Structural Evidence in the Brain for a Theory of the Evolution of Behavior

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STRUCTURAL EVIDENCE IN THE BRAIN FOR A THEORY OF THE EVOLUTION OF BEHAVIOR E. LLOYD DuBRUL, Ph.D.* I. The Thesis The gross structural sequence ofanimal evolution can be decided directly from fossil bones. But inevitably this poses the crucial question: What phylogenetic changes must concur in neural structures to qualify behavior to exploit these adapting peripheral parts'? The overt behavior of an animal is essentially some sort of muscular activity. But, because muscles move bones at the command of neurons, the logical sequence of study should be: bone-muscle-nerve. Though muscles cannot be studied directly in the fossil, the liaison between bone and muscle is imprinted as the scar ofattachment on the bone, so something can be said about the size, power, and action of a muscle. Furthermore, something is known about the structure and function ofthe muscle sense-system called "proprioceptive." It serves as an error-controlled regulator to steer muscle activity—-it is the same, inprinciple, as the electronic engineer's "servomechanism." Its sense organs are the muscle spindle and the Golgi tendon organ. When a muscle is stretched, both organs are under tension and both fire. When the muscle contracts, only the tendon organ is under tensionand only it fires. Fibers from these receptors run back to synapse directly on the motor nerves that control this muscle. Collaterals from these fibers are wired to motor nerves of associated muscles. The motor nerve in question is also receiving collaterals from the afférents of these other muscles. It is obvious that complicated combinations ofinputs from all these receptors in the periphery can feed * University of Illinois Colleges of Medicine and Dentistry, 1853 West Polk Street, Chicago 12, Illinois. 40 E. Lloyd DuBrul · Theory ofthe Evolution ofBehavior Perspectives in Biology and Medicine · Autumn i960 back a great variety ofinformation to the single motor neuron in the central nervous system (i). But information from a great many sources other than muscles converges on a motor cell. This can be seen structurally as copious boutons terminaux plugged to the surface ofa cell and its dendrites. More than a thousand have been counted on a single cell (2), and the motor nerve acts under the control of this knowing. The crucial thing about a motor nerve is that it can do only one thing: it can cause its muscle to contract. Also, because it is the sole connecting link between the central nervous system and the muscle, the motor nerve has been appropriately called "the final common path" (3). The stereotyped behavior ofthis relatively simple system has been demonstrated in decerebrated cats. When a piece ofmeat touches the mouth, thejaw drops sharply. This stretches thejaw-clamping muscles, and their receptors feed this information back to the motor neurons in the brain stem. The stimulated motor nerves fire and the muscles contract. As the teeth cut through the resistant meat, pressure is transmitted through the food to the teeth and then to their sockets in thejaw via the roots. Special receptors (4) found in the periodontium are now stimulated. These receptors also signal their information back to the motor nerves which inhibits the firing of these motor fibers and therefore releases the closing muscles from further contraction. The already tense antagonists are now freed, and thejaw opens spasmodically. This stretches the closing muscles, and again they are brought under the influence ofthe stretch receptors so that both excitatory and inhibitory circuits reverberate until there is nothing "biteable between thejaws" (5). There is generally a consistent ratio between nerves and muscles. Postural muscles like those ofthejaw have a nerve-muscle fiber ratio ofabout ? :20?. Furthermore, each so-called motor nerve has some 30 to nearly 50 per cent ofproprioceptorfibersrunning in it (6). Thereis, then, apowerful input ofmuscle information to the motor nerve (or final common path) to these muscles. I propose here that the specific information fed back from the periphery by the proprioceptors dominates in steering the activity of the motor nerve—hence its muscle—in the sort of stereotyped behavior just described. The structural sequence in human evolution can be used as a "natural experiment" to bring these notions to bear on problems of the evolution 41 ofbehavior in general...

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