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NROB60 Chapter 5IntroductionImagine you stepped on a thumbtack and how it can be converted into a neural signalFirst specialized ion channels of the sensory nerve endings allow positive charge to enter the axonIf this depolarization reaches threshold then action potentials are generated o Because the axonal membrane is excitable and has voltagegated sodium channels action potentials can propagate without decrement up the long sensory nervesFor this information to be processed by the rest of the nervous system it is necessary that these neural signals be passed on to the other neurons that lead to a coordinated reflex response eg lifting footthBy the end of the 19 century it was recognized that this transfer of information from one neuron to another occurs at specialized sites of contactIn 1897 Charles Sherrington gave these sites their names synapseThe process of information transfer at a synapse is called synaptic transmissionThe physical nature of synaptic transmission was argued over and two prominent hypotheses aroseOne suggested it was electrical current flowing from one neuron to the next o The existence of such electrical synapses was finally proven in 1959 by Edwin Furshpan and David PotterThe other hypothesis suggested it was a chemical transfer of information o This was also proven to be correct with solid support for the concept of chemical synapses by Otto Loewi in 1921Types of Synapses Electrical Synapses o Electrical synapses are relatively simple in structure and function and the allow the direct transfer of ionic current from one cell to the next o They occur at specialized sites called gap junctionsThe membrane of two cells is separated by only about 3nm and this narrow gap is spanned by clusters of special proteins called connexins Six of these combine to form a channel called a connexon and two connexons combine to form a gap junction channelIt allows ions to pass directly from the cytoplasm of one cell to the cytoplasm of the otherThe pore is relatively large 12 nm and is big enough for all the major cellular ions and many small organic molecules to pass throughMost allow ionic current to pass equally well in both directions unlike the majority of chemical synapsesCells connected by gap junctions are electrically coupled o Transmission at the electrical synapses is very fast and if the synapse is large failsafeAn action potential in the presynaptic neuron can produce almost instantaneously and action potential in the postsynaptic neuron o Also occur in the vertebrae brain and is common in every part of the mammalian CNS o When two neurons are electrically coupled an action potential in the presynaptic neuron causes a small amount of ionic current to flow across the gap junction channels into the other neuronCauses a postsynaptic potential PSP in the second neuron o Since most electrical synapses are bidirectional when the second neuron generates an action potential it will in turn induce a PSP in the first neuron o The PSP generated by a single electrical synapses in the mammalian brain is small and may not by large enough to trigger an action potential in the postsynaptic cellOne neuron usually makes electrical synapses with many other neurons so several PSPs occurring simultaneously may strongly excite a neuron synaptic integration o The precise roles of electrical synapses vary from one brain region to anotherChemical SynapsesMost synaptic transmission in the mature human nervous system is chemicalThe presynaptic and postsynaptic membranes at chemical synapses are separated by a synaptic cleft that is 2050 nm wide
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