Functional Organization of Motoneuron Pool and its Inputs

The sections in this article are: 1 Morphological Considerations 1.1 Columnar Arrangement of Motoneuron Pool 1.2 Dimensions of α-Motoneurons and Distribution of Cell Size 1.3 Scaling of Motoneurons 1.4 Initial Segment of α-Motor Axons 1.5 Axon Collaterals of α-Motoneurons 1.6 Recurrent Inhibitory Feedback from Motoneurons 1.7 Direct Synaptic Interconnections Between Spinal Motoneurons 1.8 Species of Motoneurons 1.9 Terminals of Motoneurons in Muscle 1.10 Morphology of Neuromuscular Junctions 1.11 Matching the Properties of Motoneurons and the Muscle Fibers They Supply 1.12 Concluding Comments 2 Firing Patterns of Individual Motoneurons and Motor Units 2.1 Functional Significance of Size of Motoneurons 2.2 Measurement of Total Output of Motoneuron Pools 2.3 Critical Firing Levels of Motoneurons 2.4 Relation of Critical Firing Level to Axon Diameter and Motoneuron Size 2.5 Effects of Inhibitory Inputs on Critical Firing Level and Rank Order During Repetitive Firing 2.6 Recruitment of Motor Units in Humans 2.7 Evidence Regarding Alternative Patterns of Recruitment 2.8 Evidence Regarding Voluntary Selective Control of Motor Units 2.9 Size Principle in Other Species 2.10 Modulation of Firing Rate 3 Organization of Input to Motoneuron Pools 3.1 Anatomical Studies 3.1.1 The Motoneuron 3.1.2 Anatomy of Ia-Branches to Motoneurons 3.1.3 Ia-Synapses on Motoneurons 3.2 Techniques Used to Study EPSPs Elicited by Impulses in Single Afferent Fibers 3.2.1 EPSPS Recorded from Single Motoneurons 3.2.2 EPSPS Recorded from Populations of Motoneurons 3.3 Amplitudes of EPSPs Elicited by Impulses in Single Fibers 3.3.1 Variability of EPSP Amplitudes 3.3.2 Factors Responsible for Variability in EPSP Amplitudes 3.4 Boutons of Ia-Fibers on Motoneurons 3.4.1 Physiological Analysis of Location of Boutons 3.4.2 Efficacy of Synapses on Different Parts of Motoneuron 3.5 Physiology of Ia-Terminals 3.6 Distribution of Ia Excitation to Motoneuron Pools 3.6.1 Percentage of Motoneurons Receiving Terminals from Single Ia-Fibers 3.6.2 Factors Influencing Percentage of Motoneurons Receiving Homonymous Projections 3.7 Comparison of Projections to Homonymous and Heteronymous Motoneurons 3.8 Correlations Between Morphology and Function 3.9 Latency of EPSPs 3.10 Other Examples of Divergence in Inputs to Motoneurons 3.11 Ia-Projections to Motoneurons Controlling Other Parts of the Body 3.12 Group II Input From Secondary Endings in Muscle Spindles 3.13 Inhibitory Inputs to Motoneurons 3.13.1 Ia Inhibitory Interneurons 3.13.2 Renshaw Cells 3.14 Group Ib Input From Golgi Tendon Organs 3.15 Monosynaptic Input From Descending Pathways 3.16 Topographic Factors Governing Development of Connections of Ia-Fibers to Motoneurons 3.17 Concluding Comments 4 Nonuniformity of Motoneurons 4.1 Early Classification of Tonic and Phasic Types of Motoneurons 4.2 Significance of Nonuniformity of Muscle Fibers 4.3 Motoneuron Properties Independent of Size 4.4 Differential Responses of Motoneurons to Injected Currents 4.5 Influence of Muscle on Developing and Mature Motoneurons 4.6 Evidence From Human Disease 4.7 Concluding Comments 5 How Size of Motoneurons Determines their Susceptibility to Discharge 5.1 Properties of Motoneurons That Influence Susceptibility to Discharge 5.2 Role of Input in Determining Susceptibility to Discharge 6 Some Principles Underlying Organization of Motoneuron Pools 6.1 How Sensitivity in Gradation of Tension is Achieved 6.2 Basis for Relation Between Motoneuron Size and the Force Its Motor Unit Develops 6.3 Actual Sensitivity in Grading Muscular Tension 6.4 Mathematical Derivation of a “Principle of Maximum Grading Sensitivity” 6.5 Recruitment Order and Minimum Energy Principle 6.6 Collective Action of Motoneuron Pool: Role of Input 6.7 The Size Principle in Ia and Group II Sensory Fibers 6.8 How Does the Central Nervous System Use the Motoneuron Pool?