Peripheral Nerves: Histology & Diseases PDF

‫” قالوا سبحانك ال علم لنا إال ما‬ ‫علمتنا إنك أنت العليم الحكيم “‬ ‫صدق هللا العظيم‬ ‫سورة البقرة اية ‪32‬‬ al-imam mohammed Bin saud university College of Medicine CNSS BLOCK Peripheral nerves: Histology & diseases Eman Ali Elkordy 2 OUTLINE Introduction Histological structure of the axon Connecti...

‫” قالوا سبحانك ال علم لنا إال ما‬ ‫علمتنا إنك أنت العليم الحكيم “‬ ‫صدق هللا العظيم‬ ‫سورة البقرة اية ‪32‬‬ al-imam mohammed Bin saud university College of Medicine CNSS BLOCK Peripheral nerves: Histology & diseases Eman Ali Elkordy 2 OUTLINE Introduction Histological structure of the axon Connective Tissue Investments of Peripheral Nerves Non-myelinated and myelinated axons of peripheral nerves Patterns of Peripheral Nerve Injury Peripheral Nerves: Histology & Diseases 3 Introduction  Peripheral nerves are the connections between the elements of the CNS & the target tissue  They connect the brain (cranial nerves) & the spinal cord (spinal nerves) with the target tissues  They share some histological features and disease conditions Peripheral Nerves: Histology & Diseases 4 Peripheral Nerves Bundles of nerve fibers ( axons) surrounded by connective tissue. – cranial nerves – spinal nerves A bundle has both sensory and motor components, myelinated and unmyelinated fibers together Peripheral Nerves: Histology & Diseases 5 Peripheral Nerves Bundles of axons One of processes of neuron Functional unit of nervous system Peripheral Nerves: Histology & Diseases 6 Peripheral Nerves: Histology & Diseases 7 Peripheral Nerves: Histology & Diseases 8 Peripheral Nerves: Histology & Diseases 9 Connective Tissue Investments of Peripheral Nerves: Epineurium Perineurium Endoneurium fascicle  Tiny blood vessels (vasa nervora) & sensory nerve (nervi nervora) found in these wrappings Peripheral Nerves: Histology & Diseases 10 Epineurium Covers the entire peripheral nerve Dense, irregular, collagenous connective tissue Contains blood vessels, may contain some fat cells Continuous with the dura mater Perineurium Covers each bundle (fascicle) of axons within the nerve Perineurial cells – several layers of squamous cells joined by tight junctions – constitutes blood-nerve barrier Endoneurium Surrounds individual nerve fibers (axon) It is in contact with and surrounds the basal lamina of the Schwann cells A loose connective tissue Peripheral Nerves: Histology & Diseases 11 Epineurium fascicle Perineurium Peripheral Nerves: Histology & Diseases 12 Classification of Nerves I- Functionally, the PNS is divided into: 1) Sensory (afferent) component; receives-transmits impulses from skin-viscera to the CNS 2) Motor (efferent) component; CNS to effector organs. II- According to the myelination, nerve fibers (axons) are classified into: 1) Myelinated 2) Unmyelinated Peripheral Nerves: Histology & Diseases 13 Non-myelinated axons The relationship of non-myelinated axons with their supporting Schwann cell is illustrated in diagram (a). One or more axons become longitudinally invaginated into the Schwann cell Peripheral Nerves: Histology & Diseases 14 Myelinated axons Axon Myelin sheath In peripheral nerves, myelination begins with the invagination of a single nerve axon into a Schwann cell; a mesaxon (The zone of apposition of the Schwann cell membrane is called the mesaxon ) is then formed. As myelination proceeds, the mesaxon rotates around the axon thereby enveloping the axon in concentric layers of Schwann cell plasma membrane Peripheral Nerves: Histology & Diseases 15 Internode The single axon is enveloped by many layers of fused Schwann cell plasma membrane forming the myelin sheath. The single segment of myelin produced by each Schwann cell is termed an internode; this ensheaths the axon between one node of Ranvier and the next Nodes of Ranvier are areas of the myelinated axon that are not covered by the myelin sheath. So, Each Schwann cell myelinates a single internode Peripheral Nerves: Histology & Diseases 16 Action potential propagation in myelinated neurons is faster than in unmyelinated neurons because of Saltatory conduction. Myelin is a whitish lipoprotein complex and has myelin specific proteins (MSP); Protein 0, peripheral myelin protein, myelin basic protein In myelinated fibers action potential “jumps” from node to node, a process called saltatory conduction, the myelin provides the electrical insulation of neurons Unmyelinated fibers; continuous conduction of the impulse (slower and requires more energy than the saltatory conduction) Peripheral Nerves: Histology & Diseases 17 Axon Myelin Unmyelinated axo Perineurium Epineurium Peripheral nerve Peripheral Nerves: Histology & Diseases 18 Peripheral Nerves: Histology & Diseases 19 Patterns of Peripheral Nerve Injury There are two main patterns of peripheral nerve disease, termed peripheral neuropathy, with symptoms including weakness and sensory loss. In one type, there is damage to the Schwann cells and myelin, causing reduced conduction velocity in nerves (demyelinating neuropathy). In the other main type, there is damage to the axons (axonal neuropathy). Schwann cells can regenerate after damage and remyelinate axons. Axons can also regenerate, providing the neuronal cell body is not damaged. Peripheral Nerves: Histology & Diseases 20 Peripheral Nerves: Histology & Diseases 21 Axonal neuropathies are caused by insults that directly injure the axon. The entire distal portion of an affected axon degenerates. Axonal degeneration is associated with secondary myelin loss The morphologic hallmark of axonal neuropathies is a decrease in the density of axons, correlates with a decrease in the strength of amplitude of nerve impulses. Peripheral Nerves: Histology & Diseases 22 Demyelinating neuropathies are characterized by damage to Schwann cells or myelin with relative axonal sparing, resulting in abnormally slow nerve conduction velocities. Demyelination typically occurs in individual myelin internodes randomly; this process is termed segmental demyelination Peripheral Nerves: Histology & Diseases 23 Disorders of myelin In Guillain-Barré syndrome, there is immune-mediated destruction of myelin in the PNS. Patients develop rapidly progressive weakness of limbs and weakness of respiratory muscles. Histological examination of affected nerve shows loss of myelin with preservation of axons. Conduction velocity in affected nerves is greatly slowed. Mutation in genes coding for myelin proteins is the basis for several inherited disorders of the nervous system Peripheral Nerves: Histology & Diseases 24