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Chapter 3

NROC34H3 Chapter Notes - Chapter 3: Stellate Cell, Neuron Doctrine, Dendritic Spine

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Chapter 2: Neuron and Glia
Neurons and Glia are cells in the nervous system and are broad categories, within which are
many types of cells that differ based on their structure, chemistry and function
Glia outnumber neurons by tenfold
Neurons are the most important cells for the unique functions of the brain.
o They sense the changes in the environment, communicate these changes to other
neurons, and command the body’s responses to these sensations.
Glia (glial cells) are thought to contribute to brain function mainly by insulating, supporting, and
nourishing neighboring neurons
The Neuron Doctrine
The Golgi Stain
o The Golgi Stain shows that neurons have at least two distinguishable parts: a central
region that contains the cell nucleus and numerous this tubes that radiate away from
the central region
o The swollen region containing the nucleus has several names that are used
interchangeably; most notably the cell body and soma
o The thing tubes that radiate away from the soma are called neuritis and are of two
types: axons and dendrites
o 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 can travel long distances (a meter or more); dendrites, however, rarely
extend more than 2 mm in length.
Cajal’s Contribution
o Argued that the neuritis of different neurons are not continuous with one another and
must communicate by contact, not continuity
o This became known as the neuron doctrine and the brother won a Nobel Prize
The Prototypical Neuron
The Soma
o Roughly spherical central part of the neuron
o Is about 20 µm in diameter
o Watery fluid inside the cell (cytosol) is salty, potassium rich and separated from the
o The cell body of the neuron contains the same organelles that are found in all animal
The most important:
The rough ER
The smooth ER
The Golgi apparatus
o Everything within the cell membrane, excluding the nucleus is collectively known as the
o The Nucleus
Spherical, centrally located and is contained within a double membrane called
nuclear envelope
Contains chromosomes and DNA
o The Rough Endoplasmic Reticulum
Not far from the nucleus
Enclosed stacks of membrane dotted with ribosomes
Contains an abudance of neurons
Major site of protein synthesis in neurons
Some are produced on free ribosomes
So what’s the difference?
If the protein is destined to reside within the cytosol of the
neuron, then the protein’s mRNA moves towards the free
If the protein is destined to be inserted in a membrane or an
organelle, then it is synthesized in the rough ER.
o Smooth ER and the Golgi Apparatus
Is actually quite heterogeneous and performs different functions in different
Some types of smooth ER play no direct role in the processing of protein
molecules but instead regulate the internal concentrations of
The Golgi apparatus is believed to be the sorting of certain proteins that are
destined for delivery to different parts of the neuron (e.g. axon and dendrites)
o The Mitochondrion
Very abundant in the soma
Site of cellular respiration and uses ATP as energy currency
o The Neuronal Membrane
Serves as a barrier to enclose the cytoplasm inside the neuron and to exclude
certain substances between both sides of the barrier
o The Cytoskeleton
Gives the neuron its characteristic shape
The bones that make up the cytoskeleton are the microtubules, microfilaments,
and neurofilaments
Big and run longitudinally down neuritis
Appears as a straight, thick-walled hollow pipe
About the same thickness as a cell membrane
Found throughout the neuron and are particularly numerous in
the cell membrane
Are intermediate in size
Only neurons they are called neurofilaments but exist in all cells
The Axon
o Found only in neurons and is highly specialized for the transfer of information over
distances in the nervous system
o It begins with a region called the axon hillock
o Two features distinguish the axon from the soma:
No rough ER extends into the axon, and there are few, if any, free ribosomes
The protein composition of the axon membrane is fundamentally different from
that of the soma membrane
o Since there are no ribosomes, no protein synthesis can occur in the axon and thus, all
proteins present must originate in the soma.
The different proteins in the axonal membrane allow it to serve as a telegraph
wire and send information over great distances
o Axons often branch and these branches are called axon collaterals
Those branches that return to communicate with the same cell that gave rise to
the axon or with the neighbouring cells are called recurrent collaterals.
o The diameter of an axon is variable and this variation is important
The thicker the axon, the faster the information is sent and the thinner the
axon, the slower the information is sent
The Axon Terminal
o All axons have a beginning (the axon hillock), a middle (axon proper) and an end called
the axon terminal or terminal bouton
o It is the site where the axon comes in contact with other neurons and passes info onto
o This point of contact is referred to as the synapse
o 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 collectively known as
the terminal arbor
o When a neuron makes synaptic contact with another cell, it is said to provide
o They cytoplasm of the axon terminal differs from that of the axon in several ways:
Microtubules do not extend into the terminal
The terminal contains numerous small bubbles of membrane, called synaptic
vesicles, that measure about 50 nm in diameter
The inside surface of the membrane that faces the synapse has a particularly
dense covering of proteins
It has numerous mitochondria (high energy demand)
o Has two sides: the presynaptic and postsynaptic
The presynaptic side generally consists of an axon terminal, while the
postsynaptic side may be the dendrite or soma of another neuron
o The space between the two sides is called the synaptic cleft
The transfer at the synapse from one neuron to another is called synaptic
o Information in the form of electrical impulses travelling down the axon is converted in
the terminal into a chemical signal that crosses the synaptic cleft.
On the post synaptic membrane this chemical signal is converted again into an
electrical signal