Bioelectricity:
electrical synapse
Principle
An electrical
synapse is a
mechanical and electrically conductive link, in general two-way, between two
abutting neurons that is formed at a narrow gap between the pre- and
postsynaptic cells known as a gap junction. At gap junctions, the intercellular
distance of about 25 nm narrows to about 3.5 nm of each other,
a much shorter distance than the 20 to 40 nm distance that separates cells at
chemical synapses. As opposed to chemical synapses, the postsynaptic potential
in electrical synapses is not caused by the opening of ion channels by chemical
transmitters, but by direct electrical coupling between both neurons. Electrical
synapses are therefore faster and more reliable than chemical synapses.
Electrical synapses are found throughout the nervous system, yet are less
common than chemical synapses. Electrical synapses are
abundant in the retina (e.g. horizontal cells) and the cerebral cortex (e.g. in
the vestibular and the trigeminal nucleus). Brain astrocytes show experimentally
strong gap junctions (as do horizontal cells). There seems also to be weak
neuron to glial cell coupling in some areas.
In organisms, electrical synapse-based systems co-exist
with chemical-based, but are limited to systems that require the fastest
possible response, for instance for escape mechanisms. The relative speed of
electrical synapses also allows for many neurons to fire synchronously.
There exists another cell-cell junction, the tight
junction, which has no particular electric properties. With respect to
transport it behaves in some sense opposite to the gap-junction. By coupling
cells with a dense network of tight junctions transport in the intercellular
space is prevented. See More Info
for a further description.
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Fig. 1
Electrical synapse. Right under are three neurons connected by 3
electrical synapses.
Application
Disorders caused by dysfunction
of electrical synapses are scarce. Mutations in some gap junction genes cause white
matter degeneration similar to that observed in Pelizaeus-Merzbacher disease (inhibiting
myelin grow) and multiple sclerosis (disintegration of myelin sheaths).
More info
Gap
junction
General Each gap junction (aka nexus
junction) contains numerous channels which cross the membranes of both cells.
With a lumen diameter of about 1.2 to 2.0 nm, the pore of a gap junction
channel is wide enough to allow ions and even medium sized molecules like
signaling molecules to flow from one cell to the next thereby connecting the
two cells’ cytoplasm. Thus, when the voltage of one
cell changes, ions may move through from one cell to the next carrying positive
charge with them and depolarize the postsynaptic cell.
Morphology Gap junction channels are
composed of two hemi-channels called connexons in vertebrates, one embedded in
each of the membranes. Connexons are formed by six 7.5 nm long, four-pass
membrane-spanning protein subunits called connexins. However,
in some cells, the hemichannel itself is active as a conduit between the
cytoplasm and the extracellular space. Several gap
junctions (hundreds) assemble into a macromolecular complex called a plaque. They are analogous to the
plasmodesmata that join plant cells.
Delay and direction Without the need for receptors to
recognize chemical messengers, signaling at electrical synapses is, with a synaptic
delay of about
Normally current carried by ions could travel in either
direction through this type of synapse. However, sometimes the junctions are
rectifying synapses containing voltage-dependent gates that open in response to
a depolarization and prevent current from traveling in one of the two
directions. Some channels may also close in response to increase the Ca++
or H+ ion concentration.
Long
term adaptation There is evidence for "plasticity"
at some of these synapses, i.e. that the electrical connection they establish
can strengthen or weaken as a result of long lasting activity.
Function Gap junctions allows for direct electrical
communication between cells, although different
connexin subunits can impart different single channel conductances from about
30 pS (picoSiemens; 1 S = 1 mho = 1 Ω─1) to 500 pS.
They allow also for chemical communication between cells
through the transmission of small second messengers, such as IP3 and
Ca++. Generally, molecules smaller than 1,000
Areas of electrical coupling In the myocardium the signal to contract is
passed efficiently through the gap junctions, allowing the heart muscle cells
to contract in tandem. Gap junctions occur in virtually all tissues of the
body, with the exception of mobile cell types such as sperm and blood cells. Most disorders now associated with mutations
in gap junction genes affect the skin, because this tissue is heavily dependent
upon gap junction communication for the regulation of differentiation and
proliferation.
Tight junctions
General Tight
junctions, or zonula
occludens, are the closely associated areas of two cells whose membranes
join together forming a virtual impermeable barrier to fluid. It is only
present in vertebrates. The corresponding junctions that occur in invertebrates
are septate junctions.
Structure Tight junctions are composed of a branching network of sealing, independently
acting strands, each strand from the others. Therefore, the efficiency of the
junction in preventing ion passage increases exponentially with the number of
strands. Their proteins are associated with membrane proteins located on the
intracellular side of the plasma membrane which anchors the strands to the
actin cytoskeleton. Thus, tight junctions join together the cytoskeletons of
adjacent cells. Functions They
perform three vital functions. They hold cells together. They block the
movement of integral membrane proteins between the apical and basolateral
surfaces of the cell, allowing the specialized functions of each surface to be
preserved. This aims to preserve transcellular transport. Finally, they prevent
the passage of molecules and ions through the intracellular space between
cells. So, materials must actually enter the cells (by diffusion or active
transport) in order to pass through the tissue. Active transport is at the expenditure of metabolic energy, often in the form of ATP,
to move molecules "uphill" against a concentration gradient or electric
potential.
This pathway controls what substances are allowed to
pass. Tight junctions play this role in maintaining the blood-brain barrier and
blood-retina barrier. For example, L-DOPA, the
precursor of dopamine, can cross the blood-brain barrier, whereas dopamine itself cannot. Therefore, L-DOPA is
administered for dopamine deficiencies (e.g., Parkinson’s disease) rather than
dopamine).
Epithelia are
classed as 'tight' or 'leaky' depending on the ability of the tight junctions
to prevent water and solute movement:
Tight epithelia have tight junctions that prevent most movement between cells. An example
of a tight epithelium is the distal convolutes tubule, part of the nephron in
the kidney. Leaky epithelia do
not have these tight junctions.
References
Gibson JR, Beierlein M, Connors
BW. Functional properties of electrical synapses between inhibitory interneurons of neocortical layer 4. J
Neurophysiol. 2005 93:467-80.
Hormuzdi SG et al. Electrical synapses: a dynamic signaling system that shapes the
activity of neuronal networks. Biochim
Biophys Acta. 20041662:113-37.
Kandel ER, Schwartz JH, Jessell TM. Principles of Neural Science, 4th ed., pp.178-180.