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.
Basic Structure of the Eye and Retina
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Medical Neuroscience
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Sensory Systems: The Visual System
This module will provide lessons that are designed to help you understand the basic mechanisms by which light is transduced into electrical signals that are then used to construct visual perceptions in the brain. Your studies of the visual system will benefit you at this point in the course, but also in later studies when we use the visual system as a model for understanding general principles of developmental plasticity. Lastly, it is worth noting how much of the forebrain contains elements of the visual pathways. Thus, injuries and disease in widespread regions of the brain may have a clinically important impact on visual function. All the more reason to learn these lessons well as you progress in Medical Neuroscience.
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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, we're getting ready to enter a phase
of medical neuroscience where we're going to be talking about the senses of sight,
sound, and smell. And there's no experience that brings all
that together than sitting around a camp fire, holding a brain.
So, I look forward to these next few sessions with you and if you have an
opportunity to be near a campfire over these next couple of days to weeks you
might want to have the experience I'm having right now where the sense of heat
and warmth and the sound of the crackle and the dance of the flame and the smell
of the burning wood all brings to mind these incredible feelings of, of warmth
and relaxation and comfort. Quite an amazing bit of neuroscience
behind all of that. So, I look forward to exploring some of
these topics with you and see you next time around the campfire.
Welcome again to my home. It's a beautiful day here in Central North
Carolina. And I'm pleased to begin a series of
sessions with you talking to you about the visual system.
And our focus today is on the eye. What I have to share with you today
pertains to several of our core concepts in the field of neuroscience.
We again are confronted with the complexity of the brain as the body's most
complex organ. And, we again are going to be looking at
the structure and function of circuitry that's largely determined by our genes.
And these circuits will be foundational for the operations of the visual system in
the human brain. And I can hardly think of any topics in
the whole domain of sensory neuroscience that has fascinated more human beings over
the course of our history than the sense of vision.
And so I think when we come to this topic, we certainly are in a position to
appreciate how a brain itself gives us this amazing capacity to wonder how vision
works. Our learning objectives today begin with
the factors in the mechanisms that account for focusing an image on the retina.
So, I want you to be able to describe them.
I want you to be able to identify the five basic neuronal types that are found in the
retina and I want you to be able to state the roles of each in neural processing.
I want you to be able to talk about sensory transduction in the visual system.
So, I want you to be able to characterize the molecular processes that underlie what
we call phototransduction. And lastly, I want you to be able to
discuss the responses of retinal ganglion cells to the onset and the offset of light
and the relevance of these receptive fields for the detection of light and
shadow. Well, let's begin by looking at the
anatomy of the eye. Well, I would just direct yo ur attention
to some basic anatomy here. So the, the eye is comprised of several
layers and two chambers and some structures that are important for forming
the sharply focused image on the back of the retina that are found within the eye
itself. So in the anterior side of the eye, we
have the cornea. The cornea is a structure that is
responsible for actually most of the refraction of light that happens as light
passes into the eye. So, roughly 80% of the refractive power of
the eye comes from the cornea, the remaining 20% comes as light passes
through these middle structures near the interior third of the eye including An
optical lens. And this lens can change shape through a
process that we call accommodation. We'll say more about that in just a
moment. Well, the space defined by this anterior
portion of the eye is called the anterior chamber.
And this anterior chamber is filled with a fluid called aqueous humor.
That aqueous humor is produced by some cells that sit right about in this region
here and that fluid then leaves what we call this posterior chamber between this
ciliary muscle and the lens itself. And that fluid then fills up this anterior
chamber and it helps to support the curvature of the cornea.
Now, that fluid is typically drained by a trabecular network found in this lateral
margin of this anterior chamber. Now, should there be a problem with the
absorption of this fluid from the anterior chamber, then that can build up pressure
within the eye. And that pressure has a way of damaging
the peripheral parts of the retina. And there's consequence in that eye one
might tend to lose peripheral vision. Well, let me try to illustrate the
consequences of this if I may, using a photo from my photo album.
This was a memorable trip my family took many years ago.
We're in one of the most beautiful sights in all of North America.
This is Lake Louise in Alberta, Canada. And if a person viewing my family in this
scene had glaucoma sadly they would not be able to truly appreciate the full majesty
of this scene because their visual field might very well be reduced to just the
center of their gaze. Well, let's now turn our attention to the
process of, of focusing the image on the back of the retina.
And this is a process that we call accommodation.
So, accommodation requires the active adjustment of the shape of the lens that
sits in the anterior portion of the eye. So here what we see is an illustration of
the lens in one of two conditions. In the unaccommodated vision the lens is
relatively flat. So, this would be appropriate for far
vision. And then over to the right, in the
accommodated configuration, notice how the lens is considerably fatter and more
round. So, this will increase the refractive
index of the lens, and this would be appropriate for near vision.
Well, how does this work? Well, this is where the active elements of
this ciliary body come into play. There is a muscle, the ciliary muscle,
that surrounds the lens. And this muscle is attached to the lens
through a series of fibrils called zonule fibers.
And the way this muscle works and the way it interacts with the lens is, is really
quite ingenious. As this muscle contracts, it makes the
aperture smaller that surrounds this lens. And as this muscle contracts, then the
tension on these zonule fibers can relax and consequently active contraction of the
ciliary muscle leads to a rounding up of the lens and an increase in its refractive
power. When that muscle relaxes, then it returns
to a wider aperture that causes tension on the Zonule fibers.
And as a result of that tension, then this lens ins pulled more towards a flattened
configuration. So, one other bio mechanical element here
that makes all this work, is the lens itself.
So, the lens will tend to ball up into a rounder configuration if it were to be cut
free of the zonule fiber. So, the elasticity of the lens itself is
what gives it the capacity to achieve this more rounded configuration, which is
appropriate for near vision. Now, adjustments to the shape of the lens
are not the whole story when it comes to accommodation.
There is also adjustments to the pupil itself.
So, the pupil is basically the space created by the iris.
So here you notice the, the iris. Is a muscular structure that creates the
space in the middle. And that is the dark spot that you see
when you look deeply into the eyes of another person.
Well, of course what we're looking at is darkness in the interior of the back of
the eye. The iris performs this very important
function of minimizing optical aberration. Now, the iris also serves to achieve that
optimal balance between the amount of light that's necessary to activate the
neural elements in the back of the eye, called the retina, and over activating
them all or bleaching them. So, we have an exquisite mechanism that
allows inputs from the retina to drive circuitry in the brain stem to constrict
that muscle and make moment to moment adjustments in the diameter of the pupil.
This is called the pupillary light reflex. And we'll spend a bit of time talking
about that circuitry in a different tutorial.
Well, I think we're ready now to consider those neural elements at the back of the
eye and we will spend much of the rest of the tutorial talking about those elements.
What I'm referring to, of course, is the retina.
Now, I want to make an important embryological point, as we begin to
consider the retina and that is that the retina is really considered to be truly
part of the brain. And the reason is that the retina is
diencephalic, which is to say the retina is derived from the developing forebrain,
specifically the diencephalon. So, as the diencephalon begins to
differentiate at the posterior part of the prosencephalon, something called the optic
vesicle forms. And this is an outward pouch that we see
from the lateral margins of the diencephalon.
And this pouch grows out to meet other tissues that together form the eye.
The retina and an important layer of tissue just behind it, called the
pigmented epithelium, are derived from this diencephalic tissue.
So one might wonder why then do we call the optic nerve the optic nerve, if it's
really an extension of the brain? Well, I think that's a legitimate
question. But the important point I want you to take
away here is that the retina provides us with a unique opportunity to look into the
eye of a patient and actually see tissue derived from the brain.
And that gives us an opportunity to examine that tissue as well as the
vascular supply to it and perhaps gain some insight as to the status of what's
going on deeper in the cranium. All right.
Well, let's look now at the retina itself. And as I mentioned, there are two kinds of
tissues here that are derived from this optic cup that constitute the retina.
Right at the back of the eye is a tissue called the pigmented epithelium.
This is a very dark tissue because of the presence of a pigment.
And I'll talk more about that particular layer of cells in just a moment.
The rest of this is what we call the neural retina and it's comprised of five
basic types of cells. And I want you to notice something about
the organization of those cells relative to the eye itself.
So, the back of the eye is out here somewhere.
Okay? So, the pigmented epithelium is against
the inner surface of the sclera, which is what we call the white part of the eyeball
that we see from the outside. And in order for light to activate the
cells that do the job of phototransduction, it has to pass through
this entire neural retina because the phototransduction process is out here,
right up against the pigmented epithelium. So, this is where phototransduction
happens. And that may see, seem somewhat surprising
that light would have to pass through this tissue before striking the photo pigments
that transduced that energy and generate an electrical signal.
But we think this is the way it is for a very important reason and it has to do
with the relationship between these photoreceptors and this pigmented
epithelium. So, if we look more closely at that
relationship, what we discover is that the outer tips of our photoreceptors make
intimate contact with this pigmented epithelium.
And that's critical for the function of the pigmented epithelium in the whole
process of keeping these photoreceptors competent to transduce photon energy into
neural signals. The pigmented epithelium is involved in
recycling the discs of membrane that contain the photopigments.
And we know that from experiments that allow us to trace the movement of discs
from a more proximal position to a more distal position.
And what we find is, that eventually, the discs are completely engulfed by this
pigmented epithelium. And that's important in recycling the
photopigment that's otherwise bleached by the interaction with photons.
So, this raises a rather serious problem that can happen with trauma.
And that is what can happen when the photoreceptors are detached from this
pigmented epithelium. So, this can happen with a sharp blow to
the head. It's a particular problem and people who
are in sports that are subject to a blow to the head and it's certainly something
that can happen with a motor vehicle accident or some other trauma to the head.
So, retinal detachment prevents this intimate relationship between pigmented
epithelium and photo receptor. And as a result, the photoreceptor no
longer has a chance to renew the photopigment and recycle its disk
membrane. So now, lets consider the rest of the
cells that comprise the neural retina.
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