Neurology and Special Senses: Cells of the Nervous System PDF

Neurology and Special Senses   neurology—Anatomy and Physiology SEC TION III 503 ` NEUROLOGY—ANATOMY AND PHYSIOLOGY Cells of the nervous Neurons and nonneuronal (glial) cells....

Neurology and Special Senses   neurology—Anatomy and Physiology SEC TION III 503 ` NEUROLOGY—ANATOMY AND PHYSIOLOGY Cells of the nervous Neurons and nonneuronal (glial) cells. CNS glial cells—neuroectoderm (except system Neurons—permanent, signal-transmitting cells microglia, which derive from mesoderm). of the nervous system composed of dendrites PNS glial cells—neural crest ectoderm. (receive input), cell bodies, and axons (send Myelin is a multilayer wrapping of output). Dendrites and cell bodies can be seen electrical insulation formed around axons on Nissl staining (stains RER; not present Ž  conduction velocity of transmitted signals in axons). Markers: neurofilament protein, via saltatory conduction of action potentials at synaptophysin. nodes of Ranvier ( Na+ channel density). CNS glial cells Astrocytes Physical support, repair, removal of excess Largest and most abundant glial cell in CNS. neurotransmitters, component of blood-brain Reactive gliosis in response to neural injury. barrier, glycogen fuel reserve buffer. GFAP ⊕. Oligodendrocytes Myelinate axons in CNS (including CN II). Each myelinates many axons (∼ 30). “Fried egg” appearance histologically Predominant type of glial cell in white matter. (“oleggodendrocytes”). Injured in multiple sclerosis, leukodystrophies, progressive multifocal leukoencephalopathy. Ependymal cells Ciliated simple columnar glial cells lining Specialized ependymal cells (choroid plexus) ventricles and central canal of spinal cord. produce CSF. Apical surfaces are covered with cilia (which circulate CSF) and microvilli (which help with CSF absorption). Microglia Activation in response to tissue damage Phagocytic scavenger cells of CNS. Ž release of inflammatory mediators (eg, nitric HIV-infected microglia fuse to form oxide, glutamate). Not readily discernible by multinucleated giant cells in CNS in HIV- Nissl stain. associated dementia. PNS glial cells Satellite cells Surround neuronal cell bodies in ganglia. Similar supportive role to astrocytes. Schwann cells Myelinate axons in PNS (including CN III-XII). Each myelinates a single axon (“Schwone”). S100 ⊕. Injured in Guillain-Barré syndrome. Central nervous system Peripheral nervous system Neuron Microglia Satellite cell Astrocyte Node of Ranvier Schwann cell Ependymal cell Oligodendrocyte Axon FAS1_2024_12-Neurol.indd 503 1/31/24 8:39 AM  504 SEC TION III Neurology and Special Senses   neurology—Anatomy and Physiology Neuron action potential Voltage-gated Voltage-gated Na+ channel K+ channel Na+ + 40 Extracellular Membrane potential 1 Intracellular K+ Na+ relative permeability Inactivation Activation gate 0 gate Na+ Membrane potential (mV) 2 3 2 Threshold potential K+ –55 K+ relative permeability Na + 3 Resting potential –75 K+ 1 Na+ 4 –100 4 Time (ms) K+ ➊ Resting membrane potential: membrane is more permeable to K+ than Na+ at rest. Voltage-gated Na+ and K+ channels are closed. ➋ Membrane depolarization: Na+ activation gate opens Ž Na+ flows inward. ➌ Membrane repolarization: Na+ inactivation gate closes at peak potential, thus stopping Na+ inflow. K+ activation gate opens Ž K+ flows outward. ➍ Membrane hyperpolarization: K+ activation gates are slow to close Ž excess K+ efflux and brief period of hyperpolarization. Voltage-gated Na+ channels switch back to resting state. Na+/K+ pump restores ions concentration. Sensory receptors RECEPTOR TYPE SENSORY NEURON FIBER TYPE LOCATION SENSES Free nerve endings Aδ—fast, myelinated fibers All tissues except cartilage and Pain, temperature C—slow, unmyelinated eye lens; numerous in skin A Delta plane is fast, but a taxC is slow Meissner corpuscles Large, myelinated fibers; adapt Glabrous (hairless) skin Dynamic, fine/light touch, quickly low-frequency vibration, skin indentation 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) Ruffini corpuscles Large, myelinated fiber Finger tips, joints Stretch, joint angle change intertwined among collagen fiber bundles; adapt slowly FAS1_2024_12-Neurol.indd 504 1/31/24 8:39 AM