Muscle Pain: Understanding the Mechanisms (eBook)

Foreword 6
Preface 8
Contents 10
Contents to Muscle Pain: Diagnosis and Treatment 12
Contributors 14
1: Introduction 16
Subjective Nature of Pain Terms 17
Established Pain Terms (partly after Merskey and Bogduk 1994 Loeser and Treed
General Terms 18
Nociception 18
Noxious Stimulus 18
Nociceptor 18
Nociceptive Neuron 19
Pain Matrix 19
Central Pain 19
Increased Sensitivity 19
Allodynia 19
Hyperalgesia 19
Hyperesthesia 19
Hyperpathia 20
Decreased Sensitivity 20
Analgesia 20
Hypoalgesia 20
Established, But Often Ill-Defined, Pain Terms 20
Contracture (in the Physiological Sense) 20
Muscle Spasm 20
Muscle Stiffness 21
Muscle Tone 21
Projected Pain 21
Referred Pain 21
Spread of Pain 22
General Aspects of Pain and Nociceptor Function 22
Muscle Pain Versus Skin Pain 23
Subjective Differences 24
First and Second Pain 24
Pain Localization 24
Nature of Pain 25
Referral of Pain 25
Affective Pain Component 26
Objective Differences 26
Flexion Reflexes 26
Differences in Synaptic Effectiveness and CNS Targets of Afferent Fibers 27
Descending Pain-Modulating Influences 28
Transition from Acute to Chronic Pain 28
Interactions Between Psychosocial and Somatic Factors 28
References 29
2: Functional Anatomy of Muscle: Muscle, Nociceptors and Afferent Fibers 31
Structure and Basic Function of Skeletal Muscle 32
Morphology of Muscle Nociceptors 39
Structure of Muscle Nociceptors in the Light and Electron Microscope 39
Receptor Molecules in the Membrane of Nociceptors 42
Neuropeptide Content of Nociceptors 47
The Nociceptive Afferent Fiber 49
Fiber Composition of a Muscle Nerve 51
Muscle Receptors Other Than Nociceptors 53
Free Nerve Endings in Tendon 54
Free Nerve Endings in Fascia 55
Efferent Functions of Nociceptors 55
Release of Neuropeptides from the Nociceptive Ending 55
The Axon Reflex 57
Neurogenic Inflammation 58
References 59
3: Peripheral Mechanisms of Muscle Pain: Response Behavior of Muscle Nociceptors and Factors Eliciting Local Muscle Pain 63
Methods Used for Studying the Response Behavior of Single Group III and IV Muscle Afferent Units 64
General Response Properties of Muscle Nociceptors 67
Stimuli Exciting Muscle Nociceptors 68
Mechanical Stimuli 68
Chemical Stimuli 71
General Considerations 71
ATP 72
Protons 72
Endogenous Inflammatory Substances 74
Hypertonic Saline 75
Cytokines and Neurotrophins 78
Glutamate 80
General Considerations Concerning the Chemical Excitability of Free Nerve Endings in Muscle 81
Polymodal Nociceptors 82
Acute Sensitization of Nociceptors 84
General Features of Peripheral Sensitization 84
Mechanical Sensitization by Endogenous Substances 85
Interactions Between Chemical Stimuli at the Receptive Nerve Ending 90
Responses of Free Nerve Endings in Muscle to Ischemia and Ischemic Contractions 91
Effects of Ischemia Without Contractions 91
Ischemic Contractions 92
Effects of Inflammatory Tissue Changes on the Activity of Muscle Group IV Units 95
Resting Discharge 96
Inflammation-Induced Mechanical Sensitization 97
Effectiveness of Chemical Stimulants in Chronically Inflamed Muscle 100
How Do Muscle Spindle Afferents Behave in Inflamed Muscle? 102
Effects of a Chronic Muscle Lesion on the Innervation of Skeletal Muscle 103
Response Properties and Possible Functions of Nonnociceptive Free Nerve Endings in Muscle 104
Local Causes of Muscle Pain 105
Mechanical Causes 105
Blow to Muscle 105
Rupture of Muscle and Tendon 106
Spontaneous Hematoma 107
Delayed-Onset Muscle Soreness or Postexercise Muscle Soreness 107
Local Tenderness Due to Myofascial Trigger Points 110
Chronic Work-Related Muscle Myalgia (Repetitive Strain Injury) 110
Painful Contractions of Normal Muscle 111
Metabolic Problems 111
McArdle´s Disease 112
References 112
4: Central Nervous Mechanisms of Muscle Pain: Ascending Pathways, Central Sensitization, and Pain-Modulating Systems 118
Ascending Pathways and Centers for Nociceptive Information 120
Pain Components 120
Centers and Pathways for the Information from Muscle Nociceptors 121
General Remarks 121
The Distribution of Nociceptive Information in the Spinal Cord 121
Normal Function of a Dorsal Horn Neuron 123
Dorsal Horn Cells Responding to Nociceptive Input from Muscle 126
Response Types 127
Ascending Nociceptive Tracts in the Spinal Cord 130
Spinothalamic Tract (STT, Tr. spinothalamicus) 130
Spinoreticular Tract (Tr. spinoreticularis) 132
Spinomesencephalic and Spinobrachial Tract (Tr. spinomesencephalicus and Tr. spinoparabrachialis) 132
The Thalamus:thalamus The Last Nociceptive Center Below the Cortex 134
Pain Originates in the Cortex 136
General Remarks on Pain Concepts 138
Pain Memory 139
Mechanisms of Central Sensitization 139
Introduction 139
Lesion-Induced Functional Changes 141
Events Leading to Hyperexcitability of Central Neurons 141
Sequelae of Central Sensitization 146
Increased Resting Activity of Dorsal Horn Neurons 146
Appearance of New Receptive Fields 147
Lesion-Induced Changes in the Wiring of the Dorsal Horn 150
Neuromodulatory Substances Involved in Myositis-Induced Rewiring of Spinal Connections 153
Substance P 153
Nitric Oxide 154
Neuroplasticity as a Basic Principle in Central Sensitization 157
Central Sensitization Induced by Subthreshold Synaptic Potentials in Dorsal Horn Neurons 158
The Role of Glial Cells in Central Sensitization 163
The Transition from Acute to Chronic Muscle Pain 165
Nociceptive Processing at the Trigeminal Level 167
Pain-Modulating Pathways 171
Segmental Inhibition of Dorsal Horn Neurons 171
Gate Control Theory 173
Pain-Modulating Descending Pathways 174
The Descending Pain-Inhibiting (Antinociceptive) System 174
Tonic Inhibition of Neurons Mediating Deep Somatic Pain 178
Descending Facilitation of Nociceptive Dorsal Horn Neurons 179
References 182
5: Referral of Musculoskeletal Pain 189
Introduction 190
Occurrence of Local and Referred Pain 191
Experimental Musculoskeletal Pain Referral in Humans 192
Pain Referral is a Time-Dependent Process 194
Local and/or Referred Pain 194
Need of Afferent Input from the Area of Referred Pain 197
Experimental Pain Referral in Musculoskeletal Pain Patients 198
Clinical Pain Referred from Muscle 198
Referral from Myofascial Trigger Points 198
Referral from Other Sensitive Locations in Muscle 199
Pain Referred to Muscle 199
Referral from Joints 199
Referral from Viscera 200
Referral from the Central Nervous System 200
Possible Mechanisms of Pain Referral 200
General Considerations 200
Branching of Primary Afferent Fibers 202
Input Convergence on Spinal Neurons as the Basis for the Convergence-Projection and Convergence-Facilitation Theory of Pain Re 204
The Convergence-Projection Theory by Ruch 204
The Convergence-Facilitation Theory by McKenzie 206
How can Referred Pain occur Without Simultaneous Local Pain? 209
Possible Mechanisms Underlying Changes in Dorsal Horn Connectivity 209
Referred Pain versus Head Zones 213
References 214
6: Increased Muscle Tone as a Cause of Muscle Pain 218
Nature of Muscle Tone 219
Components of Muscle Tone 220
Definitions Related to Muscle Tone 222
Viscoelastic Tone 223
Measurements of Muscle Tone 224
Clinical Applications 229
Stiffness Based on Resonant Frequency 229
Thixotropy 229
Clinically Relevant Deviations and Normal Muscle Tone 230
Hypotonia (synonym hypotonicity) 230
Hypertonia 230
Resting Muscle Tone 231
Contracture 232
Contractures in the Physiological Sense 232
Clinical Usage of Contracture 232
Identification of Contracture 233
Muscle Spasm 233
Definition of Muscle Spasm 233
Measurement of Spasm 234
Clinical Conditions with Painful Increased Muscle Tension 235
Tension-Type Headache (TTH) 235
Muscle Spasm 237
The Misconception of a Pain-Spasm-Pain Cycle 237
Spasmodic Torticollis 241
Trismus 244
Unnecessary Muscle Tension 245
Psychological Distress 245
Chronic Muscle Overload 246
Inefficient Use 247
Nocturnal Leg Cramps 248
Stiff-Man (stiff-person) Syndrome (Moersch-Woltman syndrome) 249
Muscle Stiffness of Aging 249
General Remarks on Medications 250
References 253
7: Reorganized Motor Control Due to Muscle Pain 261
Introduction 262
Relevant Pain Modalities 263
Pain-Motor Interaction 264
Resting Muscle Activity and Muscle Pain 266
Static Muscle Activity and Muscle Pain 267
Dynamic Muscle Activity and Muscle Pain 271
Motor Neuronal Excitability During Muscle Pain 273
Conclusion 274
References 274
Glossary 279
Index 328