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This article addresses the emergence of the "motoneuron concept," i.e., the idea that this cell had properties of particular advantage for its control of muscle activation. The motor function of the ventral roots was established early in the 19th C and the term "motor cell," (or "motor nerve cell") was introduced shortly thereafter by Albrecht von Kölliker and some other histologists. They knew that motor cells were among the neurons with the largest soma in vertebrates and for this reason they were, and remained for many decades, the best and most studied neuronal model. The work of clinicians like Guillaume Duchenne de Boulogne and Jean-Martin Charcot on motor degenerative syndromes began before a clear description of motor cells was available, because it was initially more difficult to establish whether the deficits of paralysis and muscle weakness were due to neuronal or muscular lesions. Next, the pioneering physiologist, Charles Sherrington, who was influenced greatly by the anatomical contributions and speculations of Santiago Ramón y Cajal, used the term, "motor neuron," rather than motor cell for the neuron that he considered was functionally "the final common path" for providing command signals to the musculature. In the early 20th C he proposed that activation of a motor neuron resulted from the sum of its various excitatory and inhibitory CNS inputs. The contraction of motor neuron to "motoneuron(e)" was put into common usage by John Fulton (among possibly others) in 1926. The motoneuron concept is still evolving with new discoveries on the horizon., (2011 Elsevier B.V. All rights reserved.)

This review addresses the history since antiquity of studies on the anatomical and functional relations between nerves and muscles, and the progressive use of newer approaches to this topic. By the Hippocratic era (almost 2500 years ago) the digestive, circulatory and nervous systems were thought to participate in the production of animal spirits. This concept had strong support for nervous conduction, even after the dawn of electrophysiology in the late 18th C. The idea that these spirits explained the nature of the motor command to muscles continued to prevail until work in the mid-to-late 19th C dispelled the concept of "fluid/spirit" transmission by measurements of nerve "action currents" and conduction velocity. In parallel with this work, the functional relations between nerves and muscles were studied with the use of curare, which continued well into the 20th C. In the late 19th C the debate was formalized about whether transmission at the motor endplate was electrical or chemical, which continued as the "soup" vs. "sparks" battle until, surprisingly, the late 1960s. The concept of the motor unit was introduced in the 1920s, this being defined as a motor neuron in the spinal cord connecting to a specific set of muscle fibers. This development accelerated work on two-way trophic relations between nerve and muscles and their essential plasticity in response to the demands of usage and disease. Thus, the relation between nerves and muscles has been on the forefront of neuroscience since antiquity., (2011 Elsevier B.V. All rights reserved.)

Bicuculline is the most commonly used GABA(A) receptor antagonist to investigate the contribution of these receptors in motor control. However, this compound has been shown recently to potentiate the burst firing of neurons in various brain regions by blocking a calcium-activated potassium current underlying the spike after-hyperpolarization (AHP). This effect may distort our understanding of the role of GABA(A) receptors at the network level. In vitro brainstem-spinal cord preparations isolated from neonatal rats were used to compare the effects of bicuculline methiodide (bicuculline-M) and picrotoxin (PTX), another GABA(A) receptor antagonist, on the AHP of lumbar motoneurons as well as on spontaneous and locomotor-like motor activities. Intracellular recordings of lumbar motoneurons showed that bicuculline-M (20 microM) reduced the AHP to 57% of control whereas PTX (20-60 microM) had no significant effect. Bath-application of increasing concentrations of PTX caused an increase in spontaneous ventral root activity, which further increased significantly when bicuculline-M was added. The effects of both antagonists were tested on fictive locomotion. The left-right alternation was disrupted in the presence of bicuculline-M. A slow synchronous bursting activity of large amplitude also appeared in the presence of PTX. This slow rhythm was superimposed on a faster rhythm which still exhibited some degree of left-right alternation. These data demonstrate that bicuculline-M may not reveal accurately the contribution of GABA(A) receptors in motor control and the intrinsic properties of disinhibited networks.

Primary afferents from a crayfish leg proprioceptor display both primary afferent depolarizations (PADs) and antidromic spikes. PADs are generated by activation of GABA receptors and produce presynaptic inhibition, while the antidromic spikes do not elicit any synaptic effect in the postsynaptic neurons. The aim of the present study was to investigate the ionic mechanisms that allow PADs to produce antidromic spikes and to test whether GABA can produce similar effects. Intracellular recordings from the sensory axon terminals within the ganglion where PAD are produced were performed. Lowering the extracellular chloride concentration resulted in an increase in PAD amplitude, which was then capable of producing antidromic spikes. Local application of GABA close to the axon terminal also resulted in production of antidromic spikes. We conclude that antidromic spikes may result from the activation of a GABA-mediated increase in chloride conductance that also produces PADs. Therefore PADs and antidromic spikes may represent two aspects of the same GABAergic inhibitory mechanism that gate sensory transmission.

An in vitro thoraco-abdominal preparation of the crayfish (Procambarus clarkii) ventral nerve cord was used to study the sites of action and the effects of proctolin and serotonin on the nervous activities of the two abdominal motor systems, namely the swimmeret and the abdominal positioning systems. In this preparation spontaneous motor activity was recorded corresponding to continuous rhythmic bursts in the swimmeret motor nerves and tonic discharge of motoneurons in both abdominal extensor and flexor motor nerves. Proctolin applied on the abdominal ganglia elicited bursts of spikes in the flexor motor nerve which were able to disturb and even stop the swimmeret activity. Increasing concentrations of serotonin applied on the thoracic ganglia were able, first, to increase the period durations of the swimmeret bursting activity and, second, to stop it. In this last condition, continuous swimmeret activity resumed by application of proctolin on the abdominal ganglia although period durations stayed slightly longer than in control. The actions of serotonin and proctolin on the two abdominal motor systems were discussed in terms of modulations and interactions between central neuronal networks and behaviors.

The modulatory effects of proctolin and a biogenic amine (5-hydroxytryptamine; 5-HT) have been investigated on a leg mechanoreceptor in the crayfish. Single afferent sensory units were recorded extra- and intracellularly during imposed sinusoidal movement to the receptor strand; all responses were facilitated by a bath application of proctolin and 5-HT at various concentrations (10(-9)-10(-6) M).

Neurograms of proximal leg motor nerves were obtained during swimming in the shore crab. Whereas excitor motoneurones fire in bursts, the common inhibitor motoneurone discharges tonically with simultaneous spikes in all the motor nerves. The average firing frequency of the common inhibitor increases as the period of the swimming cycle decreases. Moreover, greater fluctuations of the firing frequency of the common inhibitor occurs within long rather than short swim cycle periods.

Activities of individual, force-sensitive mechanoreceptors (funnel canal organs) of the terminal segment (dactyl) of the crab leg have been recorded in freely moving animals. During the stance phase of walking receptors discharge in regular bursts that are closely correlated with activity of the opener muscle of the propodite when the leg is used on the trailing side in lateral locomotion. Individual funnel canal organs also show sustained discharges to imposed cuticular strains and strains resulting from resisted muscle contractions. These receptors thus can monitor both internal and external forces that are applied to the leg in locomotion.

Motor activity similar to agonistic behaviour is obtained after dopamine (DA) injection in lobster. Specially vigorous swimmeret beatings are observed and can be compared to the 'in vitro' motor activity elicited by DA superfusion of the isolated abdominal nervous system. DA-immunoreactive neurons stained by monoclonal antibodies in abdominal ganglia may be involved in swimmeret activation during the agonistic behavior.