ReviewEncoding and decoding of reticulospinal commands
Introduction
The reticulospinal (RS) system plays an important role in the control of locomotion, steering and posture in vertebrates [23]. In higher vertebrates, the specific contribution of the RS system to the control of motor behavior is difficult to assess since this system operates along with other descending systems, the corticospinal, rubrospinal, and vestibulospinal ones. We have been studying the RS system in the lamprey (a lower vertebrate, cyclostome). This animal has a general organization of the CNS similar to that in higher vertebrates [19]. In contrast to higher vertebrates, however, the RS system in the lamprey is practically the only descending system transmitting commands to the spinal cord and responsible for the initiation of locomotion, for steering, and for postural corrections. The bilateral RS pathways originate from ∼2400 neurons located in four reticular nuclei: the mesencephalic reticular nucleus, and the anterior, middle and posterior rhombencephalic reticular nuclei [1], [4], [26], [28]. RS neurons project ipsilaterally over long distances and affect different classes of spinal neurons [2], [20], [27].
In the present study we first investigated, the commands for different forms of motor activity in the lamprey transmitted by RS neurons. Secondly, we tried to understand how these commands affect the spinal mechanisms.
Section snippets
Commands transmitted by RS neurons
The RS commands were studied by recording the responses of individual RS neurons to various sensory stimuli in the intact lamprey [6], [11], in the semi-intact preparation [10], [16], and in the in vitro preparation [7], [8], [9]. These stimuli evoke different motor behaviors such as locomotion, steering, and postural corrections.
Overlap of descending command systems
The RS system in the lamprey is practically the only descending system to transmit different motor commands to the spinal cord. This raises two questions.
(1) When the same motor pattern is used in different behavioral contexts, or is evoked by different sensory stimuli, are the corresponding motor commands transmitted by the same sub-population of RS neurons or by different sub-populations? An answer to this question for one particular case has been obtained in recent experiments [6]. It was
Decoding of RS commands
We suggest that decoding of RS commands in the spinal cord is based on two principles. (1) The commands for initiation of locomotion and regulation of its vigour, encoded as the value of bilateral RS activity, are decoded by integrating all RS signals arriving to the spinal locomotor networks, so that their integrated value determines the level of activity of these networks. (2) The commands for turns in different planes, encoded as an asymmetry in the activities of specific subdivisions of the
Motor effects of individual RS neurons
A weakness of our models of postural control is that the motor effect of RS neurons was hypothesized rather than determined experimentally. The effects of signals transmitted from the brain to the spinal networks by a population of RS neurons are determined by specific functional projections of individual neurons. In higher vertebrates, these projections are difficult to reveal [13]. We have found that, in the lamprey, motor effects of individual RS neurons can be detected by averaging the
Acknowledgements
This work was supported by grants to T.G.D. from Royal Swedish Academy of Sciences (Research Grant for Swedish–Russian scientific co-operation), Swedish Medical Research Council (no. 11554), and Curt Nilsson Foundation. P.V.Z. was supported by the Wenner-Gren Foundation.
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