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494 SecTioN iii Neurology aNd Special SeNSeS ` neurology—AnAtomy And Physiology Neurology aNd Special SeNSeS ` neurology—AnAtomy And Physiology
Myelin conduction velocity of signals transmitted Wraps and insulates axons (arrow in A ): space
A down axons saltatory conduction of action constant and conduction velocity.
potential at the nodes of Ranvier, where there
are high concentrations of Na channels.
+
In CNS (including CN II), myelin is synthesized COPS: CNS = Oligodendrocytes, PNS =
by oligodendrocytes; in PNS (including CN Schwann cells.
III-XII), myelin is synthesized by Schwann
cells.
Schwann cells Promote axonal regeneration. Derived from Each “Schwone” cell myelinates only 1 PNS
neural crest. axon.
Nucleus Schwann cell
Injured in Guillain-Barré syndrome.
Myelin sheath Node of Ranvier
Oligodendrocytes Myelinate axons of neurons in CNS. Each Derived from neuroectoderm.
oligodendrocyte can myelinate many axons “Fried egg” appearance histologically.
Node of Ranvier
(∼ 30). Predominant type of glial cell in white Injured in multiple sclerosis, progressive
Axon matter. multifocal leukoencephalopathy (PML),
leukodystrophies.
Oligodendrocyte
Sensory receptors
reCePtor tyPe sensory neuron Fiber tyPe loCAtion senses
Free nerve endings Aδ—fast, myelinated fibers All skin, epidermis, some Pain, temperature
C—slow, unmyelinated viscera
A Delta plane is fast, but a
taxC is slow
Meissner corpuscles Large, myelinated fibers; adapt Glabrous (hairless) skin Dynamic, fine/light touch,
quickly position sense, low-frequency
vibration
Pacinian corpuscles Large, myelinated fibers; adapt Deep skin layers, ligaments, High-frequency vibration,
quickly joints pressure
Merkel discs Large, myelinated fibers; adapt Finger tips, superficial skin Pressure, deep static touch (eg,
slowly shapes, edges), position sense
Ruffini corpuscles Dendritic endings with Finger tips, joints Pressure, slippage of objects
capsule; adapt slowly along surface of skin, joint
angle change
FAS1_2019_12-Neurol.indd 494 11/8/19 7:39 AM
