitely or implicitely and
it's those implicit associations that are exemplars of non-declarative memory.
There are various phenomena related to priming.
That is a form of memory that is nondeclarative and related to motor
skills there are puzzle solving skills. That are accrued through experience and
practice. That like other kinds of motor skills are
not readily accessible to our language faculties.
Now there's another dimension of memory that can be categorized and that is the
temporal dimension of memory. There's a sense of memory that's
immediate and this is memory for the previous few moments that are essential
for maintaining consciousness, conscious awareness.
conscious deliberations over cogent thought.
so this is a form of memory that most likely reflects ongoing recurrent
processing throughout cortical networks. But there's another form of memory that
helps us to maintain the goal of our behavior and so.
This is now extending over not just the last few moments but perhaps seconds to
minutes. This we call working memory.
So this is the capacity to hold online the goal that is motivating behavior.
So this also likely reflects recurrent networks that interact across
associational regions in the front and the parietal lobes at least but also the
temporal lobe. Now we think of memory probably what we
have in mind really is long term memory. So this is the kind of memory that
reflects the accrual of life experience and it's consolidation and the capacity
to then recall this information. And have it once again enter our
conscious deliberations. This kind of long-term memory is what is
clearly dependent upon the associational networks of the temporal lobe and
specifically the medial parts of the temporal lobe.
So this gets us into parts of the brain that we've just briefly encountered in
our survey of the human brain in the first unit of the course.
You may recall that there is a gyral structure on the medial edge of the
inferior temporal lobe called the parahippocampal gyrus.
This is an important gyrus for this capacity to consolidate the moment and
build long-term memory. Now deep within this gyrus on the medial
edge of the cortical mantle is a structure that we saw in cross section
called the hippocampus. And this as it turns out is the critical
brain structure for the acquisition of declarative memory.
So when we speak of long-term memory, really what we have in mind is the
storage of declarative memory. And the capacity to recall those
memories. So here in a phantom view of the human
brain we're reminded that deep to this parahippocampal gyrus we have the
structure called the hippocampus and just in front of the hippocampus is the
amygdala. Now the amygdala is associated with yet
other aspects of complex brain function namely emotion and we're going to talk
about emotion in a separate tutorial. So for this tutorial we're going to leave
the amygdala out. And focus in on the associational
networks that are associated with this fascinating region of the mammalian brain
called the hippocampus. So let's look more closely at the
hippocampus and I'll just remind you that the hippocampus is a corticol structure.
But it's found at the medial edge of this para hipicampal gyrus that folds in upon
itself forming the floor and medial wall of the lateral ventrical as it makes it's
shape into the temporal lobe. Now the hippocampus is one of those
wonderful terms in neuroanatomy that I'm sure many of you know we talked about it
briefly in unit one. The hippocampus is a Greek term for
seahorse or perhaps sea monster depending on your interpretation of the Greek.
One can imagine that this structure bears some resemblance to the body of a
seahorse. Reflecting the curving back upon itself
as this medial edge of the cerebral mantle took shape in the temporal lobe
during the third trimester of gestation. Now interestingly if you actually dissect
the hippocampus out of the human brain and just lay it out on a tray it also
resembles the body of a seahorse from that view.
And there are at least a few images out there that you're certainly free to
Google to see that view of the hippocampus as well.
perhaps for the next version of this course, I'll do that dissection myself.
And be able to show you the sea monster within our temporal lobe more directly.
But for this view, I would just emphasize that the hippocampus is a cortical
structure but it's the simplest of all cortical regions.
Philogenetically we call this archicortex, which reflects its ancient
origins in the evolution of the vertebrate brain.
but we retain this simple architecture even in our brains.
It's a simple architecture but it's evidently sufficient to do the job of
consolidating declarative memory. Now the hippocampus receives input from
this parahippocampal gyrus. And I won't go into the details of
exactly how all that works. You may remember though that there are
inputs that enter the dentate gyrus which is this curved region where we find those
dentate granule cells. That continue to harbor some neural stem
cells that differentiate into neurons throughout life.
So these dentate driver cells are being added to throuout our lives and so there
is plasticity among that cell population. Both in terms of it's members but also of
course it's synaptic weights. So the dentate gyral cells then project
to other neurons in the hippocampus and there are yet additional associational
connections. And it's those associational connections
that we studied in unit two when we looked at the synaptic basis of long-term
potentiation and long-term depression. We think that these synaptic changes are
the cellular and synaptic mechanisms that are responsible for consolidating
memories. A memory after all is simply a, a
permanent record. So something is changed in the structure
and function of circuits that enable the storage of memory.
And we know that the hypocampus is an essential structure in this medial
temporal lobe memory network that it is essential for the acquistion and perhaps
also the recall of declarative memory. So thinking about the inputs to the
hypocampus, I like to think of the hypcampus as this great funnel of the
moment. Everything that defines your sense of
right now. The sound of my voice.
The sight of my face. The background image that you see on your
device. the setting in which you find yourself
in. Perhaps there are other people around
you. All of these sensory stimuli define right
now. And this information is being processed
through the relevent sensory cortical areas.
It's being fed into accosiational cortical regions in the oxipital lobe,
the temporal lobe, the priatal lobe and even the frontal lobe.
And from these associational cortexes inputs are being funneled into this
parahypocampal gyrus. And from there they are relays from this
cortex called the interinal cortex into various divisions of this hypocampal
network. And from there through mechanisms that we
think are inclusive of long-term potentiation and long-term depression,
memories are stored. Now we know much less about the actual
storage of the memory than we do about its acquisition.
In terms of storage, we know that the hippocampus maintains back projections
through various path ways including projections that are directed back our
through the parahyppocampal gyrus. To each of these associational cortical
regions that were relevant in the processing of the sensory stimuli that
comprise the here and the now the moment. So these extensive interconnections
betweens associational cortex in each of the lobes and this parahippocampal gyral
cortex seem to be critical for the storage and the recall of information.
So when we recall a memory and I would invite you all right now to recall some
memory perhaps a memory from childhood. Or perhaps a memory of what you ate for
breakfast today. Whatever memory you may be operating with
right now reflects the back activation from this medial temporal lobe memory
system. To the relevant associational cortical
regions that first process that information.
So there must be plasticity in the connections back from the parahippocampal
gyrus cortex into these various associational regions that are associated
with vision and audition. And olfaction and even gestation.
perhaps even the vestibular sense. Although we know much less about that
sense in terms of cortical processing and memory.
So, one can imagine the kinds of problems that patients might have with damage to
this critical part of the Medial temporal lobe.
This produces a clinical phenomenon that we call amnesia.
So amnesia refers to pathological forgetting, a failure of our Medial
temporal lobe memory system to consolidate new memory.
And perhaps even to recall memories that have previously been laid down in these
vast associational networks. Amnesia can be in the entrograde
direction that is from a moment in time of injury or disease or disorder going
forward or amnesia can be retrograde. It can project back into time prior to
the moment of injury or disease and disorder.
Now for damage to the hippocampus we will find antrograde amnesia but also some
degree of retrograde amnesia. Evidently it takes some time for memories
to be consolidated and to be laid down for long term storage.
Now perhaps some of you like me have suffered a concussion at some point in
your life. perhaps some have had even more severe
forms of traumatic head injury than what we might label concussion.
But one common feature to traumatic injury of the brain is amnesia for the
moment of impact in the case of trauma. So in my case I was in a motor vehicle
accident last fall and I recall the approach of the intersection at which.
I got into my motor vehicle accident but I really don't recall the moment of
impact nor do I recall the approach of the vehicle that hit me from the side.
I have a sense of preceding as I had right-of-way into that intersection.
But the next thing I remember was coming around and realizing that I had been in
an accident and now crawling out of my vehicle.
So I had a bit of retrograde amnesia for the few moments leading up to the impact
that resulted in my concussion. Evidently those moments leading up to
that impact were peculating in the networks of the medial temporal lobe
memory system and in my own head. And the trauma of the concussion was
sufficient to alter the firing that was on going in that network so those moments
are, are simply lost. the activity evidently cannot be
reconstructed. And as a result I simply don't have
access to those few moments of my life. Now the concept of amnesia is one that
has gained quite a lot of interest especially among people that make movies.
some of you may be familiar with several movies that came out of Hollywood in
recent years on this theme of exploring the life of someone who has suffered
amnesia. some of them are rather comical and
distasteful and do a poor job representing the lives of such people.
But there have been some that, I think have done a realistic job portraying life
with limited capacity to lay down new memories.
Well, I just want to be clear. What I've been talking about thus far in
terms of the medial temporal lobe memory system associated with the
parahippocampal gyrus. And the hippocampus itself pertains to
declarative memory. The acquisition, the consolidation and
then the processing that's necessary for long term storage of declarative episodic
memories, the episodes of our lives. Well, what's been fascinating to discover
about people that have had damage to the Hippocampal system is that they retain
memories that we might categorize as non-declarative or procedural.
And that's sufficient evidence to think that there are different brain systems
outside of the medial temporal lobe that mediate procedural memories.
So let's turn our attention there now. So the important point that I want you to
be able to understand is that when it comes to nondeclarative memory to
procedural memory. these forms of memory are dependent upon
entirely different brain systems than are those that are responsible for the
acquisition, storage and recall of declarative memory.
So we've been talking about declarative memory of the hipocampus in the short
term storage of those memories. Their acquisition.
Their consolidation. And I mentioned that the long-term
storage of declarative memory engages widespread cortical regions.
Many of which were involved in the initial processing of the here and the
now. So when it comes to our memory systems
for nondeclaritive memory we're just now really beginning to do the kinds of
experimental work. And humans and non human primates that
would shed more light on to the kinds of systems that are essential for acquiring
these kinds of memories. For consolidating them, and then for long
term storage. Now our figure from the book indicates
that well we don't know really what's involved with the short term acquisition
of declarative memory. I would suggest that perhaps we do know a
little bit more than what this figure would imply.
And I certainly want to now restore the integrity of my cerebellar hemispheres so
that I can make the point that the cerebellum appears to play an important
role in the acquisition and consolidation of procedural memory.
One can think of procedural memory as a form of motor learning that requires the
generation of error correction signals in order to acquire a memory.
And that memory then takes the form of an accrued skill.
At least with respect to motor function and motor behavior.
And these are functions that we associate with the cerebellum.
And the means by which the cerebellum can modify the operation of our motor
cortical regions here in the posterior part of the frontal lobe.
So I would suggest that for the short term acquisition and storage of
nondeclarative procedural memories. The Cerebellum is among the brain
structures that will have an important role there.
Now, we know this because in functional magnetic resonance imaging studies, for
example, when human subjects are confronted with a challenging motor
skill. Activity in the cerebellum is
significantly modulated as these kinds of skills aer being aquired.
But once a certain measure of proficiency has been establish, the activity of the
cerebellum is, is much less and that would be consistent with the idea that as
we learn a new skill, the generation of error signals is quite robust.
But as the scale has been consolidated and now that scale has executed with a
certain measure of proficiency than the need for infusing additional error
signals is, is simply less. So the cerebellum is playing an important
role in the early stages of the acquisition and the consolidation of at
least procedural memory. The long term storage probably reflects
again widespread plasticity throughout the networks that are responsible for the
execution of the skill. Be it a procedural skill, a motor skill
or some other aspect of implicit human behavior.
So again if we were to look into the brain anatomy that might be responsible
we'd be looking at components of the motor system.
perhaps the cerebellum continues to play a very important role in the storage of
skill. that might be especially true for the
dentate nuclei in the lateral parts of the cerebellar hemispheres.
But it would also likely engage circuitry that run through the basal ganglia and
certainly the premotor cortex, among other sites.
So this remain a active area of ongoing study.
The acquisition of procedural memory or the acquisition of utter skill and how
that changes the brain.
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