Nervous system synapse

Kuromi H, Kidokoro Y (1998) Two distinct pools of synaptic vesicles in single presynaptic boutons in a temperature-sensitive Drosophila mutant, shibire. Neuron 20 917-25 Lamb TD, Pugh EN, Jr. (1992) A quantitative account of the activation steps involved in phototransduction in amphibian photoreceptors. J Physiol 449 719-58 Landis DM, Hall AK, Weinstein LA et al (1988) The organization of cytoplasm at the presynaptic active zone of a central nervous system synapse. Neuron 1 201-9 Lao G, Scheuss V, Gerwin CM et al (2000) Syntaphilin a syntaxin-1 clamp that controls SNARE assembly. Neuron 25 191-201... 

Neurotransmitter release is not assured in response to synaptic stimulation. Rather, the process of vesicle fusion for individual release-competent vesicles is probabilistic. This process confers a discrete probability (between 0 and 1) that a given synapse will release neurotransmitter after an action potential (the synaptic release probability). For the majority of synapses in the central nervous system, the release probability at a deflned synaptic contact is below 0.3, which leads to the often-quoted statement that the release process is reliably unreliable (1). Despite this fact, it has been demonstrated that some central nervous system synapses (in a variety of brain regions) do exhibit release probabilities as high as 0.9 (2-4). This higher synaptic release... 

After the fusion of a synaptic vesicle, the RRP is refilled from the recycling pool of synaptic vesicles. For central nervous system synapses (e.g., synapses of hippocampal neurons), the recycling pool of synaptic vesicles consists of approximately 30 vesicles, approximately three to five times the number of RRP vesicles (15, 16). During repetitive synaptic stimulation, the rapid refilling of the RRP from the recycling pool sustains continuous neurotransmitter release. An overview of the synaptic vesicle cycle is shown in Fig. 1. 

Femandez-Alfonso T, Ryan TA. The efficiency of the synaptic vesicle cycle at central nervous system synapses. Trends Cell Biol. 2006 16 413-420. 

The psychotropic effects of cocaine are similar to those of amfetamine (euphoria and excitement) but briefer and are due to blockade of the reuptake of dopamine at central nervous system synapses, which increases its concentration at receptors and produces the characteristic high. ... 

B. Mechanisms of Antidepressant Action Potential sites of action of antidepressants at central nervous system synapses are shown in Figure 30-2. By means of several mechanisms, almost all antidepressants result in a potentiation of the neurotransmitter actions of norepinephrine, serotonin, or both. The only exception is bupropion, which has an unknown mechanism of action. Long-term use of tricyclics and MAOIs, but not SSRIs, leads to down-regulation of beta receptors. 

The chemistry of the brain and central nervous system is affected by a group of substances called neurotransmitters, substances that carry messages across a synapse from one neuron to another Several of these neurotransmitters arise from l tyrosine by structural modification and decarboxylation as outlined m Figure 27 5... 

Two specialties of the nervous system are speed and localization, accompHshed using highly developed electrical signaling and close cellular apposition. At specialized points of communication, such as the synapse and the neuromuscular junction, the cells are separated by a nanometer or less. 

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. 

ChEs control the duration of ACh-mediated action on post-synaptic receptors in cholinergic synapses, and have non-hydrolytic roles in nervous systems development and plasticity. 

The amino acid glycine, a neurotransmitter at inhibitory synapses throughout the central nervous system (CNS),... 

Synaptic transmission is the transfer of biological information across synapses. Drugs that influence synaptic transmission play an eminent role in therapy, for two reasons. First, the nervous system controls all tissues. Second, with few exceptions synaptic transmission is chemical, operating by means of transmitter substances, and synapses therefore provide a large number of drug targets, such as the enzymes that synthesize the transmitter. However, the importance of... 

Rgure 22-2. Neurotransmission in the central nervous system. Neurotransmitter molecules (eg, norepinephrine), released by the presynaptic nerve, cross the synapse and bind with receptors in the cell membrane of the postsynaptic nerve, resulting in the transmission of the nerve impulse. 

The PNS has two neurohormones (neurotransmitters) acetylcholine (ACh) and acetylcholinesterase (ACliE). ACh is a neurotransmitter responsible for die transmission of nerve impulses to effector cells of die parasympathetic nervous system. ACh plays an important role in die transmission of nerve impulses at synapses and myoneural junctions. ACh is quickly... 

Neurohumoral transmitters are chemicals that facilitate the transmission of nerve impulses across nerve synapses and neuroeffector junctions. Acetylcholine is a neurohumoral transmitter that is present in the peripheral autonomic nervous system, in the somatic motor nervous system, and in some portions of the central nervous system. 

Acetylcholinesterase is a component of the postsynaptic membrane of cholinergic synapses of the nervous system in both vertebrates and invertebrates. Its structure and function has been described in Chapter 10, Section 10.2.4. Its essential role in the postsynaptic membrane is hydrolysis of the neurotransmitter acetylcholine in order to terminate the stimulation of nicotinic and muscarinic receptors (Figure 16.2). Thus, inhibitors of the enzyme cause a buildup of acetylcholine in the synaptic cleft and consequent overstimulation of the receptors, leading to depolarization of the postsynaptic membrane and synaptic block. 

Tetanus occurs when Cl. tetani, ubiquitous in the soil and faeces, contaminates wounds, especially deep puncture-type lesions. These might be minor traumas such as a splinter, or major ones such as battle injury. At these sites, tissue necrosis and possibly microbial growth reduce the oxygen tension to allow this anaerobe to multiply. Its growth is accompanied by the production of a highly potent toxin which passes up peripheral nerves and diSuses locally within the central nervous system. It acts like strychnine by affecting normal function at the synapses. Since the motor nerves of the brain stem are the shortest, the cranial nerves are the first affected, with twitches of the eyes and spasms of the jaw (lockjaw). 

Although one neuron can receive hundreds of inputs releasing a number of different NTs, the correct and precise functioning of the nervous system presumably requires that a NT should only be able to act on appropriate receptors at the site of its release. This control is, of course, facilitated to some extent by having different NTs with specific receptors so that even if a NT did wander it could only work where it finds its receptors and was still present in sufficient concentration to meet their affinity requirements. Normally the majority of receptors are also restricted to the immediate synapse. 

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