Each of the descending pathways involved in motor control has a number of anatomical, molecular, pharmacological, and neuroinformatic characteristics. They are differentially involved in motor control, a process that results from operations involving the entire motor network rather than from the brain commanding the spinal cord. A given pathway can have many functional roles. This review explores to what extent descending pathways are highly conserved across species and concludes that there are actually rather widespread species differences, for example, in the transmission of information from the corticospinal tract to upper limb motoneurons. The significance of direct, corticomotoneuronal (CM) connections, which were discovered a little more than 50 years ago, is reassessed. I conclude that although these connections operate in parallel with other less direct linkages to motoneurons, CM influence is significant and may subserve some special functions including adaptive motor behaviors involving the distal extremities.

Descending pathways in motor control

Locomotion results from intricate dynamic interactions between a central program and feedback mechanisms. The central program relies fundamentally on a genetically determined spinal circuitry (central pattern generator) capable of generating the basic locomotor pattern and on various descending pathways that can trigger, stop, and steer locomotion. The feedback originates from muscles and skin afferents as well as from special senses (vision, audition, vestibular) and dynamically adapts the locomotor pattern to the requirements of the environment. The dynamic interactions are ensured by modulating transmission in locomotor pathways in a state- and phase-dependent manner. For instance, proprioceptive inputs from extensors can, during stance, adjust the timing and amplitude of muscle activities of the limbs to the speed of locomotion but be silenced during the opposite phase of the cycle. Similarly, skin afferents participate predominantly in the correction of limb and foot placement during stance on uneven terrain, but skin stimuli can evoke different types of responses depending on when they occur within the step cycle. Similarly, stimulation of descending pathways may affect the locomotor pattern in only certain phases of the step cycle. Section ii reviews dynamic sensorimotor interactions mainly through spinal pathways. Section iii describes how similar sensory inputs from the spinal or supraspinal levels can modify locomotion through descending pathways. The sensorimotor interactions occur obviously at several levels of the nervous system. Section iv summarizes presynaptic, interneuronal, and motoneuronal mechanisms that are common at these various levels. Together these mechanisms contribute to the continuous dynamic adjustment of sensorimotor interactions, ensuring that the central program and feedback mechanisms are congruous during locomotion.

https://www.physiology.org/doi/pdf/10.1152/physrev.00028.2005

Dynamic Sensorimotor Interactions in Locomotion

Interneuronal relay in spinal pathways from proprioceptors

I. I N T R O D U C T I O N The study of the behaviour of small networks of neurones frequently requires the determination of measures of the strength of association between component neurones, an assessment of their timing relations, and the identification of which neurones may interact directly or are influenced by common inputs. In many of these studies the principal quantities available for analysis are the sequences of extracellularly recorded action potentials (neuronal spike trains). The subsequent analytical work is then based entirely on the relations between the times of occurrence of the action potentials recorded from different neurones. In these circumstances neuronal spike trains are frequently represented as the mathematical entity known as a stochastic point process. These processes are described by providing a probability law for a set of ordered times

The Fourier approach to the identification of functional coupling between neuronal spike trains.

ical, and computational techniques have provided the tools to begin uncovering concepts underlying neural network function. This review is a summary of insights into the organization and operation of neural circuits acquired through application of these techniques to invertebrate and vertebrate systems. This review is not a survey of the vast variety of known neural networks but rather concentrates on emerging concepts applicable to net­ works in general. The first section provides a brief historical perspective. The second and third sections summarize two emerging concepts about the operation and modulation of neural circuits. The first concept is that the operation of a neural network depends upon interactions among mul­ tiple nonlinear processes at the cellular, synaptic, and network levels. The second concept is that modulation of these underlying processes can alter network operation. The final section suggests potentially profitable research directions for the future.

http://www.iiserpune.ac.in/~raghav/pdfs/animalbehavior/ReadingList/getting_1989.pdf

Emerging principles governing the operation of neural networks.

The sections in this article are:


1
Methodological Considerations
1.1
Selective Stimulation of Primary Afferents

1.2
Stimulation of Central Motor Systems

1.3
Methods for Investigation of Convergence at Interneuronal Level




2
Spinal Neuronal Circuits Used in Common by Segmental Afferents and Supraspinal Motor Centers
2.1
Recurrent Inhibition

2.2
Pathways From Ia-Afferents and Their Control by γ-Motoneurons

2.3
Reflex Pathways From Group Ib Tendon Organ Afferents

2.4
Reflex Pathways From Cutaneous and Joint Afferents and From Groups II and III Muscle Afferents

2.5
Propriospinal Neurons

2.6
Presynaptic Inhibition of Transmission From Primary Afferents




3
Reticulospinal Inhibition of Segmental Reflex Transmission
3.1
Dorsal Reticulospinal System

3.2
Ventral Reticulospinal Pathways

3.3
Monoaminergic Reticulospinal Pathways

3.4
Decerebrate Preparation




4
Direct Projections of Descending Pathways to α-Motoneurons

5
Ascending Pathways that Monitor Segmental Interneuronal Activity
5.1
Evidence That Ascending FRA Pathways Monitor Activity in Interneurons of Reflex Pathways

5.2
Information Via Ascending Collaterals of Interneurons

5.3
Ventral Flexor Reflex Tracts

5.4
Ventral Spinocerebellar Tract




6
General Summary and Epilogue
6.1
General Summary

6.2
Epilogue

Fausto Baldissera

Papers

Integration in Spinal Neuronal Systems