Progress in brain research
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Historical Article
Central control of information transmission through the intraspinal arborizations of sensory fibers examined 100 years after Ramón y Cajal.
About 100 years ago, Santiago Ramón y Cajal reported that sensory fibers entering the spinal cord have ascending and descending branches, and that each of them sends collaterals to the gray matter where they have profuse ramifications. To him this was a fundamental discovery and proposed that the intraspinal branches of the sensory fibers were "centripetal conductors by which sensory excitation is propagated to the various neurons in the gray matter". In addition, he assumed that "conduction of excitation within the intraspinal arborizations of the afferent fibers would be proportional to the diameters of the conductors", and that excitation would preferentially flow through the coarsest branches. ⋯ The PAD produced by single, or by small groups of GABAergic interneurons in group I muscle afferents, can remain confined to some sets of intraspinal arborizations of the afferent fibers and not spread to nearby collaterals. In muscle spindle afferents this local character of PAD allows cutaneous and descending inputs to differentially inhibit the PAD in segmental and ascending collaterals of individual fibers, which may be an effective way to decouple the information flow arising from common sensory inputs. This feature appears to play an important role in the selection of information flow in muscle spindles that occurs at the onset of voluntary contractions in humans.
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An impressive array of neural processing appears to be dedicated to the extraction of reward-related information from environmental stimuli and use of this information in the generation of goal-directed behaviors. While other structures are certainly involved in these processes, the characteristics of activations seen in mesencephalic dopamine neurons, striatal neurons and neurons of the orbitofrontal cortex provide distinct examples of the different ways in which reward-related information is processed. In addition, the differences in activations seen in these three regions demonstrate the different roles they may play in goal-directed behavior. ⋯ As such they would be of use for the execution of appropriate goal-directed behaviors in response to known environmental stimuli, as well as for generating behaviors in response to novel stimuli that may be associated with desirable goals. Neuronal activations in the orbitofrontal cortex appear to involve less integration of behavioral and reward-related information, but rather incorporate another aspect of reward, the relative motivational significance of different rewards. These activations would serve a function similar to those striatal neurons that encode exclusively reward-related information in situations in which only a single outcome is obtainable. (ABSTRACT TRUNCATED)