--- module: 083-03 language: en chapter: 83 title: "General Histology: Epithelium, Connective Tissue, Muscle, and Nerve" module_title: "Muscle, peripheral nerve, tissue renewal, inflammation, and repair" source_sha256: 9367573ab68b30841ef6c6829536e492f04008586059eb1eafdb55adce5ef9ab --- # Muscle, nerve, renewal, inflammation, and repair ## Skeletal muscle structure ### Long multinucleated cells from fused myoblasts ### Peripheral nuclei beneath sarcolemma ### Aligned sarcomeres create cross-striations #### A band thick, I band thin filaments #### Z discs anchor thin filaments ### Contraction shortens I band and H zone #### A-band length constant ### Transverse tubules form triads #### Calcium binds troponin C #### Tropomyosin moves, myosin cycles #### Calcium reuptake allows relaxation ## Skeletal muscle in health and disease ### Endomysium, perimysium, epimysium #### Merge with tendon, carry capillaries ### Slow oxidative fibres resist fatigue ### Fast glycolytic fibres fatigue sooner ### Reinnervation after denervation: fibre-type grouping ### Satellite cells beneath basal lamina #### Small injury regenerates if lamina intact #### Extensive injury: fibrosis, fatty replacement ## Cardiac muscle ### Branching cells with central nuclei ### Intercalated discs join cells end to end #### Fascia adherens and desmosomes transmit force #### Gap junctions couple electrically ### Dyads near Z discs #### Calcium entry triggers further release ### Abundant mitochondria for aerobic metabolism ### Very limited regeneration #### Infarct heals by scar, survivors enlarge ## Smooth muscle ### Spindle cells, central nucleus, no striations ### Dense bodies and plaques instead of Z discs ### Contraction twists nuclei into corkscrews ### Calcium-calmodulin activates light-chain kinase ### Regulated by nerves, hormones, stretch ### Single-unit coordinated by gap junctions ### Multi-unit permits finer independent control ### Hypertrophy and proliferation in uterus, vessels, airways ## Neurons and peripheral nerve ### Soma, dendrites, axon ### Nissl substance is rough endoplasmic reticulum ### Axon hillock gives rise to initial segment ### Myelinating Schwann cell wraps one axon segment #### Non-myelinating cells envelop several axons ### Nodes of Ranvier enable saltatory conduction ### Endoneurium, perineurial barrier, epineurium ### Sensory ganglia: central nuclei, satellite glial ring ### Autonomic ganglia: eccentric nuclei, synapses ## Nerve injury ### Distal Wallerian degeneration ### Macrophages and Schwann cells clear debris ### Basal-lamina tubes guide sprouts ### Better with close ends and short target distance ### Misrouting, scar, prolonged denervation limit recovery ## Renewal, regeneration, and repair ### Labile: surface epithelia, haematopoietic cells ### Stable: hepatocytes, endothelium, fibroblasts ### Permanent: neurons, cardiomyocytes ### Categories are approximate ### Regeneration needs stem cells, scaffold, supply, signals ### Repair replaces tissue with connective scar ### Resolution removes cells, fibrin, temporary matrix ## Inflammation ### Acute: dilation, permeability, exudation #### Neutrophils early, then macrophages #### Suppurative inflammation forms pus ### Chronic: macrophages, lymphocytes, plasma cells #### Destruction, repair, angiogenesis, fibrosis ### Granulomas surround persistent stimuli #### Distinct from granulation tissue ### Morphology narrows mechanism, rarely identifies cause ## Wound healing and fibrosis ### Platelet-fibrin clot as provisional matrix ### Keratinocytes migrate beneath the clot ### Granulation tissue, myofibroblast contraction ### Primary versus secondary intention ### Delay: infection, ischaemia, diabetes, glucocorticoids ### Abnormal: dehiscence, keloid, contracture ### Hypoxia signalling and endothelial growth factor #### New vessels are permeable and fragile ### Fibrosis from persisting injury signals #### Transforming growth factor beta and stiffness #### Impairs lung, liver, kidney, heart function ### Read unit, injured compartment, cells, matrix