Transcript 463T022014

Biology 463 - Neurobiology
Topic 2
Neurons and Glia
Lange
Camillo Gogi & Santiago Raymon y Cajal… research rivals that
shared the 1906 Nobel Prize, but disagreed vehemently on the
organization of the microscopic level of the nervous system.
Introduction
• Cajal and Golgi were in disagreement about their ideas on how they
the nervous system operated…. be it continuous (think of the Fluid
Dynamic Model for example) or in a chain of individual cells.
• The Neuron Doctrine says that the neural network consisted of cells
that interacted together by contact. This was the major idea of
Cajal.
• Glia and Neurons
– Glia: Insulates, supports, and nourishes neurons
– Neurons
• Process information
• Sense environmental changes
• Communicate changes to other neurons
• Command body response
Obstacles to the advancement of our understanding of
building blocks of the nervous system:
• Small size requiring compound microscopy
• To examine brain TISSUE, with compound microscope,
the tissue had to be extraordinarily thin to allow light to
pass through.
• The consistency of brain, spinal cord, and other nervous
system tissue was much less firm than most other
tissues. It is often equated with having a consistency of
“Jello”.
“Brain” Jello
Yield: 11 Servings
Ingredients:
2 (6 ounce) boxes gelatin, mix any flavor (peach or
watermelon give the best color)
1 3/4 cups boiling water
3/4 cup cold water
9 ounces fat-free evaporated milk (must be fat-free or it
will curdle)
To add a greyish cast, add this additional food coloring:
15 drops red food coloring
15 drops green food coloring
15 drops blue food coloring
1.
2.
3.
4.
5.
6.
Spray or the inside of the brain mold with a small
amount of non-stick spray, then wipe out the
excess.
Put the gelatin mix in a large bowl and add the
boiling water.
Stir about two minutes until the mix is dissolved.
Stir in the cold water.
Stir in the evaporated milk and food coloring.
Pour the mixture in the brain mold, stopping about
1/4 inch from the top
In fact, the stereotypical “Brain Jello” you can often find
people making around Halloween, when used in a brain mold
is VERY realistic in texture and shape.
To overcome the issues of brain tissue texture,
specialized techniques in histology were
developed.
The microtome (“wax embedded” on the left and “freezing”
on the right) allows reserachers to slice soft tissues in very
thin sections (microns)
Yet, another major problem for studying
the nervous system tissues involved its
lack of contrast.
The first advances to overcome this obstacle
occurred in the late 1800s with the
development and identification of a series of
differentially staining dyes.
Franz Nissl – (1860 – 1919) founding father of the study of
neuropathology . “Nissl” stain is named after him and was his own creation.
Uses for Nissl Stain:
This method is used for the detection of Nissl body in the cytoplasm of neurons on
paraformaldehyde or formalin-fixed, paraffin embedded tissue sections. The Nissl body will
be stained purple-blue. This stain is commonly used for identifying the basic neuronal
structure in brain and spinal cord tissue.
A Nissl body is a large granular body found in neurons. These granules are rough
endoplasmic reticulum (with free ribosomes) and are the site of protein synthesis.
This is a coronal
section of a rat
brain with a
tissue thickness
of 10 microns.
Camillo Golgi
Developed a
specialized stain,
now named in
his honor.
Uses for Golgi Stain:
This method is used for the detection of a number of novel facts about the organization of
the nervous system, inspiring formation of the neuron doctrine:
The nervous system is made up of discrete individual cells.
•Golgi's method stains a limited number of cells at random in their entirety.
•Dendrites, as well as the cell soma, are clearly stained in brown or black and can be
followed in their entire length.
•This allowed neuroanatomists to track connections between neurons and to visualize the
complex networking structure of the brain and spinal cord.
Golgi's staining is achieved by impregnating fixed nervous system tissues with potassium
dichromate and silver nitrate.
This is a section
of a rat brain
showing the
Hippocampus
impregnated by
the Golgi stain.
• The soma or perikaryon is the more “bulbous” end of a neuron,
containing the cell nucleus.
• The soma of a neuron is often called the "cell body".
• A neurite is a generalized term that refers to any projection from
the cell body of a neuron.
Again, with Golgi staining, the entirety of a neuron could be visualized.
Cajal’s work
uncovered some of
the tremendous
complexity of the
range of cell types
seen in the brain.
Cajal differed from Golgi in adamantly suggesting that:
Neurons communicate by contact and are not continuous.
– Cajal discerned the cellular nature using Golgi’s staining method.
– However, even with the best and most careful use of the Golgi
Staining technique, it was still not definitive which theory (Cajal’s or
Golgi’s) was most accurate.
– In the 1950s, the electron microscope would definitively provided
definitive evidence for Cajal’s neuronal doctrine.
A stereotypical
representation of the
axon projecting from a
neuron. The axon will
function like a
“telegraph” wire or
“electrical wire” from
the soma carrying
impulses.
Structure of the plasma membrane according to the fluid mosaic model.
Extracellular fluid
(watery environment)
Polar head of
phospholipid
molecule
Cholesterol
Glycolipid
Glycoprotein
Carbohydrate
of glycocalyx
Outwardfacing
layer of
phospholipids
Integral
proteins
Filament of
cytoskeleton
Peripheral
Bimolecular
Inward-facing
proteins
lipid layer
layer of
containing
phospholipids
Nonpolar
proteins
tail of
phospholipid
Cytoplasm
molecule
(watery environment)
Membrane proteins perform many tasks for general cell growth, development
and survival…. but many of these tasks are also essential for neural function.
(a) Transport
A protein (left) that spans the membrane
may provide a hydrophilic channel across
the membrane that is selective for a
particular solute. Some transport proteins
(right) hydrolyze ATP as an energy source
to actively pump substances across the
membrane.
Signal Transduction
Signal
Receptor
(b) Receptors for signal transduction
A membrane protein exposed to the
outside of the cell may have a binding
site with a specific shape that fits the
shape of a chemical messenger, such
as a hormone. The external signal may
cause a change in shape in the protein
that initiates a chain of chemical
reactions in the cell.
Please note that we shall extensively
examine PLB proteins used in this
context throughout the semester.
Attachment via PLB Proteins
(c) Attachment to the cytoskeleton
and extracellular matrix (ECM)
Elements of the cytoskeleton (cell’s
internal supports) and the extracellular
matrix (fibers and other substances
outside the cell) may be anchored to
membrane proteins, which help maintain
cell shape and fix the location of certain
membrane proteins. Others play a role in
cell movement or bind adjacent cells
together.
Emzymatic Activity
(d) Enzymatic activity
Enzymes
A protein built into the membrane may
be an enzyme with its active site
exposed to substances in the adjacent
solution. In some cases, several
enzymes in a membrane act as a team
that catalyzes sequential steps of a
metabolic pathway as indicated (left to
right) here.
Intercellular joining
(e) Intercellular joining
Membrane proteins of adjacent cells
may be hooked together in various
kinds of intercellular junctions. Some
membrane proteins (CAMs) of this
group provide temporary binding sites
that guide cell migration and other
cell-to-cell interactions.
CAMs
Cell-to-Cell Recognition
(f) Cell-cell recognition
Some glycoproteins (proteins bonded
to short chains of sugars) serve as
identification tags that are specifically
recognized by other cells.
Glycoprotein
Diffusion through the plasma membrane.
Extracellular fluid
Lipidsoluble
solutes
Lipid-insoluble
solutes (such as
sugars or amino
acids)
Small lipidinsoluble
solutes
Water
molecules
Lipid
billayer
Aquaporin
Cytoplasm
(a) Simple diffusion of
fat-soluble molecules
directly through the
phospholipid bilayer
(b) Carrier-mediated facilitated
diffusion via a protein carrier
specific for one chemical; binding of
substrate causes shape change in
transport protein
(c) Channel-mediated
facilitated diffusion
through a channel
protein; mostly ions
selected on basis of
size and charge
(d) Osmosis, diffusion
of a solvent such as
water through a
specific channel protein
(aquaporin) or through
the lipid bilayer
Primary Active Transport: The Na+-K+ Pump
Extracellular fluid
Na+
Na+–K+ pump
Na+ bound
K+
ATP-binding site
Cytoplasm
1 Cytoplasmic Na+ binds to pump protein.
P
ATP
K+ released
ADP
2 Binding of Na+ promotes
phosphorylation of the protein by ATP.
6 K+ is released from the pump protein
and Na+ sites are ready to bind Na+ again.
The cycle repeats.
Na+ released
K+ bound
P
Pi
K+
5 K+ binding triggers release of the
phosphate. Pump protein returns to its
original conformation.
3 Phosphorylation causes the protein to
change shape, expelling Na+ to the outside.
P
4 Extracellular K+ binds to pump protein.
The use of the sodium –
potassium pump PLB
proteins will be shown to
be crucial to the
transmittance of the
“electrical” signal that is
the “message” carried
within a neuron.
Overview of Endocytosis & Exocytosis
Extracellular
fluid
Cytoplasm
Clathrincoated
pit
Ingested
substance
Exocytosis
of vesicle
contents
Clathrin
protein
Cytoplasm
Bacterium
or other
particle
Clathrin
protein
Pseudopod
(b) Phagocytosis
Endosome
Uncoated
vesicle
3
Transcytosis
2
To lysosome
Uncoating
for digestion
Uncoated and release
vesicle
of contents
fusing with
endosome
(a) Clathrin-mediated endocytosis
Copyright © 2010 Pearson Education, Inc.
Extracellular
fluid
Recycling of
membrane and
receptors (if present)
to plasma membrane
1
Detachment
of clathrincoated
vesicle
ClathrinPlasma
coated
membrane vesicle
Extracellular
fluid
Plasma
membrane
Membrane
receptor
Clathrin
protein
(c) Receptor-mediated endocytosis
Comparison of three types of endocytosis.
(a) Phagocytosis
The cell engulfs a large
particle by forming projecting pseudopods (“false
feet”) around it and enclosing it within a membrane
sac called a phagosome.
The phagosome is
combined with a lysosome.
Undigested contents remain
in the vesicle (now called a
residual body) or are ejected
by exocytosis. Vesicle may
or may not be proteincoated but has receptors
capable of binding to
microorganisms or solid
particles.
Phagosome
Comparison of three types of endocytosis.
(b) Pinocytosis
The cell “gulps” drops of
extracellular fluid containing
solutes into tiny vesicles. No
receptors are used, so the
process is nonspecific. Most
vesicles are protein-coated.
Vesicle
Comparison of three types of endocytosis.
Vesicle
Receptor recycled
to plasma membrane
(c) Receptor-mediated
endocytosis
Extracellular substances
bind to specific receptor
proteins in regions of coated
pits, enabling the cell to
ingest and concentrate
specific substances
(ligands) in protein-coated
vesicles. Ligands may
simply be released inside
the cell, or combined with a
lysosome to digest contents.
Receptors are recycled to
the plasma membrane in
vesicles.
Exocytosis.
Plasma membrane (a) The process
Extracellular SNARE (t-SNARE)
of exocytosis
fluid
Secretory
vesicle
Vesicle
SNARE
(v-SNARE)
Molecule to
be secreted
1 The membrane-
bound vesicle
migrates to the
plasma membrane.
Cytoplasm
2 There, proteins
at the vesicle
surface (v-SNAREs)
Fused
v- and bind with t-SNAREs
t-SNAREs (plasma membrane
proteins).
Fusion pore formed
3 The vesicle
and plasma
membrane fuse
and a pore
opens up.
4 Vesicle
contents are
released to the
cell exterior.
Exocytosis.
The endomembrane system.
Nucleus
Nuclear envelope
Smooth ER
Rough ER
Plasma
membrane
Lysosome
Golgi
apparatus
Transport
vesicle
The nucleus.
Nuclear pores
Nuclear envelope
Chromatin (condensed)
Nucleolus
Cisternae of rough ER
(a)
Nucleus
The endoplasmic reticulum.
Smooth ER
Nuclear
envelope
Rough ER
Ribosomes
(a) Diagrammatic view of smooth
and rough ER
(b) Electron micrograph of
smooth and rough ER
(10,000x)
Golgi apparatus.
Transport vesicle
from rough ER
New vesicles forming
Cis face—
“receiving” side of
Golgi apparatus
Cisternae
Secretory
vesicle
New vesicles
forming
Transport
vesicle
from
trans face
Trans face—
“shipping” side of
Golgi apparatus
Golgi apparatus
Transport vesicle
from the Golgi
apparatus
(b) Electron micrograph of the Golgi
(a) Many vesicles in the process of pinching
apparatus (90,000x)
off from the membranous Golgi apparatus.
The sequence of events from protein synthesis on the rough ER to the final distribution
of those proteins.
1 Protein-
containing
vesicles pinch
off rough ER
and migrate to
fuse with
membranes of
Golgi
apparatus.
Rough ER
ER
Phagosome
membrane
Proteins in
cisterna
Pathway C:
Lysosome containing
acid hydrolase
enzymes
2 Proteins are
Vesicle becomes
lysosome
modified within
the Golgi
compartments.
3 Proteins are
then packaged
within different
vesicle types,
depending on
their ultimate
destination.
Plasma
membrane
Golgi
apparatus
Pathway A:
Vesicle contents
destined for exocytosis
Secretory
vesicle
Secretion by
exocytosis
Pathway B:
Vesicle membrane
to be incorporated
into plasma
membrane
Extracellular fluid
This process is crucial for neurotransmitter production (and release)
from the axon terminal in neurons.
Classification of neurons can
occur from a variety of
standpoints.
The number of neurites can be
used for classification:
– Single neurite
• Unipolar
– Two or more neurites
• Bipolar- two
• Multipolar- more than
two
Classifying Neurons Based on Morphology
•
Classification based on
dendritic and somatic
morphologies
– Stellate cells (star-shaped)
and
– Pyramidal cells (pyramidshaped)
Also note that spiny or
aspinous processes may
be seen on any type of
dendtrite classification.
Further classification can be made:
– By connections within the CNS
• sensory neurons – carry sensory information to the CNS
• motor neurons – carry “commands” out from the CNS to the PNS
• Interneurons – carry signals within the CNS
– Based on axonal length
• Golgi Type I - a neuron having with a “long” axon that begins in the grey
matter of the central nervous system and may extend considerably from
there.
• Golgi Type II - a neuron having very short axons or no axon at all.
– Based on neurotransmitter type
• Cholinergic (aka Acetycholine) a neurotransmitter group that uses
acetylcholine as its primary communication chemical released at the
synapses between neurons.
• Dopamenergic (aka Dopamine) a neurotransmitter group using primarily
dopamine
Glia (Accessory Cells)– Cells That Support Neurons
1.
Astrocytes
– Most numerous glia in the
brain
– Fill spaces between
neurons
– Influence neurite growth
– Regulate chemical content
of extracellular space via
functioning as part of the
blood brain barrier
2.
Microglial cells – immune response cells specialized for the brain and
spinal cord. These are the resident macrophages of the CNS.
Microglia are constantly scavenging the CNS for damaged
neurons, plaques, and infectious agents. The brain and spinal
cord are considered "immune privileged" organs in that they are
separated from the rest of the body by a series of endothelial
cells known as the blood-brain barrier,
3. Ependymal Cells - cerebrospinal fluid producing cells (CSF)
that line the ventricles. These cells are ciliated to promote
circulation of the CSF throughout the CNS.
4. Oligodendrocytes -
myelin producing cells.
5. Schwann Cells – myelin producing cells.
The role of the myelin produced in both the oligodendrocytes and in the Schwann
Cells is to insulate neurons to promote a more rapid transmittance of a neural
signal.
The key difference between oligodendrocytes and Schwann Cells is in their
location. Oligodendrocytes are found in the CNS whereas the Schwann cells
are found in the PNS.
• Myelinating Glia
•
The production of the myelin
sheath is called myelination.
•
The production of myelin begins
in the fourteenth week of fetal
development, however only
small amounts of myelin exist in
the brain at the time of birth.
•
During infancy myelination
continues and growth occurs
quickly and does not slow until
adolescence .
•
Because of rapid myelination during
early life, it is essential that
children under the age of two
receive a diet higher in fats
than an adult.
Theodor Schwann – German physiologist who discovered
the mylenating glia in the peripheral nervous system, the
discovery and study of pepsin and the invention of the
term metabolism.
Rudolf Virchow –biologist, pathologist, and physician
Virchow discovered and described myelin in 1854 at
the age of 33.
Oligodendrocytes can also be
called “oligodendroglial
cells”
– Nodes of Ranvier
• Region where the
axonal membrane is
exposed allowing the
process of saltatory
conduction
• Found associated with
myelinated neurons in
both the PNS and CNS
(and therefore
associated with
oligodendrocytes and in
Schwann cells.)
•
The Axon & Axon Terminal
– The Axon Terminal
• Differences between the cytoplasm of axon terminal and axon
– No microtubules in terminal
– Presence of synaptic vesicles
– Abundance of membrane proteins
– Large number of mitochondria
•
The Axon
– Synapse
• Synaptic transmission
• Electrical-to-chemicalto-electrical
transformation
• Synaptic transmission
dysfunction
– Mental disorders
Differences in dendritic spine
shape and size can exert
significant effects on function.
( see Purpua, 1974 )
Notice the fewer number of
spines and their thin, longer
extensions in the affected
group.
The theory is that the changes
in the synaptic interactions that
these dendritic spine
differences lead to is the root
cause of mental retardation in
these individuals.
Structure Correlates
with Function
Structural
characteristics of a
neuron tell us about
its function
NEURONS
Soma
Axons
Dendrites
Synapse
e.g., Dense Nissl
stain = protein;
suggests
specialization
Elaborate
structure of
dendritic tree =
receiver
END.