Direct and indirect sensory input pathways to the motor cortex; Its structure and function in relation to learning of motor skills.

Abstract

It is already more than a century since FRITSCH and HITZIG [32] discovered the motor cortex in the dog. The discovery was so sensational that a vast number of experiments were carried out to further elucidate cortical motor function during the following years. In the beginning the effort was focused on delineating the localization of motor function within the motor cortex. Little attention was paid to the sensory input to the motor cortex because suitable techniques for studying it were lacking. With the progress and development of electrophysiological technique, MARSHALL et al.[53] succeeded in demonstrating evoked potentials in the monkey sensory cortex in response to tactile stimulation. Shortly following this report, ADRIAN and MoRuzzi [1] observed increase of impulses in the medullary pyramid in response to sensory stimulation in lightly anesthetized cats. The latter result suggested that sensory impulses which arrived at the motor cortex could have activated pyramidal tract cells although there was still a possibility that these PT cells were located in the sensory cortex. Several decades later, aided by the newly developed closed chamber method, MOUNTCASTLE [55] examined the details of sensory input to the somatic sensory cortex in unanesthetized cats. He found that each neuron receives precise epicritic information arising from a particular part of the body. Shortly after that discovery, BUSER and IMBERT [22] and BROOKS et al. [21] reported similar results in the cat motor cortex.We subsequently have found that there is a tight coupling between the afferent input to and the efferent outflow from the motor cortex in the cat [8]. In the monkey, however, the results were controversial. While POWELL and MOUNTCASTLE [65] demonstrated that the sensory cortex receives finely grained epicritic input, others [2,29] reported that this was not the case for the motor cortex. It was reported that the motor cortex receives input mainly from deep receptors, and not the well-localized tactile input which impinges onto the sensory cortex. These early findings differed from our own observations. We found that the monkey motor cortex also receives precise epicritic input from the periphery and that the inputoutput relationship in motor cortical neurons was precisely organized [57, 67]. From

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