BS2550 - Neuronal and Cellular Signalling Lecture 4 Synaptic Communication Synapses are specialized points of communication between a presynaptic (neuronal) and postsynaptic cell. Synaptic connections occur between: - nerve cell -> nerve cell - nerve cell -> muscle cell - nerve cell -> gland cell - sensory cell -> nerve cell Synapses are information-processing and -relaying devices. There are 2 types of synapses > chemical and electrical. Much more is known about chemical than electrical synapses. Both synapses relay information but work by very different mechanisms. Electrical synapses: - Simple - Fast - Bidirectional - Passive flow Chemical synapses: - Complex - Slow - Unidirectional - No intercellular continuity - Ligand-gated ion receptors Electrical Synapses and Gap Junctions (A] ELECTRICALSYNAPSE (B) CHEMICAL SYNAPSE Microtubule Presynaptic neuron Cytoplasm Presynaptic neuron Synaptic vesicle Mitochondrion Postsynaptic neuron Postsynaptic neuron Gap junction Ions flow through gap junction channels Presynaptic membrane Neurotransmitter released Synaptic vesicle fusing. Prosynaptic membrane Synaptic. deft Postsynaptic membrane Gap junction channels Postsynaptic neurotransmitter Postsynaptic membrane Ions flow through postsynaptic channels receptor Gap junctions are specialised intercellular channels that permit direct cell-cell transfer of small molecules (ionsff small metabolites) not DNA or big molecules like it. They are found in virtually all tissues and cellsff they have roles in developmentff homeostasisff and regeneration. In the nervous systemff gap junctions are present in the neurons and glia. 6 connexins create a connexon (hemichannel) -> homomeric or heteromeric. 2 hemichannels form a gap junction. These hemichannels meet in the middle of extracellular space -> diameter of GJs ~1.5nm. The gap size is 2- 4nm. Sodium and potassium ionsff cAMPff sucroseff small peptides etc can cross (up to 1-2kDa).
In neuronsff electrical synapses are the functional correlate of gap junctions. Electrical synapses can work bidirectional or unidirectional (rectifying). The synapses that have different hemichannels are called heteromeric. To figure out whether the electrical synapse is bidirectional or unidirectional is to inject a depolarizing current in cell Aff it should depolarize cell A and cell B. Hyperpolarizing currents hyperpolarize only cell A. Soff if only one cell (cell A) is depolarized and hyperpolarized it means its unidirectional as the hyperpolarizing current cannot pass into cell B. Main roles of electrical synapses/gap junctions: - Escape response - Synchronisation -> Oscillations in neurons are important in learning and memory. Smooth muscle (many cells contacting together like the uterus in labour) and the cardiac muscle (APs spread fast allowing the myocardium to contract as a unit). Electrical synapses are built for speed. It can trigger a jump/flight response in the escape response circuit of D. Melanogaster the giant fibre system (GFS). GFS synapses are a mix of electrical and chemical synapses. By sticking an electrode in cell 2 and cell 1 you can see the synaptic delay between each cell. It is usually less than 0.1ms. Synchronised firing shows cell B responding almost simultaneously when action potentials are fired in cell A. Synchronized firing 0 Presynaptic element Record Vm of cell 1 -25 -50 Gap junction Record Vm of