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Synapses Notes

Pharmacology Notes > BIOL10832 Excitable Cells Notes

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Excitable Cells - Lecture 8 (22/02/2018)

Information Processing in the Nervous System
Synaptic Transmission is carried out by:

Electrical synapses.
Chemical synapses.
Neurotransmitter release.
Vesicle and transmitter recycling.

The problem with Synaptic transmission is the transmission of an electrical signal across a high resistance membrane through an intercellular gap, which has a low resistance, back into a high resistance membrane.

The answer to this is the 'Spark' (electrical continuity) or 'Soup' (chemical synapses).

Electrical Synapses - 'Spark'

Electrical Synapses are made up of many Gap Junctions.

These Gap Junctions conduct the direct transfer of ions and small molecules between cells.

Electrical synapses are bidirectional as the current can pass in both directions.
The advantage of electrical synapses includes fast transmission of a signal, with no synaptic delay.
These electrical synapses are found in all multicellular animals.

Gap junctions allow direct cytoplasmic communication between cells. Each individual gap junction is very small,
only about 0.1-1% of the information is passed through by one individual gap junction

Each channel is made up of two connexons and each connexon unit is made up of six connexin subunits.

Each pore is 1-2 nm wide.

Electrical synapses 

Are fast.
Can be two-way (non-rectifying.)
Often produce one-to-one transmission.
Allow the exchange of other chemicals.
Are used by invertebrates, lower vertebrates and mammals.
Are much less common than chemical synapses.
Are used in fast pathways (eg: escape/defence)
Used to promote synchronous activity within a network.

Electrical synapses are fast and reliable but are capable of only limited plasticity, in contrast to chemical synapses.

An example of a fast escape response includes the Giant fibre system of Drosophila:

Chemical Synaptic Transmission - 'Soup'

Chemical Synaptic Transmission was first demonstrated by Otto Loewi, in 1936, at the frog heart neuromuscular junction.

Acetylcholine is released from the vagus nerve onto the cardiac muscle in order to slow down its heart rate.

The synaptic delay here is around 0.5-2.0 milliseconds.

Otto Loewi displayed in his demonstration how if one heart is stimulated through the Vagus nerve, it can stimulate another heart due to its release of Acetylcholine, which acts as a chemical transmitter.

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