The nervous system is derived from the dorsal portion of the embryonic ectoderm, which rolls into an epithelial neural tube, and from the neural crest. It has three major divisions: central, peripheral, and autonomic (Ham and Cormack, 1979). The central nervous system (CNS) is derived from the neural tube and consists of the brain and spinal cord. The CNS controls the voluntary actions of the body, as well as some involuntary actions, such as reflexes. The retina and optic nerve (cranial nerve II) are considered part of the central nervous system, since they develop from, and project to, the diencephalon of the brain. The peripheral division includes the spinal nerves, as well as cranial nerves I (olfactory) and III-XII. The motor component of the spinal nerves develops from the neural tube and the sensory component from the neural crest. Cranial nerves I and III-XII develop from ectodermal placodes and neural crest. The autonomic system, a complex subset of the peripheral nervous system, is derived from the neural crest and controls involuntary activities, such as heart rate, temperature, and the smooth muscle activity of the vascular and digestive systems.
Neural tissue in all three divisions is made up of neurons and associated glial cells (figure 5.1). Neurons consist of a cell body that sends electrical signals over axons and receives signals from the axons of other neurons through shorter dendrites. Neurons are linked at synapses, junctions where axons from one neuron meet the dendrites or cell body of another neuron. All nerve cell bodies and axons are associated with glial cells. In the brain and spinal cord, the major glial cell types are oligodendrocytes, which form insulating myelin, and astrocytes, of which several types reside in both the gray and white matter. Astrocytes are crucial for neuron survival, clearing of the neurotransmitter glutamate, maintaining acid-base balance, degrading and forming synapses, and modulating neuron responses (Travis, 1994). The glial cells associated with the optic nerve are primarily astrocytes. In the peripheral nervous system, Schwann cells are the glial counterparts of CNS oligodendrocytes and form the myelin sheath of the axons.
Mammals are able to regenerate peripheral nerve axons, neurons of the olfactory epithelium and bulb, and neurons of the hippocampus, but cannot regenerate axons or neurons of the spinal cord, optic nerve, or acoustic sensory epithelium. However, fetal stages of mammals and the adults of other vertebrates have the capacity to regenerate the neurons and axons of these tissues and are thus valuable research models to learn why regeneration fails in the mammal.
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