Junctions and Synapses Flashcards
Describe the structure of the Gap Junction?
The Gap junction is useful for DIRECT cell to cell contact
They are composed of connexins; connexins form connEXONs
Many connexons DOCK together to form a channel; many connexons, many channels.
Consists of 4 connexin subunits, Nh3 and a C00 terminal, cytoplasmic loop, extracellular loopswith cysteines (and sulphurs) which allow other molecules to adhere and communicate with the cell.
Channels are VERY DENSELY PACKED full of connexons.
They are permeable to small organic molecules, dyes, metabolites (glucose) and inorganic ions (k, Na, cl-)
What is the importance of Gap Junctions?
Diseases may arise if gene that code for Connexin protein are mutated.
Connexon26 is needed for glucose intake by fetus in pregnancy.
Can have cardiovascular, myelin related, skin diseases
They have a great influence in cell physiology as they contain thousands of channels.
They are found everywhere; they are ubiquitous (Striated muscles only cells not to have gap junction.)
What are the simplified differences between electrical and chemical synapses?
Chemical: involves transmitter to travel across the synapse (needs protein vescicle for docking and release) but in electrical contact is direct (gap junctions)
In chemical: post synaptic cells are different to pre synapses but in electrical: post and pre synapses are identical.
Transmissionis aster in electrical synapses (no delay) as there is direct neurone coupling. There is a delay in chemical synapses with the added decrease if Ca is lacking or Mg is high and inhibition by antagonists.
Electrical synapses can pass Subhreshold current but chemical synapses must have a threshold AP to impact postsynaptic cell.
What are the properties of electrical synapses?
An electrical synapse is a mechanical and electrically conductive gap junction between two neurons that is formed at a narrow gap between the pre- and postsynaptic neurons. (neurone-neurone gap)
It is composed of connexin C36.
What are the defects shown in Cx36 mice?
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How are gap junction made complex?
There are 20 different connexin genes.
Junctions can be made of heteromeric connexons where it is made of lots of different types of connexin subunits, these in turn make up varied channels and whole junctions.
Give some details of Gap junctions mediating bidirectional signaling between oocytes and granulosa cells.
In mature follicles, the oocyte, suspended in the fluid of the follicular antrum, becomes surrounded by a dense mass of granulosa cells.
Cells of the innermost layer of the cumulus extend processes through the zona pellucida (ZP), around the oocyte, forming both intermediate junctions and gap junctions with the oocyte
Gap junction channels between neighbouring granulosa cells are composed, which seems to be absent from the junctional sites linking granulosa cells to the oocyte
The oocyte provides Cx37 to form its half (connexon) of a gap junction channel with granulosa cells.
Describe the By-stander effect and its significance?
Junctions create a network of cells; one cell dies, they all die due to the interconnections
In tumorigenesis, therapeutic cancer cell killing is enhanced by this effect. Through the gene delivery of herpes simplex virus thymidine kinase (HSV-TK) followed by the administration of the pro-drug ganciclovir(GCV) is allowed passage to the surrounding cells; Only some cancer cells express required HSV-TK (incorporated into gene) and so without connections only individual cells would die.
Activated GCV mimics gluanosine and incorporated into DNA, stopping replication and causing apoptosis.
History of electrical synapses, what were the two doctrines were presented?
Neuron doctrine: there are gaps using chemical communication
Reticular doctrine: there is direct communication via junctions with no gaps.
Gap Junctions were discovered in 1967.
What is achieved by removing individual proteins from junctions and assaying?
Allows examination of abnormal synaptic function is assayed by a variety of techniques including electrical recordings, electron microscopy, confocal microscopy, live imaging.
What was show by Injecting current in the electrical synapse?
What is a spikelets?
What is the coupling coefficient?
It showed that electrical synapses are sign preserving; they can excite or inhibit depending on current; no preference for depolarizing or hyperpolarizing responses.
If positive current was injected, MP was depolarised giving EPSP (EXCITORY postsynaptic potential)
If negative current was injected, MP hyperpolarised giving INHIBITORY PSP
spikelets: Strongly attenuated postsynaptic responses formed from AP.
Coupling coefficient: The ratio between the voltage change observed in the non-injected and the injected neurons.
Detailed differences between Chemical and Electrical Synapses
1) Structural differences; close opposition of membranes for electrical
2) Electrical synapses activate faster than chemical ones: synaptic delay
3) Electrical transmission is ionic current whereas chemical requires neurotransmitter release and binding
4) Electrical synapses are almost always bidirectional; chemical synapses not.
5) Electrical synapses are sign preserving; chemical are not
6) Electrical synapses are reliable; reliability at chemical synapses varies
7) Chemical synapses are metabolically expensive.
What do the properties of electrical synapses mean
Electrical Synapses create a network of synchronously coactive neurons.
What does sign preserving mean?
depolarisation will always cause depolarisation
