W. B. Marks

The distal hindlimb musculature .of the cat

SummaryChronic recordings were made of electromyographic (EMG) activity, tension, and length of distal hindlimb muscles in six cats performing a variety of normal motor tasks. Muscles studied thoroughly or in part were medial gastrocnemius, lateral gastrocnemius, plantaris, soleus, flexor digitorum brevis, flexor digitorum longus, flexor hallucis longus, tibialis posterior, tibialis anterior, extensor digitorum longus, peroneus longus, and peroneus brevis. Postural and locomotor activities were examined, as well as jumping, landing, scratching, and paw shaking. In general, muscles could be assigned to traditional groupings (e.g. extensor, flexor) related to the demands of the motor task. Patterns of muscle activity were most often consistent with current understanding of muscle mechanics and neural coordination. However, purely functional distinctions between flexor digitorum longus and flexor hallucis longus (“anatomical synergists”) were made on the basis of activity patterns. Likewise, the activity of plantaris and flexor digitorum brevis, which are attached in series, was differentiated in certain tasks. The rhythmical oscillatory patterns of scratching and paw shaking were found to differ temporally in a manner consistent with the limb mechanics. In several cases, mechanical explanations of specific muscle activity required length and force records, as well as EMG patterns. Future efforts to study motor patterns should incorporate information about the relationships between muscle activation, tension, length and velocity.

Cat hindlimb motoneurons during locomotion. II. Normal activity patterns.

Activity patterns were recorded from 51 motoneurons in the fifth lumbar ventral root of cats walking on a motorized treadmill at a range of speeds between 0.1 and 1.3 m/s. The muscle of destination of recorded motoneurons was identified by spike-triggered averaging of EMG recordings from each of the anterior thigh muscles. Forty-three motoneurons projected to one of the quadriceps (vastus medialis, vastus lateralis, vastus intermedius, or rectus femoris) or sartorius (anterior or medial) muscles of the anterior thigh. Anterior thigh motoneurons always discharged a single burst of action potentials per step cycle, even in multifunctional muscles (e.g., sartorius anterior) that exhibited more than one burst of EMG activity per step cycle. The instantaneous firing rates of most motoneurons were lowest upon recruitment and increased progressively during a burst, as long as the EMG was still increasing. Firing rates peaked midway through each burst and tended to decline toward the end of the burst. The initial, mean, and peak firing rates of single motoneurons typically increased for faster walking speeds. At any given walking speed, early recruited motoneurons typically reached higher firing rates than late recruited motoneurons. In contrast to decerebrated cats, initial doublets at the beginning of bursts were seen only rarely. In the 4/51 motoneurons that showed initial doublets, both the instantaneous frequency of the doublet and the probability of starting a burst with a doublet decreased for faster walking speeds. The modulations in firing rate of every motoneuron were found to be closely correlated to the smoothed electromyogram of its target muscle. For 32 identified motoneurons, the units instantaneous frequencygram was scaled linearly by computer to the rectified smoothed EMG recorded from each of the anterior thigh muscles. The covariance between unitary frequencygram and muscle EMG was computed for each muscle. Typically, the EMG profile of the target muscle accounted for 0.88-0.96 of the variance in unitary firing rate. The EMG profiles of the other anterior thigh muscles, when tested in the same way, usually accounted only for a significantly smaller fraction of the variance. Brief amplitude fluctuations observed in the EMG envelopes were usually also reflected in the individual motoneuron frequencygrams. To further demonstrate the relationship between unitary frequencygrams and EMG, EMG envelopes recorded during walking were used as templates to generate depolarizing currents that were applied intracellularly to lumbar motoneurons in an acute spinal preparation.(ABSTRACT TRUNCATED AT 400 WORDS)

Action currents, internodal potentials, and extracellular records of myelinated mammalian nerve fibers derived from node potentials

The potential distribution within the internodal axon of mammalian nerve fibers is derived by applying known node potential waveforms to the ends of an equivalent circuit model of the internode. The complete spatial/temporal profile of action potentials synthesized from the internodal profiles is used to compute the node current waveforn, and the extracellular action potential around fibers captured within a tubular electrode. For amphibia, the results agreed with empirical values. For mammals, the amplitude of the node currents plotted against conduction velocity was fitted by a straight line. The extracellular potential waveform depended on the location of the nodes within the tube. For tubes of length from 2 to 8 internodes, extracellular wave amplitude (mammals) was about one-third of the product of peak node current and tube resistance (center to ends). The extracellular potentials developed by longitudinal and radial currents in an anisotropic medium (fiber bundle) are compared.

Optimal control principles for sensory transducers

Sensory transduction in physiological systems faces the double dilemma of maximizing both sensitivity and dynamic range despite limitations inherent in the transmission of information via all-or-none action potentials. The thresholds for detection in senses such as vision and hearing are close to the theoretical physical limits of uncertainty whereas these same senses frequently must discriminate well among stimuli having five or six orders of magnitude greater amplitude. All of this information must be transmitted via digital pulse-rate encoding with a maximum dynamic range of 0–500 pps. Among the mechanisms known to assist in solving this encoding problem are logarithmic response curves in the transducers themselves, accomodative processes intrinsic to the transducers, gain control systems under efferent neural control, and mechanical gates (e.g. iris of the eye, tensor tympani muscle of the ear).

Some Approaches to Quantitative Dendritic Morphology

The availability of powerful desktop computers and of a large amount of detailed data about the morphology of a wide variety of neurons has led to the development of computational approaches that are designed to synthesize such data into biologically meaningful patterns. The hope is, of course, that the emerging patterns will provide clues to the factors that control the formation of neuronal dendrites during development, as well as their maintenance in the adult animal. One class of approaches to this problem is to develop quantitative computational models that can reproduce as many aspects of the original data as possible. The development of such simulations requires analysis of the original data that is directed by the model requirements, and their relative success depends on detailed comparisons between model outputs and the original data sets. Refinement of the models may require not only new experiments, as in other scientific disciplines, but also new ways of looking at the data already in hand. This chapter discusses some examples of this process, with emphasis on spinal motoneurons.

Simulation of motoneuron morphology in three dimensions. I. Building individual dendritic trees

We have developed a computational method that accurately reproduces the three‐dimensional (3D) morphology of individual dendritic trees of six cat alpha motoneurons. The first step was simulation of trees with straight branches based on the branch lengths and topology of actual trees. A second step introduced the meandering, or wandering, trajectories observed in natural dendritic branches into the straight‐branch tree simulations. These two steps each required only two parameters, one extracted from the data on actual motoneuron dendrites and the other adjusted by comparing simulated and observed trees, using measurements that were independent of the model specifications (i.e., emergent properties). The results suggest that: 1) there is a somatofugal “tropism” (a bias introduced by the environment that affects the trajectory of dendritic branches) that tends to constrain the lateral expansion of alpha motoneuron dendrites; and 2) that most of the meandering of natural dendritic branches can be described by assuming that they are fractal objects with an average fractal dimension D of about 1.05. When analyzed in the same way, the dendrites of gamma motoneurons showed no evidence of a similar tropism, although they had the same fractal dimension of branch meandering. J. Comp. Neurol. 503:685–700, 2007. Published 2007 Wiley‐Liss, Inc.

Long-Term Peripheral Nerve Activity During Behavior in the Rabbit

peripheral nerve filaments of mammals during normal unrestrained locomotion and posture. In 20 rabbits the nerve (200 μm thick, ~175 myelinated axons) supplying the tenuissimus muscle was isolated over ~15 mm with original connectivity and blood supply kept intact, and captured in two consecutive, longitudinally slit, insulating silastic tubes (300 μm x 6 mm) mounted onto a cuff that surrounded the sciatic nerve for support. Differential recording between contacts at the midpoint and ends of each tube allowed discrimination of multiunit nerve spike activity while potentials from neighboring muscles (EMG) were effectively screened out by the addition of shunts and shields.

Simulation of motoneuron morphology in three dimensions. II. Building complete neurons

By using dendrogram data from six adult cat alpha motoneurons, we have constructed computer simulations of these cells in three dimensions (3D) by “growing” their dendritic trees from stem branches that were oriented as in the original cells. Individual trees were simulated by using the algorithms and parameters discussed in the companion paper (Marks and Burke [ 2007 ] J. Comp. Neurol. 503:685–700). It was not possible to distinguish real from simulated motoneurons by visual inspection of 3D drawings. Simulated cells were compared quantitatively with their actual exemplars by using features that were measured in spherical shells at various radii centered on the soma. These included nearest neighbor distances (NNDs) between branches, the sizes and overlaps between the territories of individual dendrites measured as convex hulls (polygons that enclose all branches passing through a shell), and the sizes of circular zones that contained no branches. We also compared the 3D fractal dimensions and lacunarity (a measure of the 3D dispersion of branches) in actual cells and their simulations. The statistical properties of these quantitative measures were not significantly different, suggesting that the simulation algorithm was quite successful. However, there were three exceptions: 1) there were more NNDs at distances < 50 μm in simulated than in actual motoneurons; 2) average overlaps between the territories of different dendrites were almost twice as large in simulated compared with actual motoneurons; and 3) estimates of lacunarity were also larger in simulated cells. These exceptions suggest that dendritic branches in actual motoneurons tend to avoid one another. We discuss possible interpretations of these results. J. Comp. Neurol. 503:701–716, 2007. Published 2007 Wiley‐Liss, Inc.

Activity Patterns of Identified Alpha Motoneurons to Cat Anterior Thigh Muscles during Normal Walking and Flexor Reflexes

The organization of the motor apparatus into anatomically and functionally defined pools of regularly recruited motor units derives from some of the earliest and most enduring observations of neurophysiology. In recent years, there has been much productive research concentrated on discovering the anatomical bases of this organization in the spinal cord circuitry and the properties of the final common pathway, the alpha motoneurons themselves (for review, see Burke, 1981a). However, almost all of the direct evidence for the function of this system has been derived from experiments on reduced or anesthetized animals and on human subjects performing highly constrained and artificial motor tasks.