Neuronal and Cellular Signalling Lecture 3 Notes: Synaptic Communication Electrical Synapses and Gap Junctions - Synapses are specialised points of communication between a presynaptic (neuronal) and postsynaptic cell. Synapses are information-processing and information-relaying devices. Synaptic connections occur between: (A) ELECTRICAL SYNAPSE Microtubule Presynaptic neuron Cytoplasm (a) Two nerve cells (b) A nerve cell and muscle cell (c) A nerve cell and gland cell (d) A nerve cell and a sensory cell Mitochondrion - Synapses can either be chemical or electrical. Both types of synapses relay information but do so via different mechanisms. Electrical synapses are: (a) Fast (b) Simple (c) Bi-directional (d) Passive flow - In neurons, electrical synapses are the functional correlate of gap junctions - Gap junctions are specialised intercellular channels that permit selective direct cell-to-cell transfer of small molecules (e.g. ions, small metabolites). They are found in virtually all tissues and cells (i.e. have roles in development, homeostasis, regeneration, etc.). In the nervous system, gap junctions are found in both neurons and glia. Postsynaptic neuron Gap junction Ions flow through gap junction channels Presynaptic membrane Postsynaptic membrane Gap junction channels Gap Junction Connexin Connexon (hemichannel) Intercellular channel 1 Axia channe Homomeric or heteromeric Kie, et øl, 2015 1 6 connexins form 1 connexon/hemichannel, 2 of which form a gap junction. A hemichannel can be homomeric (identical subunits) or heteromeric (non-identical subunits). The intercellular channel is inserted within the membrane and is connected to another hemichannel in a neighbouring cell membrane (i.e. they meet in the middle of the extracellular space). - The diameter of a gap junction is around 1.5nm, allowing it to fit within the small gap between cells (2-4nm). Small molecules up to around 1.2kDa can cross via these junctions (e.g. Na+, K+, cAMP, sucrose, small peptides). - Electrical synapses are able to pass molecules and ions bidirectionally or unidirectionally. They are open to the passage of ions from both sides of the membrane and remain open at all times to allow ions to move down their electro-chemical concentration gradients (i.e. bi- directional). - To cause uni-directional flow, there will be a non-identical (heterotypic) hemichannel which is voltage-gated and can respond to changes in electrical current across the membrane.
- To investigate whether gap junctions are bi-directional, an electrode is inserted into two cells. The depolarising current injected into cell A depolarises both cells however, a hyperpolarising current only depolarises cell A. This suggests that gap junctions have preferences for certain currents. - Electrical synapses are mostly involved in the escape response and synchronisation. Synchronisation is important in neurons (oscillations are important in learning and memory), smooth muscle (many cells contracting together e.g. in the uterus during labour) and cardiac muscle (AP spreads quickly allowing myocardium to contract as a unit). - Electrical synpases are adapted to transmit signals quickly. This is important for the escape response circuit; the 'jump-and-flight' response in D. melanogaster involves the dedicated Giant Fibre System (GFS) between the neurons and (jump/flight) muscle cells, made up of electrical, chemical and mixed