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4. Spinal Reflexes - SPINAL REFLEXES The circuits...

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SPINAL REFLEXES The circuits underlying spinal reflexes comprise only a small fraction of all spinal circuits. However they are significant because: (a) They illustrate a number of principles that are found throughout the nervous system. (b) The reflexes have important adaptive functions in their own right. (c) The reflex circuits are building blocks for circuits controlling movements. (d) Their presence or absence gives the clinician important information about the level of neurological performance. In considering the circuits involved in spinal reflexes the following facts should be remembered: (a) In a typical segment of the spinal cord, there are 5,000-10,000 motoneurons. They comprise about 5% of all neurons in the segment. The rest are interneurons. Spinal reflexes activate only a small fraction of these cells. (b) A typical large motoneuron has about 10,000 synaptic endings on its dendrites. There are at least as many inhibitory endings as excitatory endings. Each excitatory input makes a small EPSP. No single synapse is capable of firing the cell; summation of many inputs is required. A correlation has been
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