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Neurobiology II: Development and function of the nervous system W3005y
Spring 2002
Lecture 1:
Systems
1/21/02
Darcy B. Kelley
An Introduction to Nervous
Introduction
This is a course about the brain: how it develops and how it works. Our
study of the brain assumes some knowledge of how individual neurons work.
Some examples of background information include:
How does a neuron receive information?
What determines whether a neuron produces an action potential (and what is
an action potential)?
How does synaptic transmission work?
For the most part, however, we will focus not on the individual neuron but on
systems of neurons: groups of synaptically connected neurons that perform
specific functions. Most of the examples we will study come from vertebrate
nervous systems so we begin this course by examining one such system to see
how it is put together.
Slide 1 All brains, even those of identical twins, are different. This, for
example, is roughly what your brain looks like and is seen from the side.
Label: a. Anterior (front), posterior (back) b. dorsal (top), ventral c)
Cerebral cortex, cerebellum, medulla.
Slide 2 Brains of different species look different in characteristic ways that
reflect their way of life. Here is a picture of a human brain, a fish brain and a frog
brain (not to scale). Though each looks superficially very different, all are formed
from the same basic plan. In humans the cerebral cortex is hypertrophied and
growns over the entire midbrain. In some fish (electric fish for example), the
cerebellum is massively developed in connection with electrolocation and
electrocommunication. In this aquatic frog, the caudal hindbrain is hypertrophied
and represents the acoustic and lateral line system (sound and water surface
waves)..
What system of the CNS might you expect to be hypertrophied in echolocating
bats?
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Slide 3 As for real estate, a key principle of brain functions is: location, location
and location. We can pinpoint the location of brain structures or even of
individual neurons using a co-ordinate system: anterior (rostral) vs. posterior
(caudal); dorsal (top) versus ventral (bottom); medial versus lateral. Because
brains are bilaterally symmetrical, we must also specify right or left side (the
individual’s right or left).
What difference in orientation between dogs and men account for
differences in names of brain directions?
Slide 4 In man, the neuraxis (the anterior/posterior axis of the nervous
system) rotates at the border between the midbrain and the forebrain. We
distinguish a special orientation for human brains that reflects this rotation: the
coronal plane (roughly parallel to the face).
The brain is made up mostly of two kinds of cells: neurons and glia.
Slide 5 Neurons come in a large variety of shapes (morphological
classification). Specific kinds of neurons have characteristic cell body shapes,
dendritic trees and axonal arbors.
Identify a Purkinje cell, a pyramidal neuron. Injdicate the cell body, the
dendritic tree. What are their identifying features?. Suppose you rotated each
cell by 90 degrees, what would each look like?
Slide 6 Glia are more numerous than neurons. They serve a variety of
functions. Durung development, radial glia guide migrating neurons and their
processes (axons and dendrites) to more distant destinations. After injury,
reactive astrocytes engulf dead and dying neurons and remove debris. Glia also
form the fatty sheath (myelin) that wraps around the axon and speed up the
conduction of action potentials: in the brain and spinal cord (central nervous
system) these myelinating glia are called oligodendroglia. Outside of the CNS,
Schwann cells myleinate axons.
EXPERIMENTAL METHODS: How do we know what kinds of cells the CNS
contains?
Stains of sectioned materials
Almost all cells in the vertebrate nervous system are too small to be seen with
the naked eye. To examine neurons we have to first cut the CNS into thin slices
(this allows sufficient illumination) and look at the slices with a microscope (a
small neuronal c
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microscope (shorter wavelengths resolve smaller structures) or specialized
microscropic methods such as confocal microscopy or DIC (differential
interference contrast).
Slide 7 Once the brain is sectioned, we need to keep track of where in the brain
each section comes from (so that, for example, we can reassemble a three
dimensional structure from two dimensional sections) and also we need to know
how the section was cut (plane of section).
Slide 8 One popular plane of section is transverse (coronal for the human
forebrain). This plane of section is perpendicular to the neuraxis.
Slide 9 Transverse sections can come from any anterior posterior position in the
neuraxis. To reconstruct a brain structure you need to keep track of the
rostro/caudal relations of the sections.
Slide 10 The other way to recognize A/P levels from a single section is by
landmarks.
Slide 11 In the transverse plane of section, what directions are preserved; what
directions are lost?
Slide 12 Another useful plane of section is horizontal. The horizontal plane is
parallel to the neuraxis.
Slide 13 What directional information is preserved in the horizontal plane? What
is lost?
Slide 14 The final plane of section is saggital. Parallel or perpendicular to the
neuraxis?
Slide 15 A saggital section that is right at the midline (the plane of bilateral
symmetry) is called midsagittal. What directional information is preserved in the
sagittal plane? What is lost?
Slide 16 Identify the planes of section of these bread slices,
Nissl stains, myelin stains, Golgi
Once you have cut the section and are examining it under the microscope, you
will need to distinguish the cells or the fiber tracts. These can sometimes be
picked out by differences in refraction (Nomarski or differential interference
contrast microscopy), but these may only work at high magnifications (where it
may be difficult to tell exactly where in the section you are). A method that works
at both low and high magnifications is to stain the section using a dye. Some
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dyes have strong affinities for components of the cell body such as the Nissl
substance or cytoplasmic RNA (Nissl stains include cresyl violet and neutral red).
Slide 17 This is a transverse section through a song bird forebrain that has been
stained with cresyl violet. Each individual purple dot is a cell. Some groups of
cells cluster together and stain similarly. These clusters are called brain nuclei.
The appearance of brain nuclei after staining enables anatomists to use
cytoarchitectonic criteria to characterize different regions of the brain. Because,
most often, form reflects function (and vice versa) these staining characteristics
can be used reliably to identify functional brain regions (visual cortex for
example, as we will see).
Slide 18 What can stained sections tell us? These are sections through this
same part of the brain male and female song birds. The brain nucleus illustrated
is called area X. A and B illustrate area X in male and female canaries,
respectively. C. and D. illustrate area X in male and female zebra finches,
respectively. What hypotheses are suggested by these stained brain sections?
(Figure from Nottebohm and Arnold, 1976, Science).
Slide 19 Stains can also be used to visualize fiber tracts (collections of axons) or
individual axons (myleinated or not) in brain sections or in the periphery. This is
a section through a frog muscle illustrating a fiber bundle, individual myelinated
axons and neuromuscular synapses. Label each component.
Slide 21 The dyes described above allow us to visualize cell bodies and axons.
Except for the very largest neurons (motor neurons for example) dendrites are
not stained at all. In the late 1800’s, a Spanish neurologist, Santiago Ramon y
Cajal, began to apply a method of impregnation with mlietal salts developed by
Camilio Golgi from Italy, to developing and adult brains from a variety of species.
Slide 22 For reasons that are still not very well understood, the Golgi stain picks
out just a few cells but stains them in their entirety. This is a transverse section
through a frog forebrain in which a few cells and their dendrites (point to these
with arrows) have been impregnated with the rapid Golgi method.
Slide 23 Golgi stains can reveal an individual neuron in all its glory: dendrites,
cell body and axon. This feature relies on only having a few cells stain; if all cells
stained the section would be an impenetrable black field. The Golgi stain can
also reveal details of cellular morphology such as the presence of dendritic
spines (small protruberances). Do any of these cells from the frog auditory
midbrain have dendritic spines?
We can use the Golgi method to stain glia as well as neurons. For
neurons, we can describe cells according to their morphology. Some cell type
names come from shapes: basket cells, candelabra cells, pyramidal cells. We
can use other methods to characterize cells by their axonal projections. These
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methods involve the transport of substances from the cell body to the axon
terminals (anterograde transport) or from the terminals to the cell body
(retrograde transport). One of the molecular motors involved in axonal transport
is the topic of the first recitation section Using this approach we find that there
are three kinds of neurons: primary sensory neurons, motor neurons and
interneurons. The central nervous system (CNS) is almost entirely interneurons.
Slide 24 To understand neuronal function we need to know more than just the
shape of the neuron. For example: What neurotransmitters or modulators are
made? What receptors are expressed? To look at which proteins are expressed
in a neuron the most common method is immunocytochemistry. An antibody
specific to the protein of interest is generated in an animal of one species (a
rabbit for example), applied to the section and recognized by another antibody
from a second species generated against all antibodies from the first species (a
goat anti-rabbit immunoglobulin, for example). This secondary antibody is
tagged with a molecule that can be visualized (HRP or biotin are examples;
fluorescent tags can also be used). This slide shows frog vocal motor neurons
that express the clacium binding protein calbindin reveled using
immunocytochemistry.
Slide 25 This is a Golgi impregnation of a cell in the ventral horn of the cat
spinal cord . We can identify this cell in a variety of easy: its neurotransmitters,
its receptors etc. For understanding neural systems, however, we usually want
to know what other neurons it connects with synaptically. Inputs to the cell are
called afferents and targets of this cell are called efferents. There are three kinds
of neurons with regard to the central nervous system: motor neurons, primary
sensory neurons and interneurons.
Slide 26 We can identify this cell type as a motor neuron by retrograde transport
of the plant enzyme (horseradish peroxidase) from its synaptic terminals in
muscle fibers (slide 19). The cell bodies of motor neurons are located INSIDE
the central nervous system but their synaptic targets, the muscle fibers, are
located OUTIDE the nervous system.
Slide 27 This is a Golgi impregnation of an olfactory neuron, an example of a
primary sensory neuron. Primary sensory neurons convey information into the
central nervous system. Their cell bodies lie OUTSIDE the brain and spinal cord.
Olfactory neurons reside in the nose (the olfactory epithelium). Their dendrites
extend into the mucosal lining of the nose and their axons travel into the CNS to
synapse on neurons in the main olfactory bulb at the front of the brain.
Slide 28 Every other cell in the CNS is an interneuron. For interneurons, both
the cell body and all processes are inside the CNS. The pyramidal cell is an
example of an interneuron as are basket cells, candelabra cells etc.
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Slide 29 CNS interneurons are organized into systems by virtue of specific
patterns of connectivity.
Slide 30 Today, some of the most popular tracers are fluorescent dextran
amines, many of which travel both in the anterograde and the retrograde
direction. This is a frog brain split in half in the midsaggital plane. We can place
a small injection of a fluorescent dextran amine into a specific brain region and
then maintain the isolated brain for a few days while anterograde and retrograde
transport is carried out.
Slide 31 In the cleared half brain we can follow individual axons to their
terminals.
Slide 32 We can also discover the sources of input to the region in question.
Here cells in the anterior thalamus are shown after retrograde labeling from the
anterior preoptic area.
Slide 33 We can use anterograde and retrograde transport to define the
anatomical connections of interneurons. For example, here the dye Lucifer
yellow has been used to identify vocal motor neurons in the hindbrain of a frog.
Slide 34 Here we can see synaptic terminals full of a red fluorescent dye ending
on the cell body of a Lucifer-yellow filled motor neuron.
Slide 35 The holy grail for defining synaptic networks in the CNS is to find
tracers that travel across several synapses. For example, if radioactive amino
acids are placed into the eye, they are incorporated into proteins and transported
to the terminals of retinal ganglion cells in the lateral geniculate nucleus. There
the radioactive proteins are broken down again into radioactive amino acids,
these are released and taken up by LGN neurons which transport them to their
main target, primary visual cortex.
Slide 36 If only one eye is injected we can see alternating bands of synaptic
inputs, the ocular dominance columns.
Slide 37 Some label can also travel across the synapse in a retrograde direction.
Here is an example of transfer of biocytin from the post synaptic motor neuron to
its afferent interneuron.
After the geniculate to cortex synapse there is not enough tracer left travel and
be visualized at the next synapse. Current efforts at tracing circuits involve
modified neurotrophic viruses that multiply inside the cell and so are not diluted.
For small pieces of tissue we can use stimulation and calcium sensitive dyes to
look at local circuits. Of course, we can also stimulate one cell and record from
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its synaptic target to define connectivity. If we record intracellularly from the
post-synaptic neurons we can determine whether the synapse is excitatory or
inhibitory. First, however, we have to find the presynaptic inputs and it is in the
regard that tracing is most valuable.
Slide 38 To organize our thinking about brain connectivity we first have to know
what part of the brain is under discussion. A major principle here is location.
The location in which a developing cell finds itself determines its identity through
a series of complex molecular signaling events that we will discuss. The end
result if a very stereotyped set of brain regions.
Slide 39 Figuring out where you are in the brain: coordinates and landmarks.
Slide 40 The most posterior portion of the CNS is the spinal cord.
Slide 41 Organizaton of the spinal cord: sensory information enters dorsally
(from the ______ ____ _______), motor neurons are located ventrally;
everything else is interneurons and fiber tracts made up of axon bundles).
Information enters and exits the spinal cord via the spinal nerves.
Slide 42 Motor neurons are located in the ventral horn of the spinal cord.
Because humans stand on their hind legs, the ventral (bellywards) portion of the
spinal cord faces front so ventral horn cells are called anterior horn cells.
Slide 43. The medulla is located immediately in front of the spinal cord. Like the
spinal cord, sensory information comes in through dorsal nerves and motor
outflow exits through ventral nerves (from the medulla forward these nerves are
called cranial nerves).
Slide 44
Cranial nerves
I
VII
II
VIII
III
IX
IV
X
V
XI
VI
XII
Sensory information is processed serially and in parallel.
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Parallel processing: Sensory information is distributed to different regions of the
CNS when it enters; each region can perform a separate processing operation
whose results can then be combined. Parallel processing has the advantage of
speed.
Serial processing: Sensory information can be processed by a series of brain
nuclei to extract specific features of a sensory stimulus. Serial processing has the
advantage of feature enhancement or detection.
Slide 45 In the hindbrain or rhombencephalon (which includes the medulla) each
cranial nerve originates from a spoecific segment called a rhombomere.
Rhombomeres are established during development by a cascade of signalling
molecules that translate location into cellular identity.
Slide 46 The hindbrain: cranial nerves and cerebellum.
• the cerebellum, a "little brain" that works together with cortex to produce
movement and process sensory information
•various fiber (axon bundles) tracts ascending or decending to various levels of
the neuraxis
• groups of interneurons receiving sensory information and
• other groups that generate certain motor programs such as breathing.
Slide 47 The midbrain: tectum and tegmentum
• The midbrain consists of a roof (tectum) and a floor (tegmentum).
• The tectum has two major sensory processing nuclei that look, from the outside,
like little hills (colliculi). The superior (anterior) colliculus processes visual;
information and the inferior (posterior) colliculus processes auditory information.
• The tegmentum consists of various fiber tracts and some interneurons involved
in generating motor patterns.
Slide 48 The forebrain
•The hypothalamus is located in the ventral diencephalon; many of its nuclei
control the pituitary (aka "master gland").
• The thalamus is located in the dorsal diencephalon; cells in the thalamus
project to cortex and receive projections from cortex; the lateral geniculate
nucleus provides visual information to cortex and the medial geniculate nucleus
provides auditory information to cortex.
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• The basal ganglia have reciprocal connections with thalamus and motor cortex
and play important roles in motor control (Parkinson’s and Huntington’s).
• The cortex is an elaborate, crumpled, thick sheet of cells that has reciprocal
connections with thalamus and brain stem. The cortex plays an essential role in
preceiving, thinking and the planning and execution of movements.
Slide 49 The Cerebral Cortex
• In humans and primates, the cerebral flexure bends the neuraxis at the level of
the forebrain and we distinguish a new plane of section, perpendicular still to the
neuraxis, the coronal plane.
• The cortex is divided into a number of functionally and cytoarchitectonically
distinct regions.
•The main subdivisions are: frontal, parietal, occipital and temporal.
• The crumpled surface of the cortex can be viewed as hills (gyri) and valleys
(sulci). Several large sulci divide up the main subdivisions.
• The spatial representation of information is maintained in the cortex as "maps";
maps representing parts of the body are "somatotopic", visual maps are
"visuotopic" etc. Motor maps are also somatotopic. A given region of cortex can
contain multiple maps.
• In general, the larger a region of cortex devoted to a particular job (body part,
region of visual space etc.) the smaller the receptive fields of neurons in that
region.
Glossary
Ganglia - collections of nerve cell bodies. The leech nervous system is a chain of ganglia. Many
primary sensory neurons are located in the dorsal root ganglia, collections of nerve cell bodies in
a chain that runs parallel to the spinal cord.
Central nervous system - the brain and spinal cord.
Primary sensory neuron- a neurons whose cell body is located outside the CNS. The neuron itself
may transduce the sensory signal (eg. pain) or it may receive signals from a transducing cell (eg.
sound via the hair cell).
Motor neuron - a neuron whose cell body is located inside the CNS but whose axon leaves to
innervate muscle.
Interneuron- every other cell in the CNS.
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Rhombomeres- segments of the hindbrain which can be seen at embryonic stages and
encompass characteristic domains of gene expression.
Cerebellum- structure at the anterior edge of the hindbrain; in mammals attached via the
cerebellar peduncles (fiber tracts), has several complete sensory and motor maps of the brain, is
critical for motor memory (riding a bicycle) and is made up of repeated sub-structures, folia,
containing characteristic cell types with a stereotyped synaptic connections.
Sulcus- the bottom of a cerebral fold; eg. the Sylvian fissure.
Gyrus- the top of a cerebral fold.
Receptive field- the sensory field which drives changes in activity of the neuron you are recording
from.
Nuclei- a technical term in the CNS meaning a collection of cell bodies, usually subserving the
same or a related function, and distinguishable cytoarchitectonically from nearby regions.
Cytoarchitectonics- a hideous term, near and dear to the hearts of anatomists, having to do with
the ability to distinguish brain regions by their staining characteritics or the packing density of
neurons or fiber tracts. The best example is the stripe of Gennari (sp?), a feature that represents
incoming afferents from the thalamus and distinguishes striate or striped cortex (aka primary
visual cortex) from adjacent regions.
Huntington’s and Parkinson’s diseases- human diseases involving the degeneration of cells
projecting to, or found within, the basal ganglia. Huntington’s is an autosomal dominant, late
onset disease one of whose most famous victims was Woody Guthrie. Parkinson’s disease is due
to the death of dopaminergic neurons of the substantia nigra; treated with L-dopa.
Neuraxis- the plane parallel to the neural tube.
Study questions;
1) Define and give an example of serial and of parallel processing in a sensory
system(s).
2) Each of the planes of section preserves two pieces of coordinate information
and is missing one. What is present and what is absent from a transverse, an
horizontal and a saggital sction?
3) Define afferent and efferent and illustrate the uses of these terms in a
transverse section of the spinal cord.
4) Enumerate the cranial nerves; which are sensory, which are motor and which
are mixed?
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5) Where in the CNS are purkinje cells found?
6) In Latin, tectum means ____ and tegmentum means _____. Which part would
you expect to find sensory afferents in; which part would you expect to find motor
neurons in? Why?
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