Alpha, beta and gamma motoneurons: functional diversity in the motor system's final pathway.

@article{Manuel2011AlphaBA,
  title={Alpha, beta and gamma motoneurons: functional diversity in the motor system's final pathway.},
  author={Marin Manuel and Daniel Zytnicki},
  journal={Journal of integrative neuroscience},
  year={2011},
  volume={10 3},
  pages={
          243-76
        }
}
Since their discovery in the late 19th century our conception of motoneurons has steadily evolved. Motoneurons share the same general function: they drive the contraction of muscle fibers and are the final common pathway, i.e., the seat of convergence of all the central and peripheral pathways involved in motricity. However, motoneurons innervate different types of muscular targets. Ordinary muscle fibers are subdivided into three main subtypes according to their structural and mechanical… 

Figures from this paper

Simultaneous intracellular recording of a lumbar motoneuron and the force produced by its motor unit in the adult mouse in vivo.
TLDR
An extension of this original preparation was presented in which the experimentalist was able to obtain simultaneous recordings of the electrophysiological properties of the motoneurons and of the force developed by their motor unit, which is an important achievement.
Intrinsic excitability differs between murine hypoglossal and spinal motoneurons.
TLDR
While HMs and SMs have similar passive properties, they differ in their response to similar levels of depolarizing current, which suggests that each population possesses differing suites of ion channels that allow them to discharge at rates matched to the different mechanical properties of the muscle fibers that drive their distinct motor functions.
A model for self-organization of sensorimotor function: the spinal monosynaptic loop
TLDR
These findings imply a generic functionality of a musculoskeletal system to imprint important aspects of its mechanical dynamics onto a neural network, without specific preprogramming other than setting a critical period for the formation and maturation of this general pattern of connectivity.
Spinal Control of Motor Outputs Escape from homeostasis : spinal microcircuits and progression of amyotrophic lateral sclerosis
TLDR
This article proposes that should this be the case, new therapies targeted toward microcircuit dysfunction could slow the course of ALS, and suggests that this activity would ultimately lead to excitotoxicity of motoneurons, which would hasten the progression of disease.
V2a interneuron diversity tailors spinal circuit organization to control the vigor of locomotor movements
TLDR
It is reported that motor flexibility is generated through the specificity of connections between subtypes of V2a interneurons and motoneuron populations and their distinct plasticity mechanisms, and is identified as parsimonious task-specific circuit design controlling the vigor of locomotion.
Long‐lasting modifications of motoneuron firing properties by trans‐spinal direct current stimulation in rats
TLDR
Direct evidence is provided that a single polarization session can alter the electrophysiological properties of motoneurons for at least one hour and provides a basis for the further use of tsDCS techniques under conditions where the sustained modification of mot oneuron firing is desired.
Structural and Biological Basis for Proprioception
TLDR
This chapter intends to provide a comprehensive update an overview of the anatomical, structural and molecular basis of proprioception as well as of the main causes ofroprioception impairment, including aging, and possible treatments.
Molecular correlates of spinal motor neuron functional specification and plasticity
TLDR
The ability to alter motor neuron type status, by targeting molecular pathways driving motor neuron plasticity, may eventually provide novel therapeutic intervention strategies aiming at ameliorating motor neuron loss in NMDs.
Escape from homeostasis: spinal microcircuits and progression of amyotrophic lateral sclerosis
TLDR
It is proposed that motoneuron degeneration would lead to an increase in inputs to motoneurons, and, because of the pattern of neuronal degeneration, would result in an imbalance in local microcircuit activity that would overwhelm initial homeostatic responses.
Single nucleus RNA-sequencing defines unexpected diversity of cholinergic neuron types in the adult mouse spinal cord
TLDR
A targeted single nuclear RNA sequencing approach is developed and used to identify an array of cholinergic interneurons, visceral and skeletal motor neurons, providing a comprehensive transcriptomic description of this important population of neurons that control many aspects of physiology and movement and encompass the cellular substrates for debilitating degenerative disorders.
...
...

References

SHOWING 1-10 OF 161 REFERENCES
Input-output functions of mammalian motoneurons.
TLDR
There is at present no general quantitative expression for motoneuron input-output properties under dynamic conditions and the biophysical mechanisms underlying the transfer of synaptic current from the dendrites to the soma are not well understood.
A comparison of the structures of alpha and gamma-spinal motoneurones of the cat.
TLDR
The structures of seven gamma-motoneurones were compared with those of nine alpha-mot oneurones by using histochemical methods to reveal horseradish peroxidase which had previously been injected intracellularly into indentified motoneur ones in the cat lumbosacral spinal cord.
An HRP study of the relation between cell size and motor unit type in cat ankle extensor motoneurons
TLDR
Comparison of AN and motoneuron input resistance (RN) in 19 α‐motoneurons suggested that the specific resistivity of the cell membrane in type S mot oneurons was systematically higher than that characteristic of type FF or FR motoneuron.
Adult Mouse Motor Units Develop Almost All of Their Force in the Subprimary Range: A New All-or-None Strategy for Force Recruitment?
TLDR
This work recorded, for the first time, the motoneuron discharge elicited by intracellular ramps of current and the force developed by its motor unit, and showed that the motor unit developed nearly its maximal force during the subprimary range.
Beginning at the end: Repetitive firing properties in the final common pathway
Observations on the fusimotor fibres of the tibialis posterior muscle of the cat
TLDR
Whether the static and dynamic fusimotor fibres of tibialis posterior differ in conduction velocity, even though those of soleus do not (Crowe & Matthews, 1964b) is determined.
Further study of efferent small‐nerve fibres to mammalian muscle spindles. Multiple spindle innervation and activity during contraction
TLDR
It was found that the effect of small-nerve stimulation on the discharge from muscle spindles could be recorded during muscle contraction, and in this way the influence of specific spindle innervation on afferent activity was analysed during the changing tension conditions of muscle contraction.
Numbers and Contraction-Values of Individual Motor-Units Examined in some Muscles of the Limb
“The muscle with its nerve may be thought of as an additive assemblage of motor-units, meaning by motor-unit an individual motor nerve-fibre with the bunch” [or “squad” (E. L. Porter, 1929 (1))]“ of
FUNCTIONAL SIGNIFICANCE OF CELL SIZE IN SPINAL MOTONEURONS.
TLDR
The present paper is concerned with the central part of the motoneuron and the significance of its size in synaptic transmission and asks whether the cell bodies (and dendrites) connected with large and small motor fibers have different functional properties which can be recognized by their discharge characteristics.
Intrinsic activation of human motoneurons: possible contribution to motor unit excitation.
TLDR
The estimates of intrinsically mediated depolarization of human mot oneurons are consistent with values obtained for plateau potentials obtained from intracellular recordings of motoneurons in reduced animal preparations, although various alternative mechanisms are discussed.
...
...