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CHAPTER 2
Once we fully understand the individual and concerted actions of brain cells, we will understand
the origins of our mental abilities.
This chapter focuses on the structure of the different types of cells in the nervous system:
Neurons and Glia.
 There are about 100 billion neurons in the human brain. Gila outnumber neurons by ten
folds.
 Neurons are the most important cells for the unique functions of the brain. Neurons sense
changes in the environment, communicate these changes to other neurons, and command the
body’s responses to the sensations.
 Gila are thought to contribute to brain function mainly by insulating, supporting and
nourishing neighboring neurons. Glia is derived from the Greek work for "glue". We are still
very ignorant about their functions.
THE NEURON DOCTRINE
Obstacles in the study of the structure of brain cells.
1.) Small size: Cells are in the range of .01-.05mm in diameter. That is about 40-200 times
smaller than the tip of our unsharpened pencil. -Progress in cellular neuroscience was not
possible before the development of the compound microscope in the late 17th century.
2.) Brain tissue has the consistency of jello or tapioca pudding: not firm enough to make thin
slices which is required observe brain tissue under a microscope. -Early in the 19th century
scientists discovered that they could "fix" on harden brain tissue by immersing it in
formaldehyde, and they developed a special device called a microtone to make very thin slices. These technical advances spawned the field of histology, the microscopic study of the structure
of tissues.
3.) Freshly prepared brain has a uniform, cream-colored appearance under the microscope; the
tissue has no differences in pigmentation to enable histologists to resolve individual cells.
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The final breakthrough in neurohistology was the introduction of stains that could selectively
color some, but no all, parts of the cells in brain tissue.
Franz Nissl, German neurologist, showed that a class of basic dyes would stain the nuclei of
all cells and also stain clumps of material surrounding the nuclei of neurons. These clumps
are called Nissl bodies. The stain is called Nissl stain extremely useful for 2 reasons: 1.) It
distinguishes neurons and glia from one another. 2.) It enables histologists to study the
arrangement, or cytoarchitecture, of neurons in different parts of the brain.
The study of cytoarchitecture led to the realization that the brain consists of many specialized
regions, which performs a different function. The Golgi Stain In 1873, the Italian histologist
Camillo Golgi discovered that soaking brain tissue in silver chromate solution (Gogi Stain) a
small percentage of neurons became darkly colored in their entirety. This revealed that the
neuronal cell body the region of the neuron around the nucleus that is shown w/the Nissl
stain is actually only a small fraction of the total structure of the neuron.
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Golgi stain shows that neurons have at least 2 distinguishable parts: central region that
contains the cell nucleus, and numerous thin tubes that radiate away from the central region.
The swollen region containing the cell nucleus has several names that are used
interchangeably: cell body, soma, and perikaryon.
The thin tubes that radiate away from the soma are called neurites and are of 2 types: axons
and dendrites. -The cell body usually gives rise to a single axon. The axon is of uniform
diameter throughout its length, and if it branches, the branches generally extend at right
angles.
Axons call be over a meter in length. They carry the output of the neuron.
Dendrites are rarely more than 2 mm in length. Many extend from the cell body and
generally taper to a fine point. They receive in- coming signals or input. Cajal’s Contribution
-Santiago Romon y Cajal used Golgi’s stain to work out the circuitry of -many regions of the
brain.
Golgi believed that the neurites of different cells were fused together to form a continuous
network. Ramon y Cajal argued that the neurites of different neurons are not continuous with
one another and must communicate by contact. The idea that the neuron adhered to the cell
theory came to be known as the neuron doctrine.
In the 1950’s the Neuron Doctrine was finally proven.
THE PROTOTYPICAL NEURON
Neuronal Membrane: The limiting skin of the neuron.
The Soma
The Soma: The cell body of the neuron. About 20 micrometers in diameter
Cytosol: The fluid inside the cell. A salty potassium rich solution.
Organelles: the membrane-enclosed structures found within the soma.
 These are identical to those found in all animal cells. The most important organelles are the
nucleus, the rough encloplasmic reticulum, the smooth endoplasmic reticulum, the Golgi
apparatus, and the mitochondria.
Cytoplasm: Everything contained within the confines of the cell membrane, excluding the
nucleus.
The Nucleus
The Nucleus: A spherical centrally located body measuring about 5-10 mm across. It is
contained within a double membrane called the nuclear envelope
 The nuclear envelope is perforated by pores that measure about 0,1 mm across.
 Chromosomes are found within the nucleus. They contain Deoxyribonucleic acid (DNA).
Although DNA is the same in all cells in the body, the specific parts of the DNA that are
used to assemble the cell differ depending on the type of cell.
 Each chromosome contains an un-interrupted double-stained braid of DNA.
 There are 46 human chromosomes or 23 pairs.
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The "reading" of the DNA is known as gene expression. The result is the synthesis of protein
molecules.
Proteins exist in a variety of shapes and sizes, perform many different functions, and bestow
upon neurons virtually all of their unique characteristics.
Protein Synthesis occurs in the cytoplasm.
DNA never leaves the nucleus. Instead the genetic message is carried to the sites of protein
synthesis in the cytoplasm by messenger ribonucleic acid, or mRNA.
mRNA consists of four different nucleic acids strung together in various sequences to form a
chain. The detailed sequence of the nucleic acids in the chain represents the information in
the gene.
Transcription: The process of assembling a piece of mRNA that contains the information of a
gene. The resulting mRNA is called the transcript.
 The mRNA exit the nucleus via the pores in the nuclear envelope and travel to sites of
protein synthesis where a protein molecule is assembled by linking together many small
molecules into a chain.
Amino Acids: The building blocks of proteins. There are 20 different kinds. Translation: The
assembly of proteins from amino acids under the direction of the mRNA.
 The scientific study of this process is called molecular biology.
 The central dogma of molecular biology:
DNA-----Transcription-------mRNA------Translation-------Protein
Molecular Neurobiology: A new field within neuroscience. Uses the information contained in the
genes to determine the structure and functions of neuronal proteins.
Rough Endoplasmic Reticulum
 These are stacks of membrane dotted with dense globular structures called ribosomes.
 Located not far from the nucleus.
 These abound in neurons more than in glia or most other non- neuronal cells.
 Also called Nissl bodies.
 Rough ER is a site of protein synthesis in neurons. RNA transcripts bind to the ribosomes,
which translate the instructions in the mRNA to assemble a protein molecule.
 Free ribosomes are not attached to the rough ER but are free floating.
 Polyribosomes are free ribosomes attached by a single thread of mRNA. The associated
ribosomes are working on it to make multiple copies of the same protein.
 Those proteins synthesized by free ribosomes are destined to reside within the cytosol of the
neuron.
 If the protein is destined to be inserted into the membrane of the cell or an organelle, then it
is synthesized on the rough ER.
Smooth Endoplasmic Reticulum and the Golgi Apparatus.
 The remainder of the cytosol of the soma is crowded with stacks of membranous organelles
that look a lot like rough ER without the ribosomes. Thus one type is called smooth ER.
 Performs different functions in different locations.
 Some regulate the internal considerations of substances such as calcium.
 Golgi apparatus: The stack of membrane-enclosed disks in the soma that lies farthest from
the nucleus.
 This is a site of extensive "post-translational" chemical processing of proteins.
 Believed to be responsible for sorting certain proteins that are destined for delivery to
different parts of the neuron, such as the axon and the dendrites.
Mitochondrion
Mitochondrion: Sausage shaped organelles. Within the enclosure of the outer membrane are
multiple folds of inner membrane called cristae. Between the cristae is an inner space called
matrix.
 Mitochondria are the site of cellular respiration. When a mitochondrion "takes a breath"
pyruvic acid and oxygen enter the mitochondrion. Pyruvic acid is derived form sugars and
digested proteins and fates. Once in a mitondrion pyruvic acid enters into a complex series of
biochemical reactions called the Krebs Cycle.
 These reactions result in the addition of phosphate to adenosine diphonphate (ADP), yielding
adenosine triphosphate (ATP), the cells energy source. When a mitochondrion "exhales", 17
ATP are released for every puruvic acid molecule "inhaled".
The Neuronal Membrane
Encloses the cytoplasm inside the neuron. The membrane is studded with proteins. Some of these
pump substances from the inside to the outside. Others form pores that regulate which substances
enter the neuron. The protein composition of the membrane varies depending on whether it is in
the soma, the dendrites, or the axon.
The Cytoskeleton
The staffolding of the neuron, which gives it its characteristic, shape. Composed of microtublues,
microfilaments, and neurofilaments. These elements are dynamically regulated and are likely in
continual motion.
Microtubles: Run longitudinally down neurites. Appears as a straight, thick-walled hollow pipe.
The walls are formed small strands of the protein tubulin.
Polymerization: The process of joining small proteins to form a long strand.
Polymer: The resulting strand.
MAP Microtubule
 Associated Proteins: One class of proteins that participate in the regulation of microtubule
assembly and function.
 MAPS anchor the microtubules to one another and to other parts of the neurons.
 Pathological changes in an axonal MAP called tau have been implicated in the dementia that
accompanies Alzheimer’s Disease.
Microfilaments
 Found throughout the neuron, but particularly numerous in the neurites.
 Composed of braids of 2 thin strands of polymers of the protein actin.
 Actin is one of the most abundant proteins in cells of all types believed to play a role in
changing cell shape. Critically involved in the mechanism of muscle contraction.
 Microfilaments are constantly undergoing assembly and disassembly. Run longitudinally
down the core of the neurites.
Neurofilaments
 Intermediate in size between microtubules and microfilaments
 Exist in all cells of the body where they are called intermediate filaments.
 Neurofilaments most closely resemble the bones and ligaments of the skeleton.
The Axon
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Found only in neurons. Highly specialized for the transfer of information over distance n the
nervous system.
Axon begins with a region called the axon hillock, which tapers to form the initial segment of
the axon proper.
Because there are no ribosomes in the axon there is no protein synthesis in the axon.
The protein composition of the axon enables it to serve as the "telegraph wire" that sends
information over great distances.
Axons may extend from less that a millimeter to over a meter long.
Axons often branch. The branches are called axon collaterals.
Recurrent collaterals are axon branches, which will return to communicate with the same cell
that gave rise to the axon or with the dendrites of neighboring cells. Axons in humans: 1 mm
to 25mm in diameter In the squid: 1 mm in diameter.
Size is important because the speed of the nerve impulse that is the electrical signal that
sweeps down the axon, varies depending on axonal diameter.
The thicker the axon, the faster the impulse travels.
The Axon Terminal
Axon hillock: The beginning of the axon.
Axon proper: The middle of the axon.
Axon terminal or terminal bouton: The end of the axon
 The axon terminal is a site where the axon comes in contact with other neurons and cells and
passes information on to them.
 The point of contact is called the synapse.
 Sometimes axons have many branches at their ends and each branch forms a synapse on
dendrites or cell bodies in the same region. These branches are called terminal arbor.
 Some axons have synapses at swollen regions along their length. These swellings are called
boutons en passant.
 When a neuron makes synaptic contact with another cell, it is said to innervate that cell.
 Axon terminal contains no microtubules, but does contain numerous small bubbles of
membrane called synaptic vesicles. The terminal has numerous mitochondria, indicating a
high-energy demand.
The Synapse
 Two sides: Presynaptic and Postsynaptic.
 Presynaptic side generally consists of an axon terminal.
 Postsynaptic side may be the dendrite or soma of another neuron.
Synaptic Cleft: The space between the presynaptic and postsynaptic membranes.
Synaptic transmission: The transfer of information at the synapse from one neuron to another.
 At most synapses, information in the form of an electrical impulse, which has traveled down
the axon, is converted in the terminal into a chemical signal, which crosses the synaptic cleft.
 The chemical signal is transformed back into an electrical signal on the postsynaptic side of
the cleft.
 The chemical signal is called a neurotransmitter. It is stored and released from the synaptic
vesicles within the terminal different neurotransmitters are used by different types of
neurons. The electrical-to-chemical-to electrical transformation of information makes
possible many of the brains computational abilities. Modification of this process is involved
memory and learning and synaptic transmission dysfunction accounts for certain mental
disorders. The synapse is also the site of action for nerve gas and for most psychoactive
drugs.
Axoplasmic Transport
Because axons do not contain ribosomes, the proteins needed must be synthesized in the soma
and tin transported down the axon. If an axon is disconnected from its soma it will degenerate.
This is called Wallerian Degeneration after the English physiologist Augustus Waller. The
movement of material down the axon is called axoplasmic transport, which was just
demonstrated in the 1940s by neurobiologist Paul Weiss and his colleagues. The process works
in the following way: materials are enclosed in vesicles and walked down the microtubules by
the protein leinesin. It is fueled by ATP. All movement form the soma to the axon terminal is
called anterograde transport. Movement form the terminal to the soma is called retrograde
transport. This process signals changes in metabolic needs of the axon terminal. The process is
similar to anterograde transport except the protein involved is called dynein.
Dendrites
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Derived from the Greek word for tree, which reflects the fact that dendrites resemble the
branches of a tree as they extend from the soma.
Dendritic Tree: The dendrites of a single neuron.
 The wide variety of shapes and sizes of dendritic trees are used to classify different groups of
neurons.
 Dendrites function as the antennae of the neuron and are covered with thousands of synapses.
Receptors: Specialized protein molecules located on the postsynaptic membrane that detects
neurotransmitters in the synatic cleft.
Dendritic Spines: Specialized structures that cover the dendrites of some neurons. They receive
some types of synaptic input.
 Spine structure is sensitive to the amount and type of synaptic activity.
 Unusual changes in spines have been shown to occur in the brains of individuals with
cognitive impairments.
 Cytoplasm of dendrites resembles that of axons. Difference: polyribosomes can be found in
dendrites.
CLASSIFYING NEURONS
Neurons in the brain can be divided into a small number of categories. The goal is to understand
the unique contribution of each category rather than each cell.
Classification Based on the Number of Neurites
Unipolar: A neuron with one neurite.
Bipolar: A neuron with 2 neurites
Multipolar: A neuron with 3 or more neurites.
 Most neurons are multipolar.
Classification Based On Dendrites
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Dendritic trees call vary widely from one type of neuron to another.
Classification is often unique to a particular area of the brain.
 In the cerebral cortex there are two broad classes: Pyramidal cells and Stellate cells (star
shaped).
Classification also based on whether or not the dendrites have spines: Spiny (spines) and
Aspinous (no spines).
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The classification schemes overlap.
 Pyramidal cells are spiny.
 Stellate cells can be either.
Classification Based on Connections
Primary Sensory Neurons: Neurons that have neurites in the sensory surfaces of the body
transmit sensory information.
Motor Neurons: Neurons that have axons that form synapses with the muscles and command
movements.
Interneurons: Neurons, which form connections with other neurons. Theses are the most
numerous in the brain.
Classification Based on Axon Length
Golgi Type I Neurons: Have long axons that extend from one part of the brain to the other.
Golgi Type II Neurons (Local Circuit Neurons): Neurons with short axons that do not extend
beyond the vicinity of the cell body.
Classification Based on Neurotransmitter
Neurons can also be classified by the specific neurotransmitter used by the cell.
GLIA
Current evidence indicates that Glia contribute to brain function mainly by supporting neuronal
functions.
Astrocytes
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The most numerous glia in the brain
Fill the spaces between neurons.
Essential role of astiocytes is regulating the chemical content of the extiacelluar space thus,
they restrict the
spread of neurotransmitter molecules that have been released.
 Astrocytic membranes also possess neurtransmitter receptors that can trigger electrical and
biochemical events
inside the glial cell.
 Also tightly controls the extracellular concentration of several substances that have the
potential to interfere
with proper neuronal function.
Myelinating Glia: Oligodendrigal and Schwann Cells
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Primary Function: Provide layers of membrane ha insulate axons.
This wrapping celled myelin, spinals around axons in the brain.
The wrapping is called the myelin sheath. The sheath is periodically interupted leaving a
short length where the
membrane is exposed. This region is called a Node of Ranuier. Myelin serves to speed the
propagation of nerve
impulses down the axon.
 Oligodendroglia are found only in the peripheral nervous system.
 Oligodendroglial cells contribute myelin to several axons.
 Schwann Cells myelinates only a single axon.
Other Non-Neuronal Cells
Ependymal Cells: Provides the lining of fluid-filled ventricles within the brain, and they also
play a role in directing cell migration during brain development.
Microglia: Remove debris left by dead or degenerating neurons and glia.