Prof. U.J. McMahan
IBRO Lecture
STRUCTURE OF SYNAPSES: OVERVIEW
Traditionally, a synapse is considered to be a site of apposition between two
neurons, where an ion conductance change (impulse) across the plasma membrane of one
brings about an ion conductance change across the plasma membrane of the other, in a
process all together referred to as synaptic transmission. The term synapse now also
applies to functionally similar sites of apposition between receptor cells and neurons,
between neurons and muscle fibers and between neurons and gland cells. Because
virtually all aspects of animal behavior are dependent on the synaptic transmission of
impulses, much of neuroscience is devoted to understanding how synapses function.
There are "electrical" synapses, at which there is simply a passive spread of ions
from one cell to the other. And there are “chemical” synapses, where the impulse in the
presynaptic cell brings about that cell’s release of a chemical neurotransmitter which
diffuses across a narrow gap (synaptic cleft) to bind to specific receptors in the plasma
membrane of the postsynaptic cell. It is the interaction of the neurotransmitter with its
receptors in the postsynaptic plasma membrane that leads to the change in the membrane's
conductance. Chemical synapses are the most abundant type in all animal species.
Electrical synapses are more abundant in invertebrates than in the vertebrates, and in
mammals chemical synapses seem to be the predominant type by far. At both chemical
synapses and electrical synapses, there are structural features distinct from those found in
other parts of the pre- and postsynaptic cell. These specializations have provided and
continue to provide important information as to how the synapses work. This lecture
describes prominent specializations together with an overview of their function.
ELECTRICAL SYNAPSES
When thin sections from from tissues containing electrical synapses are examined
by electron microscopy, the plasma membranes of the apposed cells at the synapses are
seen to lie much closer to each other than in non-synaptic regions. Indeed the distance
separating the bimolecular lipid leaflets of the membranes at these synapses is 2-3nm
instead of 15-20nm. Electron microscopy of freeze fracture replicas of the interior of each
of the plasma membranes at the synapses show that both contain a hexagonal array of
macromolecules. Biochemistry has revealed that the macromolecules are a distinct class of
proteins now known as connexons. Each connexon is made up of six subunits, connexins.
Moreover, each of the six connexins traverses the plasma membrane, so that it has
cytoplasmic and extracellular domains as well as as a membrane domain, and all together
they line a transmembrane channel. The connexins of a connexon in one plasma
membrane bind to the connexins of a connexon in the adjacent plasma membrane so that
the channel of the one is continuous with that of the other. It is these conjoined proteinlined channels between apposed plasma membranes that mediate the passive spread of
ions from one cell to the other during electrical transmission. Aggregates of connexons are
characteristic of gap junctions, which occur between many types of non-neuronal cells
(such as liver cells, where they are involved in the intercellular transfer of ions and small
peptides necessary for the maintenance of a tissue’s functional integrity.
CHEMICAL SYNAPSES
At chemical synapses, there are structural specializations unique for the pre- and
postsynaptic cell; those in the presynaptic cell being involved mainly in the storage and
release of neurotransmitter, those in the postsynaptic cell being involved in the reception
of the neurotransmitter. Moreover, there are specializations in the synaptic cleft involved
in maintaining the efficacy of the synapse. Although these specializations are similar for
all synapses, there are variations depending on the type of pre- and postsynaptic cell and
the animal species. In the following account I describe the structural specializations at
common neuron-to-neuron synapses in the vertebrate central nervous system.
1
December, 2003
Prof. U.J. McMahan
IBRO Lecture
December, 2003
Presynaptic Specializations
A neuron has three principal parts based on sturctural and functional differences: a cell body,
which contains the nucleus, multiple dendrites and an axon. There are recorded examples of synapses
between two neuron cell bodies, between two dendrites, and between two axons. But the most common
type of neuron-to-neuron synapse involves an axon as the presynaptic component and a neuron cell body or
dendrite as the postsynaptic component.
The axonal arborization
A neuron is generally innervated by the axons of many neurons. Each of these
parent axons extends from the cell body without branching. However, as it approaches
the neuron or neurons it innervates, it forms multiple branches. When each of these
preterminal branches is within a few microns of the neurons it innervates, it gives rise to
an arborization of terminal branches that extend over the neurons’ surface. The number
and length of terminal branches varies from one neuron type to another, extending up to
many tens of micrometers over a target neuron’s surface. In all cases, however, the
terminal branches release transmitter, and the release occurs at multiple sites along the
terminal branch. Accordingly, the terminal arborization is an axonal specialization that
provides distributed sources of transmitter to the postsynaptic neuron.
Varicosities in terminal branches
Each terminal branch in the terminal arborization has numerous varicosities
(swellings). The varicosities are directly apposed to the postsynaptic neuron. Glial cells
can form myelin sheaths on the parent motor axons and their preterminal branches.
Terminal branches, on the other hand, are unmyelinated but the processes of glial cells are
closely associated with them. While the terminal glial cells can completely wrap around
the connectives between varicosities, they are associated only with that portion of the
varicosity surface that faces away from the postsynaptic neuron. Indeed there is no
intervening cell between the plasma membrane of the varicosity and the plasma membrane
of the postsynaptic cell. Although the varicosities directly face the postsynaptic neuron,
they are separated from it by a narrow space of ~25 nanometers (nm). Based largely on
evidence discussed below (see Active zones) the, neurotransmitter release that occurs
during impulse transmission takes place at the varicosities and from that portion of the
varicosities that face the postsynaptic neuron. Thus, varicosities provide the terminal
branches with transmitter release sites, and these sites are positioned a very short distance
from the postsynaptic neuron that is uninterrupted by any other cell, which, altogether,
accounts in part for the constant and brief time course between the release of transmitter
and the postsynaptic response as recorded electrophysiologically. The portion of the
varicosities' plasma membrane that faces the postsynaptic cell is the presynaptic
membrane and the 25nm wide space between the presynaptic membrane and the plasma
membrane of the postsynaptic neuron (postsynaptic membrane) is the synaptic cleft.
Composition of varicosities
Each varicosity can be divided into two distict regions: the axon backbone and
the varicosity proper. The axon backbone is largely the continuation of axonal components
that fill the parent axon, the preterminal axon branches and the connectives between the
varicosities. It's main constituents includes mitochondria, lysosomes, microtubules and
filaments, and smooth endoplasmic reticulum. In the varicosity proper the most
conspicuous components are vesicles and actin filaments. The vesicles fall into several
categories according to their form and function. Principal among them are the following:
1) agranular vesicles, that are spherical, ~35-50nm diameter (depending on the
synapse), and contain neurotransmitter. These are also known as synaptic vesicles and
they are by far the most abundant vesicle in the varicosity proper. They are in clusters that
are focused on the presynaptic membrane. They contain the so-called small molecule
neurotransmitters that include amino acids (such as glutamate or glycine), catecholamines
(such as norepinephrine and dopamine), serotonin or ACh. When the nerve impulse arrives
2
Prof. U.J. McMahan
IBRO Lecture
at the varicosity, some of the synaptic vesicles fuse with the presynaptic membrane
releasing their content into the synaptic cleft to act on receptors specific for the
neurotransmitters in the postsynaptic membrane. Thus synaptic transmission relies on the
vesicle mediated exocytosis of neurotransmitter. There is usually only one small molecule
transmitter for a specific neuron type, but examples of different transmitters being released
from the same varicosity have been observed.
2) granular vesicles. These range from 80nm-160nm and they contain an electron
dense granule when viewed by conventional electron microscope methods. There are
much fewer granular vesicles in a varicosity than synaptic vesicles. The granular vesicles
are known to contain peptides, which at many synapses include large molecule
neurotransmitters. The peptides are also released from the varicosity by exocytosis. They
act on receptors in the postsynaptic neuron to modulate the action of the small molecule
neurotransmitters.
3) coated vesicles, cisternae and endosomes. Coated vesicles are about the size
of synaptic vesicles and are agranular but their external surface is covered by a network of
the protein, clathrin. Cisternae are flattened agranular vesicles of varying dimensions;
endosomes are rounded agranular vesicles of varying dimensions, but usually larger than
synaptic vesicles. Shortly after fusion of synaptic vesicles with the presynaptic membrane,
portions of the membrane are internalized by the varicosity which leads to the local
generation new synaptic vesicles loaded with neurotransmitter and available for release
during impulse transmission. The coated vesicles, cisternae and endosomes are thought to
play a role in the recycling of synaptic vesicle membrane.
Actin filaments. The vesicles are not free to move in just any direction within a
varicosity. If they were, one might expect to find them uniformly distrubuted. But this is
not the case. Rather they are arranged in clusters which are focused on distinct sites on the
presynaptic membrane. Among the vesicles are actin filaments. It is not known whether
the F-actin is focused on the presynaptic membrane, but there are in varcosities a class of
proteins known as synapsins which can cross-link synaptic vesicles to F-actin, inhibiting
their movement. The movement of vesicles toward the presynaptic membrane, which must
occur as vesicles that have fused with the presynaptic membrane are replaced, may rely on
the regulation of interactions between synapsin, synaptic vesicles and actin.
Active zones. The sites on the presynaptic membrane at which the vesicle groups
are focused are characterized by one or more dense aggregates of proteins that stud the
cytoplasmic surface of the memrane. At common neuron-to-neuron synapses the
aggregates are more or less conical and they extend about 75nm into the cytoplasm. The
base of the cone, about 50 nm diameter, is attached to the presunaptic membrane. The
protein aggregates are often in clusters. Synaptic vesicles next to the protein aggregates
are docked to (held at) the presynaptic membrane. The presynaptic membrane just beneath
each protein aggregates contains aggregates of macromolecules that include voltage
dependent calcium channels and calcium activated potassium channels. When a nerve
impulse arrives at a varicosity, the depolariztion of the presynaptic membrane causes the
calcium channels to open. Calcium that enters the varicosity interacts with proteins that
bring about the fusion of docked vesicles with the presynaptic membrane and exocytosis
of neurotransmitter. It also interacts with the calcium activated potassium channels, which
open to allow the passage of potassium out of the varicosity. This repolarizes the
presynaptic membrane, thus closing the calcium channels and terminating the exocytosis
of neurotransmitter. The region(s) along the presynaptic membrane of varicosities
characterized by one or more protein aggregates, their associated docked vesicles and the
subjacent presynaptic membrane with aggregates macromolecules (calcium channels and
calcium activated potassium channels) are called axtive zones. It is at the active zones that
the initial events in synaptic transmission take place. The aggregates of proteins in the
active zones are known as active zone material. The active zone material helps dock the
synaptic vesicles and anchor the macromolecules, and it may contain the proteins that
mediate the calcium-induced fusion of the docked vesicles with the presynaptic
membrane.
3
December, 2003
Prof. U.J. McMahan
IBRO Lecture
Postsynaptic Specializations
Postsynaptic Plasma membrane and subsurface cytoskeleton (Postsynaptic Density )
In the plasma membrane of the postsynaptic cell, just opposite each active zone
of the varicosity, there is a high concentration of receptor proteins for the
neurotransmitter. At some synapses the concentration of receptors opposite the active zone
is more than a thousand times greater than it is anywhere else on the surface of the
postsynaptic cell. A dense aggregate of proteins, including cytoskeleton, lines the
cytoplasmic surface of the postsynaptic membrane subjacent to the receptor aggregates. It
is called the postsynaptic density (PSD). The PSD helps anchor the receptors in the
postsynaptic membrane. At certain synapses it also contains proteins that mediate the
opening of ion channels in response to the neurotransmitter-receptor interaction.
Synaptic Cleft
The 25nm wide region of extracellular space between the active zone of the varicosity and
the receptor-rich region of the postsynaptic membrane is slightly wider than elsewhere
between the pre- and postsynaptic membrane (~15nm). It is referred to as the synaptic
cleft. There is little obvious material between neurons and glial cells in the central nervous
system and antibodies to extracellular matrix proteins found in abundance in other tissues,
such as muscle or kidney, show little to no staining by immunohistochemistry. But at
synapses a conspicuously dense aggregate of proteins extends across the synaptic cleft
from the pre-to postsynaptic membrane. These proteins do not impede the diffusion of
neurotransmitter across the cleft. They include the extracellular portions of receptors and
channels and probably proteins involved in maintaining a tight adherence between the preand postsynaptic membrane at active zones.
SUMMARY
The preceding account describes in brief the structural specializations at electrical
and chemical synapses. Those at chemical synapses are more numerous and varied than
those at electrical synapses. However, with the exception of synaptic vesicles at chemical
synapses, the specializations of both types of synapses consist of aggregates of proteins.
At electrical synapses there are aggregates of connexons in the pre and postsynaptic
membrane. At chemical synapses there are aggregates of calcium channels and calcium
activated potassium channels in the presynaptic membrane, aggregates of receptors for
neurotransmitter in the postsynaptic membrane, aggregates of proteins on the cytoplasmic
surface of the presynaptic membrane (active zone material), aggregates of proteins on the
cytoplasmic surface of the postsynaptic membrane (PSD) and aggregates of proteins in the
synaptic cleft. I have pointed out that each specialization plays a specific role in synaptic
function. In subsequent lectures I will discuss the evidence for such conclusions as they
pertain to the specializations at chemical synapses, and how the specializations are formed
and maintained.
General References
Cowan, M.W., Sudhöf, T.C. and Stevens, C.F. (eds) 2000 Synapses, Johns Hopkins
University Press/ Baltimore.
Kandel, E.R., Schwartz, J.H. and Jessell, T.M. 1991 Principles of Neuroscience (3rd. ed),
Elsevier/New York.
Nicholls, J.G., Martin, A.R.,Wallace, B.G. and Fuchs, P.A. 2001 From Neuron to Brain
(4th ed), Sinauer Associates, Inc/ Sunderland.
4
December, 2003
Prof. U.J. McMahan
IBRO Lecture
Peters, A., Palay, S.L. and Webster, H. deF. 1991 The Fine Structure of the Nervous
System, Oxford/Oxford.
.
5
December, 2003