PS271 Lecture Notes - Gap Junction, Axon Terminal, Postsynaptic Density

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1 Feb 2013
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Types of Synapses
Electrical Synapses
o Electrical synapses are relatively simple in structure and function, and the allow the direct transfer of
ionic current from one cell to the next.
o They occur at specialized sites called gap junctions
The membrane of two cells is separated by only about 3nm and this narrow gap is spanned by
clusters of special proteins called connexins. Six of these combine to form a channel called a
connexon and two connexons combine to form a gap junction channel
It allows ions to pass directly from the cytoplasm of one cell to the cytoplasm of the other.
The pore is relatively large (1-2 nm) and is big enough for all the major cellular ions and many
small organic molecules to pass through
Most allow ionic current to pass equally well in both directions (unlike the majority of chemical
synapses).
Cells connected by gap junctions are electrically coupled
o Transmission at the electrical synapses is very fast and, if the synapse is large, fail-safe.
An action potential in the presynaptic neuron can produce almost instantaneously, and action
potential in the postsynaptic neuron.
o Also occur in the vertebrae brain and is common in every part of the mammalian CNS
o When two neurons are electrically coupled, an action potential in the presynaptic neuron causes a small
amount of ionic current to flow across the gap junction channels into the other neuron
Causes a postsynaptic potential (PSP) in the second neuron
o Since most electrical synapses are bidirectional, when the second neuron generates an action potential,
it will in turn induce a PSP in the first neuron
o The PSP generated by a single electrical synapses in the mammalian brain is small and may not by
large enough to trigger an action potential in the postsynaptic cell
One neuron usually makes electrical synapses with many other neurons so several PSPs
occurring simultaneously may strongly excite a neuron (synaptic integration)
o The precise roles of electrical synapses vary from one brain region to another.
Chemical Synapses
Most synaptic transmission in the mature human nervous system is chemical
The presynaptic and postsynaptic membranes at chemical synapses are separated by a synaptic cleft that is
20 50 nm wide.
o It is filled with a matrix of fibrous extracellular proteins.
o One function is to make the pre- and post-synaptic membranes adhere to each other
o The presynaptic side of the synapse is usually an axon terminal and typically contains dozens of small
membrane-enclosed spheres called synaptic vesicles.
These vesicles store neurotransmitter, the chemical used to communicate with the postsynaptic
neuron.
Many axon terminals also contain larger vesicles called secretory granules
They contain soluble protein that appears dark in the electron microscope and sometimes
called dense-core vesicles.
o Dense accumulations of protein adjacent to and within the membranes on either side of the synaptic
cleft are collectively called membrane differentiations.
On the presynaptic side, proteins jutting into the cytoplasm of the terminal along the intracellular
face of the membrane sometimes look like a field of tiny pyramids.
These are the actual sites of neurotransmitter release called active zones
Synaptic vesicles cluster adjacent to the active zones (Fig. 5.3)
o The protein thickly accumulated in and just under the postsynaptic membrane is called the
postsynaptic density.
It contains the neurotransmitter receptors, which convert the intercellular chemical signal into an
intracellular signal in the postsynaptic cell
o CNS Synapses
Different types of synapse may be distinguished by which part of the neuron is postsynaptic to
the axon terminal
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