ReviewBranched thalamic afferents: What are the messages that they relay to the cortex?
Research Highlights
►Most axons carrying messages for thalamic relay to cortex have a branch going to a motor center. ►Some of these axons come from ascending pathways, others come from cortex. ►The thalamic branches, therefore, also carry copies of current motor instructions and ►all thalamic relays pass information to cortex about ongoing messages for motor control. ►Thalamocortical messages are about ongoing senorimotor contingencies.
Section snippets
Introduction: early evidence from Golgi preparations
The patterns of axonal branching revealed by the Golgi method and described in significant detail by Cajal form the subject of this essay. They were early recognized as important. The Golgi method allowed investigators to trace axons for long distances, find branching points and often trace individual branches to several distinct end-stations, demonstrating functionally significant links. However, the full functional implications of these branching patterns could not be understood until the
Relating the ascending “sensory” axon to the innervation of spinal motor centers
Fig. 1 shows one of Cajal's drawings of the intraspinal branching patterns of the dorsal root inputs and Fig. 2 shows another of his drawings for the trigeminal nerve. In Fig. 1 the axons ascending towards the top of the figure on their way to the posterior column nuclei are shown with many spinal branches within the cord. Fig. 2 shows that nerve cells of the spinal nucleus of the trigeminal nerve (F) send axons towards the top of the figure on their way to the thalamus and also innervate two
Efference copy or corollary discharge
Copies of motor instructions have a long history in neuroscience (see Grüsser, 1995). They have played an important role in accounting for an organism's ability to distinguish whether changes in sensory inputs are produced by changes in the environment or by movements produced by the organism that affect how a receptor responds to stimuli: a good example is in the effects that eye movements have on retinal receptors. In 1950 two important articles (Sperry, 1950, von Holst and Mittelstaedt, 1950
The common occurrence of branched driver inputs to the thalamus and their possible functional significance
For the ascending spinal pathways considered so far there is no question that the message they carry is relayed to the cortex. Although the terminals of thalamic afferents that carry messages for relay to the cortex, the “drivers” (Sherman and Guillery, 1998), represent only a minority of the afferents in any one thalamic nucleus (generally less than 10%), they have a characteristic light and electron microscopical appearance in all major thalamic nuclei and are characterized by comparable
What happens when these messages reach the cortex?
This question clearly requires an answer. The following represents a suggestion that needs significant experimental study before it can be taken seriously. It is an attempt to follow through the implications of the observations reported so far. If the observations reported in 1 Introduction: early evidence from Golgi preparations, 2 Relating the ascending “sensory” axon to the innervation of spinal motor centers, 3 Efference copy or corollary discharge, 4 The common occurrence of branched
Conclusions
The main point of our review is that, although there are many efference copy messages in the lower motor mechanisms involved in stabilizing the sensory world of a moving organism, the efferent copies that represent most, possibly all, of the driver afferents to the thalamus function primarily to convey to the cortex a model of what the lower levels are doing. They connect to these mechanisms but do not repeat the functions of the efferent copies that characterize lower levels in brainstem and
Acknowledgments
We thank Michele Basso, Jeff Hawkins, Donata Oertel, Leslie Osborne, Luis Populin, Fritz Sommer and Brian Theyel for many helpful comments on an earlier draft. SMS was supported by USPHS Grant DC008794.
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