les4_13102008 - Synaptic transmission 1. Introduction...

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28 Synaptic transmission 1. Introduction Synaptic transmission is the process by which nerve cells signal one another. An average neuron forms and receives about 1000 synaptic connections and the human brain contains at least 10E11 neurons, thus 10E14 synaptic connections are formed in the brain. There are more neurons and synapses in one brain than the several billion stars in our galaxy! With some exceptions, the synapse consists of the terminus of a presynaptic axon apposed to a postsynaptic cell. Based on the structure of the apposition, synapses are categorized into two major groups: electrical and chemical. At electrical synapses, the presynaptic terminal and the postsynaptic cell are not completely separated and the current generated by an action potential in the presynaptic neuron flows directly into the postsynaptic cell through specialized channels called gap junctions, which physically connect the cytoplasm of the presynaptic and postsynaptic cells. At chemical synapses a cleft separates the two cells and the cells do not communicate through bridging channels. At the chemical synapse, a change in the membrane potential of the presynaptic cell leads to the release of a chemical transmitter from the nerve terminal. The transmitter diffuses across the synaptic cleft and binds to receptor molecules on the postsynaptic membrane, thus opening ion channels through which current flows. Receptors for transmitters can be classified into two major groups depending on how they control ion channels in the postsynaptic cell: 1. ionotropic receptors: ion channels that open when the transmitter binds 2. metabolic receptors: act indirectly on ion channels by activating a second- messenger system within the postsynaptic cell. Both types of receptor can result in excitation or inhibition. The sign of the signal depends on the properties of the receptor with which the transmitter interacts, not on the identity of the transmitter. A single transmitter can produce several distinct effects by activating different types of receptors. Thus receptor diversity permits a relatively small number of transmitters to produce a wide variety of synaptic actions. Most transmitters are low-molecular-weight molecules, but certain peptides also can act as messengers at synapses. 2. Synapses are either electrical or chemical All neurons make use of one of two basic forms of synaptic transmission: electrical or chemical. Moreover, the strength of both forms of synaptic transmission can be enhanced or diminished by cellular activity. This plasticity in nerve cells is crucial to memory and other higher brain functions.
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29 In the brain, electrical synaptic transmission is rapid and rather stereotyped. Electrical synapses are used primarily to send simple depolarizing signals; they do not lend themselves to producing inhibitory actions or making long-lasting changes in the electrical properties of postsynaptic cells. In contrast, chemical synapses are capable of more variable signaling and thus can produce electrical changes in the
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les4_13102008 - Synaptic transmission 1. Introduction...

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