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Nervous System SUMMARY OF KEY POINTS Development of Nerve Tissue

  • Nervous tissue develops in the early embryo when the dorsal ectoderm neural plate folds lengthwise to form the neural tube, the precursor of the CNS, and releases neural crest cells, precursors for much of the PNS.

  • There are many kinds of neurons, but all consist of a cell body (perikaryon) containing the nucleus, a long cytoplasmic extension called the axon, and one or more shorter processes called dendrites.

  • Neurons use the common cell property of excitability to produce and move an action potential (nerve impulse) along the axon to excite another neuron or other effector cell.

  • Such nerve communication is transmitted to another neuron or effector cell via a synapse, where neurotransmitter is released at the presynaptic membrane and binds receptors on the postsynaptic cell, initiating a new action potential there.

Glial Cells
  • Glial cells (glia), required to support neurons in many ways, consist of six major types:

  • Oligodendrocytes wrap processes around portions of axons in the CNS, forming myelin sheaths that insulate the axons and facilitate nerve impulses.

    • Astrocytes, the most numerous cell of the CNS, all produce hundreds of processes to cover and provide regulated microenvironments for neuronal perikarya, synapses, and capillaries.

    • Ependymal cells are epithelial-like cells, lacking basement membranes, which line the fluid-filled cerebral ventricles and central canal of the spinal cord.

    • Microglia differs from all other glial cells in originating from blood monocytes, not from neural tissue precursors; they mediate immune defense activity within the CNS.

    • Schwann cells (neurolemmocytes) enclose all axons in nerves of the PNS, producing myelin sheaths around large-diameter axons, whose impulse conductivity is augmented at the nodes of Ranvier between successive Schwann cells.

    • Satellite cells are located within PNS ganglia, aggregated sensory or autonomic neuronal cell bodies, where they enclose each perikaryon and regulate its microenvironment.

Central Nervous System
  • Within the brain and spinal cord, regions rich in neuronal perikarya and astrocytes comprise the gray matter and regions containing tracts of myelinated axons comprise white matter.

  • Hundreds of different neurons make up the CNS; large, unique Purkinje neurons characterize the cortex of the cerebellum, and layers of small pyramidal neurons form the cerebral cortex.

  • The CNS is completely enclosed by three connective tissue layers called meninges: (1) the tough external dura mater; (2) the middle arachnoid layer; and (3) the delicate pia mater that directly contacts neural tissue.

  • The arachnoid layer contains much CSF, which helps cushion the CNS within its bony enclosure.

  • The choroid plexus consists of elaborate folds of vascularized pia mater covered by ependyma that project from walls of the cerebral ventricles; the water is removed from capillaries and transferred into the ventricles as CSF.

  • In most CNS regions, neurons are also protected by the blood-brain barrier, consisting of the perivascular feet of astrocytic processes and the nonfenestrated capillary endothelial ...

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