Signal across membrane

The influx of Ca(Il) across the presynaptic membrane is essential for nerve signal transmission involving excitation by acetylcholine (26). Calcium is important in transducing regulatory signals across many membranes and is an important secondary messenger hormone. The increase in intracellular Ca(Il) levels can result from either active transport of Ca(Il) across the membrane via an import channel or by release of Ca(Il) from reticulum stores within the cell. More than 30 different proteins have been linked to regulation by the calcium complex with calmoduhn (27,28). 

The members of Group 1 are called the alkali metals. The chemical properties of these elements are unique and strikingly similar from one to another. Nevertheless, there are differences, and the subtlety of some of these differences is the basis of the most subtle property of matter consciousness. Our thinking, which relies on the transmission of signals along neurons, is achieved by the concerted action of sodium and potassium ions and their carefully regulated migration across membranes. So, even to learn about sodium and potassium, we have to make use of them in our brains. 

According to Fig. 6.17 the nerve cell is linked to other excitable, both nerve and muscle, cells by structures called, in the case of other nerve cells, as partners, synapses, and in the case of striated muscle cells, motor end-plates neuromuscular junctions). The impulse, which is originally electric, is transformed into a chemical stimulus and again into an electrical impulse. The opening and closing of ion-selective channels present in these junctions depend on either electric or chemical actions. The substances that are active in the latter case are called neurotransmitters. A very important member of this family is acetylcholine which is transferred to the cell that receives the signal across the postsynaptic membrane or motor endplate through a... 

We must remember too that all the gradients of ions (or molecules) across membranes, represented sometimes by electrical potentials, are bulk sources of energy, not of specific chemical use but are of general value in uptake/rejection and signalling (see Chapter 9). 

Lipid modified proteins are often attached to cell membranes. In many cases, they play crucial roles in the transduction of extracellular signals across the plasma membrane and into the nucleus. A particularly important example are the N-, K-, and H-Ras proteins. All Ras proteins terminate in a fame... 

Each of their receptors transmits its signal across the plasma membrane by increasing transmembrane conductance of the relevant ion and thereby altering the electrical potential across the membrane. For example, acetylcholine causes the opening of the ion channel in the nicotinic acetylcholine receptor (AChR), which allows Na+ to flow down its concentration gradient into cells, producing a localized excitatory postsynaptic potential—a depolarization. 

There is a space that exists between neurons known as the synapse. The transmission of the nerve signal across this synaptic cleft is a chemical phenomenon. Molecules generically referred to as neurotransmitters are produced in the neuron and released from the axonal membrane into the synapse. They diffuse to the dendrites of the next nerve cell and combine with receptors. This combination of neurotransmitter (agonist) and receptor produces a response which results in the propagation of the nerve signal down the next neuron. We will discuss this activity in much more detail shortly. 

In this chapter we first describe the composition of cellular membranes and their chemical architecture— the molecular structures that underlie their biological functions. Next, we consider the remarkable dynamic features of membranes, in which lipids and proteins move relative to each other. Cell adhesion, endocytosis, and the membrane fusion accompanying neurotransmitter secretion illustrate the dynamic role of membrane proteins. We then turn to the protein-mediated passage of solutes across membranes via transporters and ion channels. In later chapters we discuss the role of membranes in signal transduction (Chapters 12 and 23), energy transduction (Chapter 19), lipid synthesis (Chapter 21), and protein synthesis (Chapter 27). 

A variety of signals can be transmitted across membranes without the actual flow of a substance from one side of the membrane to the other. We saw in chapter 12 that some hormones bind to specific receptor sites on the outer surface of the plasma membrane, thereby triggering metabolic changes on the cytosolic side of the membrane. Hormonal systems that function in this way are discussed in greater detail in chapter 24. Other membrane proteins mediate specific cell-cell interactions. Sometimes these interactions merely stimulate particular types of cells to bind to one another, but often they also trigger reactions that result in proliferation or differentiation of the interacting cells. We discuss signals of this type when we consider the interaction between the B and T cells of the immune system (see Supplement 3). 

Transfer by transduction and reading of information and signals across a barrier such as a membrane couldbeperformedfollowingtheprinciple illustrated inFigure28. 

Photoprotonic signals can be generated by light induced proton release, this might happen for the quaternary N-R+ derivatives of 117, for instance [8.233]. The photoproduction of proton gradients across membranes could serve as a light-powered proton pump for inducing vectorial processes such as the transport of protons [8.233, 8.234] or H+-ATPase model reactions [8.274]. 

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