Nervous Tissue |
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NEURONS
Neurons are cells that are morphologically and functionally polarized so that information may pass from one end ofthe cell to the other.
Neurons may be classified by the form and number oftheir processes as bipolar, unipolar, or multipolar. The cell body of the neuron contains the nucleus and membrane-bound cytoplasmic organelles typical of a eukaryotic cell, including endoplasmic reticulum (ER), Golgi apparatus, mitochondria, and lysosomes. The nucleus and nucleolus are prominent in neurons.
The cytoplasm contains Nissl substance, clumps ofrough ER with bound poly somes. The cytoplasm also contains free polysomes; free and bound polysomes in the Nissl substance are sites ofprotein synthesis.
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Chapter 5 • Nervous Tissue
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Figure 1-5-2. Neural Tissue with Nissl stain that stains rough
ER in cell body (arrowhead) and proximal parts of dendrites (B)
The axon (A) lacks Nissl substance. The nucleus of an adjacent neuron has a prominent nucleolus (arrow).
Clinical Correlate
CNS Disease and Cytoplasmic Inclusions in Neurons
Lewy bodies are cytoplasmic inclusions of degenerating neurons ofthe substantia nigra, pars compacta, evident in patients with Parkinson's disease and in cortical and brain-stem neurons in patients with certain forms of dementia.
Negri bodies are eosinophilic cytoplasmic inclusions seen in degenerating neurons in the hippocampus and cerebellar cortex in patients with rabies.
Cytoskeleton
The cytoskeleton ofthe neuron consists ofmicrofilaments, neurofilaments, and microtubules.
Neurofilaments provide structural support for the neuron and are most numer ous in the axon and the proximal parts ofdendrites.
Microfilamentsform a matrixnear the peripheryofthe neuron. A microfilament matrix is prominent in growth cones ofneuronal processes and functions to aid in the motility ofgrowth cones during development. A microfilament matrix is prominent in dendrites and forms structural specializations at synaptic mem branes. Microtubules are found in allparts ofthe neuron, and are the cytoplas mic organelles used in axonal transport.
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Section I • Hi tology and Cell Biology
Copyright McGraw-Hill Companies. Used with permission.
Figure 1-5-3. EM of Neuropil including the Cell Body of a
Neuron with Rough ER (arrowheads) and Golgi (arrows)
Surrounding neuropil has myelinated axons(M) and unmyelinated bare axons (box).
Clinical Correlate
Microtubules and Neuronal Degenerative Diseases
In degenerative neuronal diseases of the CNS, a tau protein becomes excessively phosphorylated, which prevents crosslinking of microtubules. The affected microtubules form helical filaments and neurofibrillary tangles and senile plaques in the cell body and dendrites of neurons. Neurofibrillary
tangles are prominent features of degenerating neurons in Alzheimer's disease, amyotrophic lateral sclerosis, and Down syndrome patients.
Dendrites taper from the cell body and provide the major surface for synaptic contacts with axons of other neurons. which are small cytoplasmic extensions that dramatically increase the surface area of den drites. Dendrites maybe highlybranched; thebranchingpattern ofdendrites may be used to define a particular neuronal cell type.
Theaxonhas auniformdiameterandmaybranchatrightanglesinto collaterals along the length ofthe axon, in particular near its distal end. The proximal part of the axon is usually marked by an a tapered extension ofthe cellbody that lacks Nissl substance.
The initial segment is adjacent to the axon hillock. The membrane ofthe initial segment contains numerous voltage-sensitive sodium ion channels. The initial segment is the "trigger zone" ofan axon where conduction ofelectrical activity as an action potential is initiated.
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Section I • Histology and Cell Biology
remove the neurotransmitter glutamate and K+ ions from the extracellular space. Astrocytes have foot processes that contribute to the blood-brain bar rier by forming a glial-limiting membrane. Astrocytes hypertrophy and pro liferate after an injury to the CNS; they fillup the extracellular space left by degenerating neurons by forming an astroglial scar. Radial glia are precursors of astrocytes that guide neuroblast migration during CNS development.
Microglia cells are the smallest glial cells in the CNS. Unlike the rest of the CNS neurons and glia, which are derived from neuroectoderm, microglia are derived from bone marrow monocytes and enter the CNS after birth. Microglia provide a link between cells of the CNS and the immune system. Microglia proliferate and migrate to the site of a CNS injury and phagocytose neuronal debris after injury. Pericytes are microglia that contribute to the blood-brain barrier.
GLIAL CELLS
Microglia and CNS Disorders
Microglia determine the chances of survival of a C
S tissue graft and are the cells in the CNS that are targeted by the HIV- 1 virus in patients with AIDS. The affected microglia may produce cytokines that are toxic to neurons.
CNS microglia that become phagocytic in response to neuronal tissue damage may secrete toxic free radicals. Accumulation offree radicals, such as superoxide, may lead to disruption of the calcium homeostasis of neurons.
Oligodendrocytes form myelin for axons in the CNS. Each of the processes of the oligodendrocyte can myelinate individual segments of many axons. Unmy elinated axons in the CNS are not ensheathed by oligodendrocyte cytoplasm.
Schwann cells are the supporting cells of the peripheral nervous system (PNS), and are derived from neural crest cells. Schwann cells form the myelin for axons and processes in the PNS. Each Schwann cell forms myelin for only a single inter nodal segment of a single axon. Unmyelinated axons in the PNS are enveloped by the cytoplasmic processes of a Schwann cell. Schwann cells act as phagocytes and remove neuronal debris in the PNS after injury. A node ofRanvier is the region between adjacent myelinated segments of axons in the CNS and the PNS.
In all myelinated axons, nodes ofRanvier are sites that permit action potentials to jump from node to node (saltatory conduction). Saltatory conduction dramati cally increases the conduction velocity of impulses in myelinated axons.
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Chapter 5 • NervousTissue
CopyrightMcGraw-Hill Companies. Used with permission.
Figure 1-5-4. Section of a Peripheral Nerve
Endoneurial border of an individual axon (arrowhead) and two Schwann cell nuclei (arrows) are shown.
Myelin
Schwann cell nuclei
Figure 1-5-5. Axons Cut in Cross-Section
Multiple sclerosis (MS) is marked by the presence of plaques, which are sharply demarcated areas of demyelination. MS plaques tend to form in axons that course near the surfaces of the lateral ventricles, in the floor of the fourth ven tricle, or near the pial surfaces of the brain stem or spinal cord.
•In patients with MS, multiple lesions appear over time, but the signs and symptoms may undergo exacerbation and remission
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Section I • Histology and Cell Biology
Clinical Correlate
Drugs of Addiction and the
Blood-Brain Barrier
Heroin, ethanol, and nicotine are lipid soluble compounds that readily diffuse across the blood-brain barrier.
Clinical Correlate
Immune-Related Demyelinating Diseases, MS, and Guillain-Barre
Syndrome
In MS, myelin formed by oligodendrocytes undergoes an inflammatory reaction that impairs impulse transmission in axons in the CNS.
•Commonly, 2 or more CNS sensory or motor neural systems are affect ed in separate attacks.
•The only cranial nerve or spinal nerve affected by MS is the optic nerve because all of the myelin sheaths of its axons are formed by oligoden drocytes.
In Guillain-Barresyndrome, myelin formed by Schwann cells in the PNS under goes an acute inflammatory reaction after a respiratory or gastrointestinal illness. This reaction impairs or blocks impulse transmission ofaxons in the PNS, result ing in a polyneuropathy. Motor axons are always affected, producing weakness in the limbs. Weakness of cranial nerve (CN) innervated muscles (most commonly those innervated by CNs VI and VII) or respiratory muscles may be seen. Sen sory deficits are mild or absent.
Ependymalcellsline the ventricles in the adult brain. Some ependymal cells dif ferentiate into choroid epithelial cells, forming part of the choroid plexus, which produces cerebrospinal fluid (CSF). Ependymal cells are ciliated; ciliary action helps circulate CSF.
Tanycytes are specialized ependymal cells that have basal cytoplasmic processes in contact with blood vessels; these processes may transport substances between a blood vessel and a ventricle.
Blood-Brain Barrier
•The blood-brain barrier restricts access of micro-organisms, proteins, cells, and drugs to the nervous system.
•The blood-brain barrier consists of capillary endothelial cells, an under lying basal lamina, astrocytes, and pericytes.
•Cerebral capillary endothelial cells and their intercellular tight junctions are the most important elements of the blood-brain barrier.
•Astrocytes and pericytes are found at the blood-brain barrier outside the basal lamina. Astrocytes have processes with "end feet" that cover more than 95% of the basal lamina adjacent to the capillary endothelial cells.
Substances cross the blood-brain barrier into the CNS by diffusion, by se lective transport, and via ion channels. Oxygen and carbon dioxide are lipid soluble gases that readily diffuse across the blood-brain barrier. Glucose, amino acids, and vitamins K and D are selectively transported across the blood-brain barrier. Sodium and potassium ions move across the blood-brain barrier through ion channels.
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Chapter 5 • Nervous Tissue
Chapter Summary
•Neurons are composed of a cell body, dendrites, and an axon. They contain pigments such as melanin and lipofuscin.
•The cell body (soma or perikaryon) contains a nucleus, other cellular components, and rough endoplasmic reticulum (ER). Microtubules and neurofilaments form the cytoskeleton. They are important for axonal transport.
•Dendrites receive and transmit information to the cell body.
•Axons arise from the perikaryon or proximal dendrite. They contain microtubules and neurofilaments. Rapid axonal transport utilizes microtubules. Kinesins promote anterograde transport, whereas dynein promotes retrograde transport.
•Myelin is the covering of axons and is composed of phospholipids and cholesterol. Axons may be myelinated or nonmyelinated. Schwann cells myelinate a single peripheral nervous system (PNS) axon.
•Neurons may also associate with several axons (unmyelinated) without forming myelin. Oligodendrocytes form myelin in the central nervous system (CNS). One oligodendrocyte myelinates many axons.
•The node ofRanvier is a collar of naked axon between a proximal and distal bundle of myelin that has myelinated the axon. Its purpose is to allow rapid signal transport by rapidly moving from one node to the next. This process is called saltatory conduction.
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