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. 

From the lesson

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. 

Meet the Instructors

Leonard E. White, Ph.D.

Associate ProfessorDepartment 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

Welcome back.  In this tutorial, I'd like to talk to you

briefly about the circuitry that's responsible for the pupillary light

reflex.  again, our journey into the visual system

is an attempt to make it a bit more simple, some of the most complex pathways

and function of the brain itself.  We will once again consider circuitry,

that is genetically determined and provides a foundation for one very useful

aspect of nervous system function.  One that is exploited from a clinical

point of view to assess the integrity of cranial nerves and brain stem circuits,

that is the pupillary light reflex.  So my learning objectives for you, are to

describe the distribution of retinal axons, from ganglion cells to their major

processing centers in the forebrain in the brainstem. 

And we have some particular brain stem targets in mind in this tutorial. 

And I want you to be able to use that knowledge to discuss the neuroanatomical

basic for the pupillary light reflex.  So let's consider again, the distribution

of the axons of our retinal gangling cells. 

Those that reside on the temporal side of the retina, grow their axons out through

the optic nerve, and they remain on the ipsilateral side of optic chiasm, being

distributed to the appropriate targets in the diencephalon and to the mid-brain. 

those axons that arise from nasal retinal ganglion cells, that sit here in this

location, close to the nose on what we call the nasal side of the retina. 

They grow their axons through the optic nerve. 

And when they get into the optic chiasm, now these axons cross the midline. 

And they give rise to projections to a variety of targets in the diencephalon

and in the midbrain.  including the hypothalamus, and the

lateral geniculate nucleus of the diencephalon. 

And the superior colliculus and the pretectum and the midbrain. 

And for this discussion today, the projection of interest is this projection

to the pretectum, that I would highlight here from the retina into a set of nuclei

that sit in a transitional region between the diencephalon and the midbrain. 

And in that region of the dorsal part of the midbrain, we have the superior and

inferior colliculi.  That, collectively, is called the tectum. 

Tectum means roof.  It's the roof over the cerebral aqueduct. 

So the region just in front of the roof is the area of our focus in this session. 

That's what we call the pretectum.  That's where we find a collection of

nuclei that receive direct input from the retina. 

And it's those cells that will coordinate the distribution of that sensory

information to the motor structures that result in the constriction of the pupils. 

So let's look now at a view just specifically of this reflex circuitry. 

So now what we see are the sensory and the motor limbs of this reflex arc. 

And let's first consider the sensory limb. 

Now the retinas are projecting into the pretectum. 

And notice that the pretectum in one side of the mid line is getting input derived

from both retinas.  So the temporal retina of one eye, the

nasal retina of the other, are both contributing input into that pretectum. 

So that's important.  so you need to understand that the

sensory signals going into one side of that circuitry are derived from both

eyes.  Now, the motor limb of this reflex

circuitry arises out of the pretectum.  Now notice another very important fact

about this circuitry here, that pertains to sidedness. 

The projections from these pretectum nuclei to the motor nuclei in question

are bilateral.  So here is our pretectal nucleus and it

gives rise to axons that are directed to both sides of the midline. 

And specifically the target there is a small parasympathetic preganglionic

nucleus called the Edinger-Westphal nucleus. 

It sits just dorsal and slightly lateral to our somatic motor nucleus or ocular

motor nucleus and it's in this Edinger-Westphal nucleus that we have the

preganglionic neurons that grow axons out through the third cranial nerve. 

So, here in green is a Edinger-Westphal nucleus, and it's growing an axon out

through the third nerve, which makes a synapse on a post-ganglionic neuron, in

the ciliary ganglion.  And then that neuron grows its axons,

into the eye itself, to supply ganglionic parasympathetic innervation of the

constrictor muscle of the iris.  So, when this motor signal is conveyed

down that third nerve, the iris will constrict. 

Now, notice one other important fact that's worth emphasizing about the

organization of the motor limb of this reflex. 

Once the signals leave the Edinger Westphal nucleus, they remain on the same

side.  That is, they remain ipsilateral. 

So there is bilateral distribution of sensory signals into this circuitry. 

And then from the pretectum to the motor output there is once again bilateral

distribution of information.  But once we get to the parasympathetic

motor nucleus the information is unilateral. 

Specifically it remains ipsilateral.  Okay, now, I'm going to want you to think

through systematically all the various perturbations of the pupillary light

reflex.  Imagine shining a light in one eye, and

looking for constriction of the pupil.  Shining a light in the opposite eye and

doing the same.  And imagine what you might see in a

variety of patients.  There will be hopefully, all of your

patients, but we know sadly that won't be the case. 

Hopefully all of your patients will show symmetrical, bilateral constriction of

the pupils.  And this bilateral constriction is an

indication that the signals are being processed in the pretectum, distributed

bilaterally to the motor nuclei.  And these signals are being sent along

the efferent pathway to the constrictor muscles of the iris, leading to the

symmetrical constriction of the pupils.  Now if any of that is not observed, then

you very well may have an injury to the eye, to the optic nerves, to the optic

tract, to the circuitry of the pretectum, to the region of the Edinger-Westphal

nucleus in the nerve roots of the third nerve. 

Or the path taken by those preganglionic or postganglionic axons out to the

constrictor muscles of the iris.  And it's your job as a clinician to

reason through this circuitry and make a judgement as to whether you think there

is a localized lesion.  Now, our focus here is on the

constriction of the pupil, but let me just say in passing, that there's also a

dilation of the pupil that's under the governance of the sympathetic nervous