Synaptic gap

The answer is that there are two ways in which neurotransmitter levels might be increased. One is to inhibit their destruction after they have been released into the synaptic gap. That is how MAOIs are supposed to work. Recall, however, that after a neurotransmitter is released, some of its molecules are reabsorbed by the presynaptic neuron that released them in a process that is called reuptake . Blocking this reuptake process should also increase the level of neuro transmitters in the brain. In 1961, Julius Axelrod, who later received the Nobel Prize in Medicine for his work on the release and reuptake of neurotransmitters, reported that imipramine, as well as a few other drugs, inhibited the reuptake of norepinephrine in cats. Two years later he reported that these drugs also inhibited the reuptake of serotonin.13... 

Keywords action potential axon ion channel mechanoreceptor membrane potential neurons neurotransmitter olfaction proteins synaptic gap. 

The sequence of events that result in neurotransmission of information from one nerve cell to another across the s)mapses begins with a wave of depolarization which passes down the axon and results in the opening of the voltage-sensitive calcium charmels in the axonal terminal. These charmels are frequently concentrated in areas which correspond to the active sites of neurotransmitter release. A large (up to 100 M) but brief rise in the calcium concentration within the nerve terminal triggers the movement of the synaptic vesicles, which contain the neurotransmitter, towards the synaptic membrane. By means of specific membrane-bound proteins (such as synaptobrevin from the neuronal membrane and synaptotagrin from the vesicular membrane) the vesicles fuse with the neuronal membrane and release their contents into the synaptic gap by a process of exocytosis. Once released of their contents, the vesicle membrane is reformed and recycled within the neuronal terminal. This process is completed once the vesicles have accumulated more neurotransmitter by means of an energy-dependent transporter on the vesicle membrane (Table 2.3). 

A magnified view of the synapse, shown in the bottom portion of the figure, illustrates the release of neurotransmitters when a vesicle fuses with the cell s membrane and spills the molecules into the synaptic gap. Diffusing randomly, some neurotransmitter molecules bind and activate certain receptors embedded in the recipient neuron s membrane. 

It inactivates many of the neurotransmitters in the synaptic gap or in the synapse if the latter are not protected by synaptic vesicles. The metabolism of NE, DA, 5-HT, tyramine, and histamine is thus taken care of by MAO as well as by some other enzymes. 

Monoamine oxidase inhibitors (MAOIs) are useful as thymoleptic (antidepressant) drugs, especially since the action of some of these agents is very rapid, as compared to the lag period of days or even weeks shown by tricyclic antidepressants. All MAOIs act by increasing the available concentration of the neurotransmitters NE and 5-HT which, because they are not metabolized, accumulate in the synaptic gap and exert an increased postsynaptic effect. The drugs show hypotensive activity as a side effect, and some MAOIs are used as hypotensive drugs. 

Release of acetylcholine When an action potential propagated by the action of voltage-sensitive sodium channels arrives at a nerve ending, voltage-sensitive calcium channels in the presynaptic membrane open, causing an increase in the concentration of intracellular calcium. Elevated calcium levels promote the fusion of synaptic vesicles with the cell membrane and release of acetylcholine into the synapse. This release is blocked by botulinum toxin. By contrast, black widow spider venom causes all of the cellular acetylcholine stored in synaptic vesicles to spill into the synaptic gap. 

Neurotransmission in adrenergic neurons closely resembles that already described for the cholinergic neurons (p. 37), except that norepinephrine is the neurotransmitter instead of acetylcholine. Neurotransmission takes place at numerous beadlike enlargements called varicosities the process involves five steps the synthesis, storage, release, and receptor binding of the norepinephrine, followed by removal of the neurotransmitter from the synaptic gap (Figure 6.3). 

Neurotransmitters are chemicals that mediate the propagation of nerve signals across the synapse. They are released at the presynaptic nerve terminal and cross the synaptic gap to react with a receptor on the surface... 

Nerve impulses are transmitted through the synaptic gap via chemical signals in the form of a specialized group of chemicals termed neurotransmitters. Neurotransmitters can also pass the neural impulse on to glands and muscles. Except where the neural synapses terminates on a muscle (neuromuscular synapse) or a gland (neuroglandular synapse), the synaptic gap is bordered by a presynaptic terminal portion of one neurcn and the dendrite of the postsynaptic neurcn. 

As the action potential sweeps into presynaptic region, there is a rapid influx of calcium from the extra cellular fluid into a qtecialized area of the presynaptic terminus termed the maptic knob. Via the process of exocytosis, specific neurotransmitters are then released from synaptic vesicles into the synaptic gap. The neurotransmitters drfiuse across the synaptic gap and specifically bind to specialized receptor sites cn the dendrite of the post aptic neurcn. 

Dendrites are the (filamentous) terminal portions of neuron that bind neurotransrrritter chemicals migrating across the synaptic gaps separating neurons. Depending on the type and function of a particular neuron, neurotransmitters may cause or inhibit the transmission of neural impulses. The cell body contains the cell nucleus and a concentration of cellular organelles. The cell body is the site of the normal metabolic reactions that allow the cell to remain viable. Neurotransmitters synthesized within the cell body are transported to the axon terminus by microfilaments and microtubules. 

At the axon terminus, neurotransmitters are released into the synaptic gap. Through aptic gaps, a typical neuron may intercormect with thousands and tens of thousands of other neurons. Axon terminals have knob-like swellings at the very end called synaptic knobs or end buttons. Each synaptic knob communicates with a dendrite or cell body of another neuron, the point of contact being a apse. Under very high magnification, a very tiny space, the aptic clefl or gap (about one millionth of an irKh, or mm), can be delected between the synaptic knob and dendrite or cell body. Synaptic knobs contain hundreds of neurovesicles that contain a transmitter substance (or neurotransmitter). 

When a nerve impulse reaches the synaptic knob the neurotransmitter is ejected into the synaptic cleft and serves as a stimulus to the next adjacent neuron. The vast majority of all impulses transmitted occur at the synaptic gaps, although recent research indicates that chemical transmission can occur at other points along the axon. Many neurological diseases and psychiatric disorders result from a disturbance or alteration of synaptic activity. Drugs such as tranquilizers, anesthetics, nicotine, and caffeine target the synapse and can cause an alteration of impulse transmission. 

Neurotransmitters are chemical agents secreted at the end of axons of nerve cells that diffuse across the synaptic gap and transmit information to adjoining cells such as neurons, muscle cells, and glands, by altering their electrical state or activity. There are many neurotransmitters with a variety of structures and functions two of the principle ones are acetylcholine and norepinephrine. Since the neurotransmitters convey information, anything that affects their behavior affects the function of the organism. 

The neurotransmitters are stored in tiny sac-like structures called vesicles at the end of axons. When an impulse, or nerve signal, reaches the end of the axon, the vesicles release a neurotransmitter into the small space between the adjoining cells (synaptic gap). Neurotransmitters diffuse across the synapse and bind to receptors in the receiving cell that are specific for the... 

---