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HISTOLOGY_LECTURE_9_Nerve_tissue_nervous_system - Mescher...

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Mescher AL, Chapter 9.Mescher AL, Chapter 9.Nerve Tissue & the NervousNerve Tissue & the NervousSystem: Junqueira's BasicSystem: Junqueira's BasicHistology: Text & Atlas, 12e:Histology: Text & Atlas, 12e:
Nerve Tissue & the Nervous System:Nerve Tissue & the Nervous System: thenervous system can be divided into thefollowing:Central nervous system(CNS), consisting of the brain and spinalcordPeripheral nervous system (PNS),composed of the cranial, spinal, andperipheral nerves conducting impulses toand from the CNS (motor and sensorynerves respectively) andgangliawhichare small groups of nerve cells outside theCNS (Figure 9–1; Table 9–1).
Figure 9-1. The general organization of the nervous system.
Both central and peripheral nerve tissueconsists of two cell types:nerve cells,orneurons,which usually show numerouslong processes; and variousglial cells(Gr.glia,glue), which have shortprocesses, support and protect neurons,and participate in neural activity, neuralnutrition, and defense of cells in thecentral nervous system.Neurons respond to environmentalchanges (stimuli) by altering the ionicgradient that exists between the inner andouter surfaces of their membranes.
All cells maintain such a gradient, alsocalled an electrical potential, but cells thatcan rapidly change this potential inresponse to stimuli (eg, neurons, musclecells, some gland cells) are said to beexcitableorirritable.Neurons reactpromptly to stimuli with a reversal of theionic gradient (membranedepolarization) that generally spreadsfrom the place that received the stimulusand is propagated across the neuron'sentire plasma membrane. Thispropagation, called theaction potential,thedepolarization wave, or thenerveimpulse.
Development of Nerve Tissue:The nervous system develops from the outerembryonic layer, the ectoderm, beginning in the thirdweek of human embryonic life (Figure 9–2). Withsignals from the notochord, the underlying axialstructure, ectoderm along the mid-dorsal side of theembryo thickens to form the epithelialneural plate.The lateral sides of this plate fold upward, bend andgrow toward each other medially and within a fewdays fuse to form theneural tube. Cells of this tubegive rise to the entire CNS, including neurons, mostglial cells, ependymal cells, and the epithelial cells ofthe choroid plexus. As the folds fuse and the neuraltube separates from the now overlying ectoderm thatwill form epidermis, a large population of importantcells called theneural crestseparates from theneuroepithelium and becomes mesenchymal
Figure 9-2. Neurulation in the early embryo.
Neurons:The functional unit in both the CNS and PNS isthe neuron or nerve cell. Most neurons consistof three parts (Figure 9–3):
Neurons can be classified according to thenumber of processes extending from the cellbody (Figure 9–4):Multipolar neurons,whichhave one axon and two or many dendrites;Bipolar neurons,with one dendrite and oneaxon; andUnipolarorpseudounipolar neurons,which

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