Seminars in Neuroscience最新文献

In-vivo imaging of the development of complex retinotectal axon arbors indicates that arbors branches can form and extend in the absence of growth cones. A variety of imaging protocols were used to observe arbor elaboration over a range of time intervals and total observation periods. Growth cones are observed relatively rarely in the growing axon arbors. Branch addition, or backbranching, and branch extension occur without apparent specializations at growing branchtips. Branch addition within the elaborating axon arbor is discussed with respect to the process of interstitial branching of axon collaterals.

During axonal pathfinding, the direction of nerve fiber extension is established by the growth cone, the motile structure at the distal tip of an elongating axon. It is the growth cone that navigates and directs axonal outgrowth by detecting and responding to complex molecular cues in the nervous system environment. Changes in growth cone behavior and morphology that result from contact with these cues depend on the regulated assembly and dynamic reorganization of actin filaments and microtubules. Therefore, an understanding of growth cone guidance requires resolution of the cytoskeletal rearrangements that occur as navigating growth cones respond to stimulatory and inhibitory molecular signals in their milieu. In this review, we discuss the role of the cytoskeleton in growth cone navigation.

Axons in the adult central nervous system (CNS) of higher vertebrates are in general not capable of regeneration after injury. This is in contrast to the situation in lower vertebrates (fish and in part amphibia) and the mammalian peripheral nervous system (PNS), where severed axons can regenerate, correct synaptic connections can be formed again, and function can be restored. This enigma has been the subject of extensive studies in the last decades and a large amount of data has been accumulated. This article reviews recent developments in experimental approaches to axonal regrowth in the mammalian CNS focusing mostly on in-vivo systems.

The distinction between memory and habit has been of great historical importance in theories of animal learning. The reality of animal memory is now firmly established, but it is only recently becoming clear how memory systems can influence action. In explaining the interactions between memory systems and action systems it is no longer necessary to invoke the concept of habit, since the same mechanisms appear to apply to habitual action and to non-habitual action. A review of recent ablation studies shows that, both in visual reward-association memory and visual recognition memory, the visual association cortex controls action through a direct output to the corpus striatum.

Experimental lesion studies in monkeys have demonstrated that the cortical areas surrounding the hippocampus, including the entorhinal, perirhinal and parahippocampal cortices play an important role in declarative memory (i.e. memory for facts and events). A series of neuroanatomical studies, motivated in part by the lesion studies, have shown that the macaque monkey entorhinal, perirhinal and parahippocampal cortices are polymodal association areas that each receive distinctive complements of cortical inputs. These areas also have extensive interconnections with other brain areas implicated in non-declarative forms of memory including the amygdala and striatum. This pattern of connections is consistent with the idea that the entorhinal, perirhinal and parahippocampal cortices may participate in a larger network of structures that integrates information across memory systems.

Recent studies examining the neural substrates of stimulus memory in monkeys have found that the ‘rhinal’ cortex (i.e. the entorhinal and perirhinal cortex), makes a pivotal contribution to memory. Indeed, the rhinal cortex appears to be the only critical medial temporal lobe structure for stimulus recognition and certain kinds of associative memory as well. Thus, the mnemonic contributions of certain medial temporal structures, especially the amygdala and hippocampus, appear to have been overemphasized, and should be reconsidered.

Recognition memory requires identification of a stimulus plus judgement concerning its prior occurrence. Information concerning the prior occurrence of stimuli is signalled by neurons in the anterior inferior temporal and perirhinal cortex. Typically these neurons respond less when a visual stimulus is seen for the second time than when it is first seen, even if the two occurrences of the stimulus are widely separated in time. The properties of such responses, together with those of similar responses in related brain regions, are discussed in relation to recognition memory.

Recent studies of the mnemonic effects of selective lesions within the lateral frontal cortex in the monkey have suggested that the mid-dorsolateral region may be a specialized system for the monitoring and manipulation of information within working memory. This impairment appears against a background of normal performance on several basic mnemonic tasks. By contrast, a more severe impairment follows damage to the mid-ventrolateral frontal region. Functional activation studies with normal human subjects have confirmed the suggestion from the animal work that the mid-dorsolateral region is involved in the monitoring and manipulation of information within working memory.