Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 70

Cellular secretion is the process of releasing substances that cause changes in other cells, usually, or even the secreting cell. Secretion may have evolved from excretion (the release of waste products). Although many secretion mechanisms exist, secretion typically involves vesicles transiently docking and fusing with the cell membrane, thereby releasing their contents into the extracellular space. Some secretion occurs from porosomes, which are permanent secretory structures at the cell plasma membrane.

Secreted signals are often hormones — chemicals released by a cell in one part of the organism that act as messages to cells in other parts of the organism. Hormones may travel in the extracellular space (which is called paracrine signaling) or be transported in the blood. Target cells that express a specific hormone receptor can respond when that hormone binds the receptor. This activates a cascade of signals, producing hormone-induced responses. Mechanisms exist to degrade the hormone so the cycle can begin again.

Proteins that are to be secreted are synthesized by ribosomes located on the rough ER. Sugar molecules are then added in a process called glycosylation, and the protein begins to fold with the help of special proteins called molecular chaperones. Vesicles containing the secretory proteins bud from the ER membrane and travel to the Golgi apparatus. In the Golgi apparatus, where further post-translational modifications may occur, the proteins are encapsulated in secretory vesicles. These vesicles are transported via the cytoskeleton to their destination at the plasma membrane. (Sometimes post-translational modifications occur within the vesicles.) Finally, the vesicle contents are released outside the cell (the process is called exocytosis) when the vesicle fuses with the cell membrane.

Secretion probably evolved from excretion when the detection of excreted products from one cell became a signal for other cells whose behavioral changes based on that signal were adaptive to the organism. Most, but not all, neurotransmitters are released by calcium-mediated exocytosis (the movement of material out of a cell using a sac or vesicle) after an action potential at the axon terminal.

The sequence of neurotransmission is quite similar to hormonal communication. Neurotransmission differs, however, in that neurons actually contact other specific neurons — or muscles or gland cells — directly at synapses, where information flows from one neuron to another across the synaptic cleft, the gap between a pre- and post-synaptic cell. The neurotransmitter released usually activates only receptors directly across the synaptic cleft in a single postsynaptic cell. This allows for significantly more complex communication with neurons than is possible with hormones. However, some specialized cells that are directly driven by neurons in brain areas like the hypothalamus do secrete hormones that circulate in the bloodstream. (See Chapter 4 for details about synapses and synaptic function.)

In some cases, however, a single presynaptic terminal can activate more than one post-synaptic receptor region. Extra-synaptic neurotransmission also happens, where neurotransmitter molecules escape from the synaptic cleft and activate distal sites on the postsynaptic neuron, or even other neurons.