PSYC 4750 Lecture Notes - The Terminal, Neuroglia, Microglia

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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
outside
o The cell body of the neuron contains the same organelles that are found in all animal
cells
The most important:
Nucleus
The rough ER
The smooth ER
The Golgi apparatus
Mitochondria
o Everything within the cell membrane, excluding the nucleus is collectively known as the
cytoplasm
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
ribosome
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
locations
Some types of smooth ER play no direct role in the processing of protein
molecules but instead regulate the internal concentrations of
substances
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
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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
Microtubules
Big and run longitudinally down neuritis
Appears as a straight, thick-walled hollow pipe
Microfilaments
About the same thickness as a cell membrane
Found throughout the neuron and are particularly numerous in
the cell membrane
Neurofilaments
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
them
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
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