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The lowest level of this modifying intermediate network is the spinal cord. The cord still possesses many features that were first developed in the segmented earthworm. It is largely made up of neurons completely contained within it, which form bridges between the sensory and motor elements throughout the whole body. Each peripheral nerve trunk still innervates a specific segment of the body, and still joins the cord at a specific level, creating a ganglion. Sensory signals entering into a single segment may be processed by its own ganglion, and cause localized motor response within the segment; or the signals may pass to adjacent segments, or be carried even further up or down the line, involving more ganglia in a more widely distributed response. In this way, the cord can monitor a large number of sensorimotor reactions without having to send signals all the way up to the brain. Thus stereotyped responses can be made without our having to “think” about them on a conscious level. Most of these localized and segmentally patterned responses are not the result of experience or training, but of genetically consistent wiring patterns in the internuncial network of the cord itself. These basic wiring patterns unfold in the foetus during the “mapping” process of the nervous system, and they have been pre-established by millions of years of development and usage. The spinal cord can be surgically sectioned from the higher regions of the internuncial net, and the experimental animal kept alive, so that we can isolate the range of responses that are primarily controlled by these cord reflexes. Almost all segmentally localized responses can be elicited, such as the knee jerk caused by tapping the tendon below the knee cap, or the elbow jerk caused by tapping the bicep tendon. These simple responses can also be spread into other segments, so that a painful prick on a limb causes the whole body to jerk away in a general withdrawal reflex. The bladder and rectum can be evacuated. A skin irritation elicits scratching, and the disturbance can be accurately located with a paw. Some of the basic postural and locomotive reflex patterns seem to reside in the wiring of the cord as well. If an animal with only its cord intact is assisted in getting up, it can remain standing on its own. The sensory signals from the pressure on the bottoms of the feet are evidently enough to trigger postural contractions throughout the body and hold the animal in the stance typical of its species. And if the animal is suspended with its legs dangling down, they will spontaneously initiate walking or running movements, indicating that the fundamental sequential arrangements of the basic reflexes necessary for walking are in the cord also. All of these localized and intersegmental responses are rapid and automatic, follow specific routes through the spinal circuitry, and elicit stereotyped patterns of muscular response. Most of them appear to consistently use the same neurons, synapses, and motor units every time they are initiated.
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