(For further discussion, see below Transmission at the synapse.) Dendritesīesides the axon, neurons have other branches called dendrites that are usually shorter than axons and are unmyelinated. Rapid neuronal communication at these junctions is probably electrical in nature. At these synapses there is no synaptic gap instead, there are gap junctions, direct channels between neurons that establish a continuity between the cytoplasm of adjacent cells and a structural symmetry between the pre- and postsynaptic sites. While the chemically mediated synapse described above forms the majority of synapses in vertebrate nervous systems, there are other types of synapses in vertebrate brains and, in especially great numbers, in invertebrate and fish nervous systems. Other neurotransmitter receptors do not have the same structure, but they are all proteins and probably have subunits with a central channel that is activated by the neurotransmitter. This receptor, called the end plate, is a glycoprotein composed of five subunits. Most knowledge of postsynaptic neurotransmitter receptors comes from studies of the receptor on muscle cells. In nerve-muscle junctions the synaptic cleft contains a structure called the basal lamina, which holds an enzyme that destroys neurotransmitters and thus regulates the amount that reaches the postsynaptic receptors on the receiving cell. The presynaptic terminal is unmyelinated and is separated from the neuron or muscle cell onto which it impinges by a gap called the synaptic cleft, across which neurotransmitters diffuse when released from the vesicles. The thickened areas are called presynaptic dense projections, or active zones. The most numerous of these are synaptic vesicles, which, filled with neurotransmitters, are often clumped in areas of the terminal membrane that appear to be thickened. Presynaptic terminals, when seen by light microscope, look like small knobs and contain many organelles. This process is known as axoplasmic flow it occurs in both directions along the axon and may be facilitated by microtubules.Īt the terminal of the axon, and sometimes along its length, are specialized structures that form junctions with other neurons and with muscle cells. The synthesis of these substances can occur in the terminal itself, but the synthesizing enzymes are formed by ribosomes in the soma and must be transported down the axon to the terminal. While the axon mainly conducts nerve impulses from the soma to the terminal, the terminal itself secretes chemical substances called neurotransmitters. In the central nervous system the myelin sheath is formed from glial cells called oligodendrocytes, and in peripheral nerves it is formed from Schwann cells ( see below The neuroglia). Concentric layers of these lipids separated by thin layers of protein give rise to a high-resistance, low-capacitance electrical insulator interrupted at intervals by gaps called nodes of Ranvier, where the nerve membrane is exposed to the external environment. Myelin is composed of 80 percent lipid and 20 percent protein cholesterol is one of the major lipids, along with variable amounts of cerebrosides and phospholipids. Large axons acquire an insulating myelin sheath and are known as myelinated, or medullated, fibres. This is the region where the plasma membrane generates nerve impulses the axon conducts these impulses away from the soma or dendrites toward other neurons. The axon arises from the soma at a region called the axon hillock, or initial segment.
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