Peripheral Nervous System And Skeletal Muscle

Peripheral Nervous System and Skeletal Muscle

▪Perimysium:

groups muscle fibers into primary and secondary bundles (fasciculi) ▪Epimysium:envelops single muscles or large groups of fibers

▪Functional unit: motor unit ▪Motor unit consists of: o o o

Lower motor neuron: ant horn of spinal cord/ cranial nerve motor nucleus in brainstem Axon Muscle fibers

▪Lower Motor Neurons: arranged somatotopically ▪Muscles with highly refined movements (eg eye): neuron-to-muscle fiber ratio = 1:10 ▪Muscles with coarse and stereotyped movements (calf) = 1:1800 Normal Peripheral Nerve ▪Principal structural component: nerve fiber

▪Nerve

fiber = axon + Schwann cells + myelin sheaths ▪Nodes of Ranvier: between Schwann cells

▪Myelin Protein Zero (MPZ): major protein of myelin ▪3 major connective tissue components of PN: Epineurium: encloses entire nerve Perineurium: encloses each fascicle Endometrium: surrounds indiv nerve fibers ▪Perineurial barrier, blood-nerve barrier, nervecerebrospinal fluid barrier: regulates nerve microenvironment o o o

Normal Skeletal Muscle ▪Muscle fibers, myocytes are syncitia derived from the fusion of a contiguous column of indiv embryonic cells  multinucleated ▪Satellite cells: stem cell population located adjacent to sarcolemma; covered by basement membrane ▪Myofilaments: form the contractile apparatus of myofibrils ▪Myofibrils: consist of sarcomeres

▪Sarcomeres: Actin: longitudinally directed thin filaments, binds to dystrophin o Myosin: thick filaments o Z-bands (α-actinin): perpendicular to sarcomere ▪T-tubule system: calcium release during excitation o

▪2 major types of fibers: o Type 1: “one slow fat red ox” o Type 2: rich in glycolytic enzymes, involved in rapid, phasic contractions

▪All fibers of a single unit are of the same type ▪Muscle spindles: fusiform structures that respond to stretch in muscles  role in maintaining tone ▪Endomysium: surrounds indiv muscle fibers

Pathology PNS

▪Target of insult: Schwann cels or axon Path – Dr. Yabut ▪Diseases thatGen affect Schwann cells:

loss of myelination segmental demyelination PNS or and Skeletal muscle ▪Primary involvement of neuron and its axon: Feb 02, 2008 axonal degeneration  may be followed by axonal regeneration and reinnervation of muscle Muscle ▪Denervation atrophy: due to loss of axons

▪Primary abnormality of muscle fiber: myopathy Segmental Demyelination ▪Occurs when there is dysfunction of the Schwann cell ▪Guillain-Barre syndrome o Damage to myeln sheath (hereditary motor and sensory neuropathy) o There is no primary abnormality of the axon o The denuded axon provides a stimulus for remyelination o Newly formed myelinated internodes are shorter than normal and several are required to bridge the demyelinated region o New myelin sheath is thin in proportion to diameter of the axon o Sequential episodes of demyelination and remyelination  onion bulb appearance Axonal Degeneration and Muscle Fiber Atrophy ▪Axonal degeneration: result of primary destruction of the axon with secondary disintegration of its myelin sheath ▪Wallerian degeneration: axonal injury occurs as a result of a focal lesion (trauma, ischemia)  axon breaks down, Schwann cells catabolize myelin and engulf axon fragments, formation of myelin ovoids ▪Neuronopathy and axonopathy: axonal degeneration is scant  only few fibers actively degenerate at any given time ▪When axonal degeneration occurs, muscle fibers w/in the affected motor unit lose their neural input and undergo denervation atrophy (cells remain viable) ▪Type specific Atrophy: atrophy of disuse (common in type 2 fibers)

Nerve Regeneration and Reinnervation of Muscle ▪Regenerating cluster: multiple closely aggregated, thinly myelinated small caliber axons  evidence of regeneration (a slow process) ▪Reinnervation: occurs when axons belonging to an unaffected neighboring motor unit extend sprouts to reinnervate the denervated myocytes and incorporate them in to the healthy motor unit Reactions of the Muscle Fiber ▪Segmental necrosis: destruction of a portion of the length of a myocyte ▪Vacuolation, alterations in structural proteins or organelles and accumulation of intracytoplasmic deposits ▪Regeneration

▪Fiber hypertrophy: in response to increased load