Synapse, nerve cell

Inside the end of each nerve cell are the potassium and other electrolytes, while outside are the sodium and chloride, creating an electric potential at the gap (synapse) that a nerve impulse has to jump over as it travels from one nerve cell to the next. When that electric nerve impulse arrives, sodium ions (electrically charged sodium atoms) rush into the nerve cell, potassium ions rush out of the nerve cell into the synapse, and the cell membrane is depolarized. The depolarization allows the nerve impulse to jump across the synapse and enter the next nerve cell, usually with the help of a chemical called a neurotransmitter. When the nerve impulse has crossed the gap between nerve cells (synapse), the potassium ions rush back into the cell, the sodium ions rush out into the synapse (outside the cell), and the normal polarization of the nerve ceU is restored. It is again ready to respond to the next nerve impulse that arrives. 

This team exposed slices of rat brain to microwaves. They found that the exposure reduced electrical activity and weakened response to stimuli. The brain slices were taken from the hippocampus, a part of the brain with a role in learning. However Tattersall has indicated that he thinks the hippocampus is too deeply buried within the brain to be affected by mobile phones. His more recent research has shown that nerve cell synapses may become more receptive to changes linked to memory when they are exposed to microwaves. 

Choline functions in fat metaboHsm and transmethylation reactions. Acetylcholine functions as a neurotransmitter in certain portions of the nervous system. Acetylcholine is released by a stimulated nerve cell into the synapse and binds to the receptor site on the next nerve cell, causing propagation of the nerve impulse. 

Long nerve-cell process transmitting the action potential and ending as the synapse. 

Intracellular motility is also of vital importance in the lives of cells and the organisms they form. Material and organelles are transported within cells along microtubules and microfilaments an extreme example of this are the axons of nerve cells which transport materials to the synapses where they are secreted—another motile event. Other examples of intracellular motility include phagocytosis, pino-cytosis, the separating of chromosomes and cells in cell division, and maintenance of cell polarity. 

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... 

Fig. 9.2 A general impression of the construction of nerve cells (a) cross-section of a nerve trunk, (b) a neuron, (c) sensory and motor nerve connection via a synapse.

Chemical transmission between nerve cells involves multiple steps 167 Neurotransmitter release is a highly specialized form of the secretory process that occurs in virtually all eukaryotic cells 168 A variety of methods have been developed to study exocytosis 169 The neuromuscular junction is a well defined structure that mediates the presynaptic release and postsynaptic effects of acetylcholine 170 Quantal analysis defines the mechanism of release as exocytosis 172 Ca2+ is necessary for transmission at the neuromuscular junction and other synapses and plays a special role in exocytosis 174 Presynaptic events during synaptic transmission are rapid, dynamic and interconnected 175... 

The taste bud is a polarized structure with a narrow apical opening, termed the taste pore, and basolateral synapses with afferent nerve fibers. Solutes in the oral cavity make contact with the apical membranes of the TRCs via the taste pore. There is a significant amount of lateral connectedness between taste cells within a bud both electrical synapses between TRCs and chemical synapses between TRCs and Merkel-like basal cells have been demonstrated to occur [39]. Furthermore, there are symmetrical synapses between TRCs and Merkel-like basal cells [39]. In addition, these basal cells synapse with the afferent nerve fiber, suggesting that they may function in effect as interneurons [39]. The extensive lateral interconnections... 

The communication between neurons occurs at either gap junctions (electrical synapses) or chemical synapses with release of neurotransmitters from a presynaptic neuron and their detection by a postsynaptic nerve cell (Fig. 17.1). Neurotransmitters not used in the synaptic cleft are removed promptly by either uptake into adjacent cells, reuptake in the presynaptic neuron, or are degraded by enzymatic systems. 

What is a synapse In the brain, the nerve cells or neurons are connected at special functional junctions called synapses, which depend on many proteins, including large complexes. They participate in basic functions with important roles in coordinating every characteristic of the nervous system, including physiology, emotions, learning, sleep, memory, and pain signal transmission. 

Similar considerations would apply at the synapse between the preganglionic fibre and the nerve cell of the postganglionic fibre of the sympathetic and parasympathetic systems and curare is the blocking agent (fig. 7 A). 

Figure 1.2 Serotonin is one of the brain s neurotransmitters. This image depicts serotonin transmission between neurons and the drug Ecstasy s effects on that transmission. Serotonin is normally removed from the synapse shortly after being released. Ecstasy blocks this mechanism, increasing the amount of serotonin in the synapse. This causes the postsynaptic neuron to be overstimulated by serotonin. Serotonin is one of many neurotransmitters that nerve cells can secrete. Other common neurotransmitters include dopamine, glutamate, gamma aminobutyric acid (GABA), noradrenaline, and endorphins.

Because the membrane is partially depolarized in the dark, its neurotransmitter glutamate is continuously released. Glutamate inhibits the optic nerve bipolar cells with which the rod cells synapse. By hyperpolarizing the rod cell membrane, light stops the release of glutamate, relieving inhibition of the optic nerve bipolar cell and thus initiatii a signal into the brain. 

The nervous system consists of two main units the central nervous system (CNS), which includes the brain and the spinal cord and the peripheral nervous system (PNS), which includes the body s system of nerves that control the muscles (motor function), the senses (the sensory nerves), and which are involved in other critical control functions. The individual units of the nervous system are the nerve cells, called neurons. Nenrons are a nniqne type of cell becanse they have the capacity to transmit electrical messages aronnd the body. Messages pass from one nenron to the next in a strnctnre called a synapse. Electric impnlses moving along a branch of the nenron called the axon reach the synapse (a space between nenrons) and canse the release of certain chemicals called neurotransmitters, one of which, acetylcholine, we described earlier in the chapter. These chemicals migrate to a nnit of the next nenron called the dendrites, where their presence canses the bnild-np of an electrical impnlse in the second nenron. 

Neurotransmitter/Receptor Binding. At this point, the neurotransmitter chemical is free in the synapse (extracellular fluid) and drifts (diffuses) in all directions. Some of the neurotransmitter molecules float across the synapse and bind to receptors on the surface of the adjacent nerve cell. Each neurotransmitter has its own unique three-dimensional shape and binds with certain receptors but not others. The binding between a neurotransmitter and a receptor is similar to fitting a key into a lock. When the neurotransmitter binds the receptor, the signal has been passed to the neighboring nerve cell. This is the process of neurotransmission. 

Stopping Neurotransmission. Turning off the neurotransmitter signal once it has been released into the synapse is critical to successful communication between nerve cells. This is of paramount importance because unbridled stimulation can be harmful to nerve cells. For example, one of the problems in the minutes and hours following a stroke is that nerve cells near the stroke area can literally be stimulated to death. In fact, some of the new medications used to minimize damage to the brain after a stroke act by literally calming the cells in the brain. Thus, signal termination is a critically important aspect of neurotransmission. 

As we noted earlier, when the neurotransmitter is released from the axon terminal into the synapse, it is free to diffuse across the synapse to bind the receptors on the neighboring nerve cell. However, other fates may await the neurotransmitter once it s released into the synapse. In general, these other processes act to terminate neurotransmission by preventing the neurotransmitter from reaching the receptor on the adjacent nerve cell. There are, in fact, five distinct mechanisms for terminating the neurotransmitter signal once it has been released into the synapse. 

Negative Feedback. Some of the neurotransmitter diffuses back to the surface of the nerve cell that released it. There are also receptors that tit the neurotransmitter here. When a neurotransmitter binds a receptor (called an autoreceptor) at the axon terminal of the nerve cell that released it, it tells the nerve cell that there s plenty of neurotransmitter already in the synapse. So don t release anymore This process is called negative feedback and is analogous to the way a thermostat works in your home to control room temperature. 

Reuptake. The nerve cell that released the neurotransmitter also has what are called reuptake sites on its surface. These reuptake sites are actually transporter proteins that are specific to each type of neurotransmitter. They act like miniature vacuum cleaners to retrieve the neurotransmitter from the synapse. The neurotransmitter is removed from the synapse at the reuptake site and returned to the inside of the nerve cell s axon terminal. Although the reuptake process recycles the neurotransmitter molecules for future use, the process does, in fact, serve to terminate the current neurotransmitter signal. 

When we talk about what a psychiatric medication does, we are invariably discussing its effect on neurotransmission between nerve cells across the synapse. Psychiatric medications act by modulating chemical neurotransmission in the synapse. However, as you probably know, it often takes several days or weeks for depression or psychosis to respond to treatment. Clearly, psychiatric medications work. Why, however, is there often a delay before they begin to do so ... 

Blocking Enzymes. Remember that there are enzymes both in the synapse and in the cytoplasm of the nerve cells that metabolize and thereby inactivate neurotransmitter molecules. One way to promote neurotransmission is to increase the supply of available neurotransmitter. Blocking (or inhibiting) the enzymes that destroy neurotransmitter will do just that. Certain antidepressants known as monoamine oxidase inhibitors (MAOIs) and some medications used to treat Alzheimer s disease act in this manner. 

Stimulating Postsynaptic Receptors. Neurotransmitters carry their signals by diffusing across the synapse from the axon terminals to receptors on the neighboring, so-called postsynaptic, nerve cell. Some medications stimulate postsynaptic receptors and thereby act by mimicking the action of the neurotransmitter itself. 

Medications that enhance norepinephrine activity can do so in one of several ways. First, they can block the reuptake of norepinephrine back into the nerve cell once it has been released. This keeps the norepinephrine in the synapse longer and therefore makes it more active. The tricyclic antidepressants (TCAs), duloxetine (Cymbalta), and venlafaxine (Effexor) act in this manner, as does paroxetine (Paxil) at higher doses. Atomoxetine (Strattera), a treatment for ADHD, also works in this way. 

Dopamine activity can be enhanced in one of four main ways. Medications can stimulate dopaminergic nerve cells to release dopamine into the synapse. This is the way that stimulants such as methylphenidate (Ritalin), dextroamphetamine (Dexe-drine), and dextroamphetamine/amphetamine (Adderall) work. In addition, certain drugs of abuse, notably cocaine and methamphetamine, act in part in this way. Providing more of the raw material that nerve cells use to manufacture dopamine can also increase dopamine activity. This is the approach that neurologists use when they prescribe L-DOPA (Sinemet) to patients with Parkinson s disease. Nerve cells convert L-DOPA into dopamine. L-DOPA otherwise has little place in the treatment of psychiatric disorders. Dopamine activity can also be increased by medications that directly stimulate dopamine receptors. Bromocriptine, another medication used to... 

The cyclase produces cAMP which results in opening of a Na" ion channel in the membrane of the sensory cell. If a sufficient number of Na " ions enter, this depolarises the membrane and initiates an action potential along the axon to the olfactory nerve. Further effects depend upon interaction between the nerves and synapses within the olfactory centre in the brain. This can result in physiological effects in other parts of the body which define the function of the pheromone. The effects of pheromones on the sexual responses of men and women are discussed in Chapter 19 (see Figure 19.17). 

The concept of chemical transmission in the nervous system arose in the early years of the century when it was discovered that the functioning of the autonomic nervous system was largely dependent on the secretion of acetylcholine and noradrenaline from the parasympathetic and sympathetic nerves respectively. The physiologist Sherrington proposed that nerve cells communicated with one another, and with any other type of adjacent cell, by liberating the neurotransmitter into the space, or synapse, in the immediate vicinity of the nerve ending. He believed that transmission across the synaptic cleft was unidirectional and, unlike conduction down the nerve fibre, was delayed by some milliseconds because of the time it took the transmitter to diffuse across the synapse and activate a specific neurotransmitter receptor on the cell membrane. 

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