les5_20102008 - 2.5.1.4 Excitatory and inhibitory signals...

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54 2.5.1.4 Excitatory and inhibitory signals are integrated into a single response by the cell In the central nervous system a single neuron can receive inputs from thousands of other neurons. Several thousand nerve terminals, for example, make synapses on an average motor neuron in the spinal cord; its cell body and dendrites are almost completely covered with them. Some of these synapses transmit signals from the brain or spinal cord, others bring sensory information from muscles or from the skin. The motor neuron must combine the information received from all these sources and react either by firing action potentials along its axon or by remaining quiet. Neuronal integration reflects at the level of the cell the task that confronts the nervous system as a whole: decision making . A cell at any given moment has two options: to fire or not to fire an action potential. Of the many synapses on a neuron, some will tend to excite it, others to inhibit it. Neurotransmitter released at an excitatory synapse causes a small depolarization in the postsynaptic membrane called an excitatory postsynaptic potential (EPSP). The EPSPs produced in a motor neuron by most stretch-sensitive afferent neurons are only 0.2-0.4 mV in amplitude. If the EPSPs generated in a single motor neuron were to sum linearly, at least 25 afferent neurons would have to fire together in order to depolarize the trigger zone by the 10 mV required to reach threshold. At the same time the postsynaptic cell is receiving excitatory inputs, it may also be receiving inhibitory inputs that tend to prevent the firing of action potentials. The net effect of the inputs at any individual excitatory of inhibitory synapse will therefore depend on several factors: location, size, shape of the synapse, and the proximity and relative strength of other synergistic or antagonistic synapses. Because neuronal integration involves the summation of synaptic potentials that spread passively to the trigger zone, it is crucially affected by two passive membrane properties of the neuron: 1. the time constant helps to determine the time course of the synaptic potential and thereby affects temporal summation, the process by which consecutive synaptic potentials at the same site are added together in the post-synaptic cell. 2. the length constant of the cell determines the degree to which a depolarizing current decreases as it spreads passively. In cells with a larger length constant, signals spread to the trigger zone with minimal decrement, in cells with a small length constant the signals decay rapidly with distance. Because the membrane of the dendrites and cell body of most neurons contains few voltage-gated Na+ channels, an individual EPSP generally does not trigger an action potential. Instead, each incoming signal is reflected in a local PSP of graded magnitude, which decreases with distance from the site of the synapse. If signals arrive simultaneously at several synapses in the same region of the dendritic tree, the
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les5_20102008 - 2.5.1.4 Excitatory and inhibitory signals...

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