Types of chemical messenger

As knowledge is gained about the variety and types of chemical messengers, a definitive classification becomes more challenging some terminology has already become rednndant. 

In a complex organism like the human, different cell types carry out different functions. This specialization of function requires cells to communicate with each other. One of the ways they communicate is through secretion of chemical messengers that carry a signal to another cell. In Chapter 11, we consider some of the principles of cell signaling and describe some of the chemical messenger systems. 

The actions of chemical messengers are often classified as endocrine, paracrine, or autocrine (Fig. 11.4). Each endocrine hormone is secreted by a specific cell type... 

The neurotransmitters of the ANS and the circulating catecholamines bind to specific receptors on the cell membranes of effector tissue. Each receptor is coupled to a G protein also embedded within the plasma membrane. Receptor stimulation causes activation of the G protein and formation of an intracellular chemical, the second messenger. (The neurotransmitter molecule, which cannot enter the cell, is the first messenger.) The function of intracellular second messenger molecules is to elicit tissue-specific biochemical events within the cell that alter the cell s activity. In this way, a given neurotransmitter may stimulate the same type of receptor on two different types of tissue and cause two different responses due to the presence of different biochemical pathways within each tissue. 

During the process of transcription, the information stored in a gene s DNA is transferred to a similar molecule called RNA (ribonucleic acid) in the cell nucleus. Both RNA and DNA are made up of a chain of nucleotide bases, but they have slightly different chemical properties. The type of RNA that contains the information for making a protein is called messenger RNA (mRNA) because it carries the information, or message, from the DNA out of the nucleus into the cytoplasm. 

RNA has a variety of functions within a cell for each function, a specific type of RNA is required. Messenger RNA (mRNA) serves as intermediaries for carrying genetic messages from the DNA to the ribosomes where protein synthesis lakes place. Ribosomal RNA (rRNA) serves both structural and functional roles in the ribosome it is diverse, both in terms of its size and structure. Transfer RNAs (tRNAs) are small molecules that have a central role in protein synthesis. Other RNA molecules, called ribozymes, function as enzymes to catalyze chemical transformations. Although ribozymes most often catalyze cleavage of the... 

Discussion of vulnerability of the axon and its terminal to toxicants must be based on an appreciation of specialized aspects of the metabolism of the neuron. This cell type has been discussed in the chapter dealing with the CNS. The concepts of chemically mediated neurotransmission, second messenger systems, specialized ion channels, and conduction of waves of depolarization with consequent high demand for energy to restore Na+ and K+ ion gradients are applicable to the PNS as well as the CNS. Of particular relevance is the fact that the neuromuscular junction is cholin-... 

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