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EYE BRAIN AND VISION - PART 2

Chất trắng trong não và tủy sống bao gồm các sợi trục có bao myelin nhưng không có cơ quan tế bào thần kinh, các sợi nhánh, hoặc các khớp thần kinh. Xám vấn đề là thực hiện chủ yếu là các cơ quan tế bào, sợi nhánh, thiết bị đầu cuối sợi trục, và các khớp thần kinh | result is that the nerve impulse in effect jumps from one node to the next rather than traveling continuously along the membrane which produces a great increase in conduction velocity. The fibers making up most of the large prominent cables in the brain are myelinated giving them a glistening white appearance on freshly cut sections. White matter in the brain and spinal cord consists of myelinated axons but no nerve cell bodies dendrites or synapses. Grey matter is made up mainly of cell bodies dendrites axon terminals and synapses but may contain myelinated axons. The main gaps remaining in our understanding of the impulse and also the main areas of present-day research on the subject have to do with the structure and function of the protein channels. SYNAPTIC TRANSMISSION How are impulses started up in the first place and what happens at the far end when an impulse reaches the end of an axon The part of the cell membrane at the terminal of an axon which forms the first half of the synapse the presynaptic membrane is a specialized and remarkable machine. First it contains special channels that respond to depolarization by opening and letting positively charged calcium ions through. Since the concentration of calcium like that of sodium is higher outside the cell than inside opening the gates lets calcium flow in. In some way still not understood this arrival of calcium inside the cell leads to the expulsion across the membrane from inside to outside of packages of special chemicals call neurotransmitters. About twenty transmitter chemicals have been identified and to judge from the rate of new discoveries the total number may exceed fifty. Transmitter molecules are much smaller than protein molecules but are generally larger than sodium or calcium ions. Acetylcholine and noradrenaline are examples of neurotransmitters. When these molecules are released from the presynaptic terminal they quickly diffuse across the 0.02-micrometer synaptic gap to the postsynaptic .