A study and model of the role of the Renshaw cell in regulating the transient firing rate of the motoneuron

Influence of proprioceptive feedback on the firing rate and recruitment of motoneurons

We investigated the relationships of the firing rate and maximal recruitment threshold of motoneurons recorded during isometric contraction with the number of spindles in individual muscles. At force levels above 10% of maximal voluntary contraction, the firing rate was inversely related to the number of spindles in a muscle, with the slope of the relationship increasing with force. The maximal recruitment threshold of motor units increased linearly with the number of spindles in the muscle. Thus, muscles with a greater number of spindles had lower firing rates and a greater maximal recruitment threshold. These findings may be explained by a mechanical interaction between muscle fibres and adjacent spindles. During low-level (0% to 10%) voluntary contractions, muscle fibres of recruited motor units produce force twitches that activate nearby spindles to respond with an immediate excitatory feedback that reaches maximal level. As the force increases further, the twitches overlap and tend towards tetanization, the muscle fibres shorten, the spindles slacken, their excitatory firings decrease, and the net excitation to the homonymous motoneurons decreases. Motoneurons of muscles with greater number of spindles receive a greater decrease in excitation which reduces their firing rates, increases their maximal recruitment threshold, and changes the motoneuron recruitment distribution.

Realistic simulation of the Aplysia siphon-withdrawal reflex circuit: roles of circuit elements in producing motor output

The circuitry underlying the Aplysia siphon-elicited siphon-withdrawal reflex has been widely used to study the cellular substrates of simple forms of learning and memory. Nonetheless, the functional roles of the different neurons and synaptic connections modified with learning have yet to be firmly established. In this study we constructed a realistic computer simulation of the best-understood component of this network to better understand how the siphon-withdrawal circuit works. We used an integrate-and-fire scheme to simulate four neuron types (LFS, L29, L30, L34) and 10 synaptic connections. Each of these circuit components was individually constructed to match the mean or typical example of its biological counterpart on the basis of group measurements of each circuit element. Once each cell and synapse was modeled, its free parameters were fixed and not subject to further manipulation. The LFS motor neurons respond to sensory input with a brief phasic burst followed by a long-lasting period of tonic firing. We found that the assembled model network responded to sensory input in a qualitatively similar fashion, suggesting that many of the interneurons important for producing the LFS firing response have now been identified. By selectively removing different circuit elements, we determined the contribution of each of the LFS firing pattern. Our first finding was that the monosynaptic sensory neuron to motor neuron pathway contributed only to the initial brief burst of the LFS firing response, whereas the polysynaptic pathway determined the overall duration of LFS firing. By making more selective deletions, we found that the circuit elements responsible for transforming brief sensory neuron discharges into long-lasting LFS firing were the slow components of the L29-LFS fast/slow excitatory postsynaptic potentials. The inhibitory L30 neurons exerted a significant braking action on the flow of excitatory information through the circuit. Interestingly, L30 lost its ability to reduce the duration of LFS firing at high stimulus intensities. This was found to be due to the intrinsic nature of L30's current-frequency relationship. Some circuit elements, including interneuron L34, and the electrical coupling between L29 and L30 were found to have little impact when subtracted from the network. These results represent a detailed dissection of the functional roles of the different elements of the siphon-elicited siphon-withdrawal circuit in Aplysia. Because many vertebrate and invertebrate circuits perform similar tasks and contain similar information processing elements, aspects of these results may be of general significance for understanding the function of motor networks. In addition, because several sites in this network store learning-related information, these results are relevant to elucidating the functional significance of the distributed storage of learned information in Aplysia.

An improved muscle-reflex actuator for use in large-scale neuro-musculoskeletal models

This paper extends the systematic approach described in Winters and Stark (62) for developing muscle models. The underlying motivation is our finding that for larger scale shoulder and head-neck postural systems to be mechanically stable, open-loop muscle properties are often not sufficient. There are three primary contributions. First, the previous muscle mechanical model structure and parameter estimation process of (62) is updated to reflect recent experimental findings. Second, an intrafusal (IF) muscle model is developed that includes a gamma static motoneuron (MN) drive, a Hill muscle model, and a muscle spindle sensor across the IF series element; this provides a more appropriate muscle spindle output signal, especially for studies of posture. Third, the conceptual cut between the neuro-control input and the actuator is raised from just below the MN summing junction to a higher location, allowing a "muscle-reflex actuator" to be defined that satisfies the formal theoretical requirement for possessing passive spring-like behavior when the neurocontrol input is constant, alpha-gamma MN coactivation is assumed, and three types of intrinsic autogenic reflex responses (spindle, Golgi tendon organ, Rhenshaw cell) are developed. Default feedback gains are set based on the criteria that inherent feedback should not sculpt the feedforward excitation drive by more than +/- 10% of maximum. This new actuator model only mildly affects voluntary goal-directed dynamic performance, but enhances spring-like performance around the postural equilibrium state, in line with available animal and human studies and with several theories on postural regulation.