Plasticity in the Somatosensory System of Developing and Mature Mammals - The Effects of Injury to the Central and Peripheral Nervous System

1 Introductory Remarks.- 2 Plasticity in the Peripheral Somatosensory Nervous System.- 2.1 Aspects of Plasticity in the Peripheral Nervous System.- 2.2 Survival and Loss of Sensory Neurons After Lesions of the Peripheral Nervous System.- 2.2.1 Effect of Crush or Transection of Peripheral Nerve on Neurons of Sensory Ganglia.- 2.2.2 Trophic Dependence of Immature Sensory Neurons on the Periphery.- 2.2.3 Effect of Peripheral Nerve Transection on Different Types of Sensory Neurons in Dorsal Root Ganglia.- 2.2.4 Effect of Peripheral Nerve Section on Fibre Composition of Dorsal Roots.- 2.2.5 Fate of the Lost neurons.- 2.2.6 Sensory Cell Loss After Chemical Lesions of Afferent Fibres.- 2.3 Collateral Sprouting of Primary Afferent Fibres in the Periphery.- 2.3.1 Collateral Reinnervation of the Skin in Adult Mammals.- 2.3.2 Collateral Sprouting in Neonates.- 2.3.3 Effect of Neural Activity on Collateral Sprouting.- 2.3.4 Collateral Sprouting of Trigeminal Afferents.- 2.3.5 Collateral Sprouting and Sensory Recovery in Man.- 2.3.6 Fate of Collateral Sprouts After Regeneration of Original Nerve.- 2.4 Regeneration of Somatic Sensory Afferent Fibres.- 2.4.1 Numbers of Axons in Nerves Regenerating After Crush or Transection.- 2.4.2 Size of Regenerated Axons.- 2.4.3 Effect of Denervation on Specialized Cutaneous Mechanoreceptors.- 2.4.4 Reinnervation of Cutaneous Receptors by Regenerating Sensory Fibres.- 2.5 Modality Specificity of Somatosensory Nerve Regeneration.- 2.5.1 Regeneration of Myelinated Afferent Fibres to Hairy Skin.- 2.5.2 Regeneration of Myelinated Afferent Fibres to Glabrous Skin.- 2.5.3 Regeneration of Unmyelinated Afferent Fibres..- 2.6 Major Conclusions.- 3 Plasticity and the Mystacial Vibrissae of Rodents.- 3.1 General Account of Pathway.- 3.2 Normal Development of the Vibrissae and Their Neural Connections to the Cerebral Cortex.- 3.3 Effects of Lesions and Manipulations in Prenatal, Neonatal and Developing Animals.- 3.3.1 Damage of the Infraorbital Nerve.- 3.3.2 Lesions to One or More Vibrissae.- 3.3.3 The Effects of Supernumerary Vibrissae.- 3.3.4 The Effects of Lesioning Unmyelinated Afferents.- 3.3.5 Hyper- and Hypostimulation of Vibrissa Afferents.- 3.3.6 Cortical Alterations.- 3.4 Plasticity in the Vibrissa System of Adult Animals.- 3.4.1 The SI Cortex.- 3.4.2 The Ventral Posterior Medial Nucleus.- 3.5 Major Conclusions.- 4 Plasticity and the Spinal Dorsal Horn (with Notes on Homologous Regions of the Trigeminal Nuclei).- 4.1 Experimental Strategies for Demonstration of Plasticity in the Dorsal Horn of the Spinal Cord and Trigeminal Nuclei.- 4.2 Overview of Dorsal Horn Organization.- 4.2.1 Laminar Cytoarchitectonic Organization.- 4.2.2 Laminar Organization of the Termination of Primary Afferent Fibres.- 4.2.3 Microanatomical Organization of Low-Threshold Cutaneous Afferents.- 4.2.4 Relation of Functional Properties to Lamination of the Dorsal Horn.- 4.2.5 Inhibitory Receptive Fields.- 4.3 Somatotopic Organization of the Dorsal Horn.- 4.3.1 Dorsal Horn Neurons.- 4.3.2 Somatotopy and Lamination.- 4.3.3 Relation of Primary Afferent Projections to Dorsal Horn Somatotopy.- 4.3.4 Relation Between Dorsal Horn Cell Dendritic Morphology and Receptive Field.- 4.4 Effect of Lesions on Somatotopic Organization.- 4.4.1 Dorsal Rhizotomy.- 4.4.2 Chronic Spinal Lesions.- 4.4.3 Peripheral Nerve Transection or Crush.- 4.5 Mechanisms Underlying the Somatotopic Reorganization of Dorsal Horn Neurons.- 4.5.1 Physiological and Pharmacological Evidence for the Existence of Normally Ineffective.- Afferent Connections.- 4.5.2 Spontaneous Changes of Receptive Fields.- 4.5.3 Plasticity of Receptive Fields Induced by Afferent Activity.- 4.5.4 Involvement of Unmyelinated Afferents in the Somatotopic Reorganization After Peripheral Nerve Injury.- 4.5.5 Sprouting of Primary Afferent Fibres and Other Neurons as a Basis for Somatotopic Reorganization.- 4.6 Plasticity of the Developing Dorsal Hor.- 4.6.1 Development of Dorsal Horn Neurons and Primary Afferents.- 4.6.2 Functional Plasticity in Development.- 4.6.3 Somatotopic Reorganization Following Neonatal Peripheral Nerve Lesions.- 4.6.4 Anatomical Plasticity of Neonatal Afferent Projections.- 4.7 Major Conclusions.- 5 Plasticity and the Dorsal Column Nuclei.- 5.1 Advantages of the Dorsal Column Nuclei for Studies of Plasticity.- 5.2 Organization of the Dorsal Column Nuclei.- 5.2.1 Cytoarchitectonics.- 5.2.2 Ascending Afferent Pathways.- 5.2.3 Responses of Neurons to Natural Stimulation.- 5.2.4 Core and Shell Organization.- 5.2.5 Somatotopic Organization.- 5.3 Alterations of Inputs to the Nuclei.- 5.3.1 Section of Ascending Pathways.- 5.3.2 Effects of Dorsal Rhizotomy.- 5.3.3 Peripheral Nerve Section.- 5.4 Evidence for Ineffective Afferent Connections.- 5.4.1 Projections of Dorsal Roots and Peripheral Nerves.- 5.4.2 Projections of Single Afferent Fibres.- 5.4.3 Dendritic Spread of Cuneate Neurons.- 5.4.4 Electrical Stimulation and Widefield Neurons..- 5.4.5 Pharmacological Alterations of Receptive Fields.- 5.5 Recovery from Sensorimotor Deficits Following Dorsal Column Lesions.- 5.6 Plasticity of the DCN During Development.- 5.6.1 Effects of Prenatal Lesions.- 5.6.2 Effect of Neonatal Destruction of Unmyelinated Afferents.- 5.7 Major Conclusions.- 6 Plasticity and the Somatosensory Thalamus.- 6.1 Experimental Strategies and Plasticity in the Ventral Posterior Nuclei of the Thalamus.- 6.2 Anatomical Organization of Inputs and Outputs of the Ventral Posterior Nuclei.- 6.2.1 Primate and Cat.- 6.2.2 Raccoon.- 6.2.3 Rat.- 6.3 Responses of Neurons to Cutaneous Stimulation and the Effects of Anaesthetics and Other Drugs.- 6.4 Somatotopic Organization of the VPL and VPM.- 6.5 Effects of Alteration of Input on Somatotopic Organization.- 6.5.1 Reversible Blockade of Afferents and the Immediate Expression of New Inputs.- 6.5.2 Chronic Lesion of Afferent Pathways and Sprouting of Thalamic Afferents.- 6.6 Major Conclusions.- 7 Plasticity and the Somatosensory Cerebral Cortex.- 7.1 Experimental Strategies and Cortical Plasticity..- 7.2 Plasticity in the Cortex of Adult and Developing Primates.- 7.2.1 Multiple Representations.- 7.2.2 Thalamic Input and Intracortical Connectivity..- 7.2.3 Responses of Cortical Neurons to Natural Stimulation.- 7.2.4 Somatotopic Representation of the Hand in Areas 3b and 1.- 7.2.5 Anatomy and Innervation of the Monkey Hand.- 7.2.6 Anaesthetics and the Representation of the Hand.- 7.2.7 Injury and Subsequent Regeneration of Peripheral Nerves.- 7.2.8 Section and Ligation of Peripheral Nerves.- 7.2.9 Effects of Repeated Stimulation on Cortical Representations.- 7.2.10 Cortical Damage.- 7.3 Plasticity in the Cortex of Adult and Developing Cats.- 7.3.1 Somatotopic Organization, Cytoarchitectonics and Neuronal Responses.- 7.3.2 Thalamic Input and Ineffective Thalamocortical Connections.- 7.3.3 Effects of Anaesthetics and Other Drugs.- 7.3.4 Cordotomy and Section of Ascending Tracts.- 7.3.5 Blockage of Primary Afferent Input in Specific Dorsal Roots.- 7.3.6 Damage to Peripheral Nerves and Effects of Usage on Cortical Representation.- 7.3.7 Cortical Damage.- 7.4 Plasticity in the Cortex of Adult and Infant Raccoons.- 7.4.1 Somatotopic Organization and Cytoarchitectonics.- 7.4.2 Neuronal Responses in SI Cortex and the Effects of Anaesthetics.- 7.4.3 Ineffective Afferent Connections.- 7.4.4 Effects of Amputation on Cortical Somatotopy..- 7.5 Plasticity in the Cortex of Adult and Developing Rodents.- 7.5.1 Somatotopic Organization and Cytoarchitectonics.- 7.5.2 Section and Ligation of Peripheral Nerves in the Adult.- 7.5.3 Effects of Perinatal Nerve Section or Limb Amputation.- 7.5.4 Pharmacological Mechanisms Underlying Somatotopic Reorganization.- 7.5.5 Cortical Damage.- 7.6 Major Conclusions.- 8 Concluding Remarks.- 8.1 Plasticity During Development.- 8.1.1 Disruption of a Growing System and the Influence of the Periphery.- 8.1.2 The Influence of Afferent Axons and the Target Tissue.- 8.2 Evaluation of Experimentally Induced Plasticity in Adult Animals.- 8.2.1 Plasticity in the Peripheral Nervous System.- 8.2.2 Somatotopic Organization in Intact Animals as a Baseline for Assessing Altered Connections.- 8.2.3 Somatotopic-Artifacts in Regions Deprived of Their Normal Input.- 8.2.4 Plasticity and the Level of the Neuraxis.- 8.3 The Case for Ineffective Connections.- 8.3.1 Elucidation of Sub-Threshold Inputs.- 8.3.2 Somatotopically Inappropriate Projections of Afferent Axons.- 8.4 Spatial Extent of Immediate and Long-Term Changes in Somatotopic Organization.- 8.4.1 Distance Limits of Somatotopic Reorganization.- 8.4.2 Sprouting and Synaptogenesis in the Mature System.- 8.4.3 Recovery of Function.- 8.5 Normal Physiological Mechanisms and Plasticity.- 8.5.1 Inhibitory Receptive Fields and Partial Deafferentation.- 8.5.2 Neurotransmitters and Neural Systems That Regulate Sensory Input.- 8.6 Role of Plasticity in the Mature Somatosensory System.- References.