Synapse transmission

Events or structures occurring proximal to the synapse. Transmissible spongiform encephalopathy is an example, together with Creutzfeldt-Jakob disease and bovine spongiform encephalopathy. Caused by abnormal proteins. 

The cholinergic nervous system is a network of neurones spread through both the central and peripheral nervous systems which are characterised by synapses. Transmission of signals within the network is electrical except at the synapses where acetylcholine (ACh) is released to carry the impulses across a small gap to the next neurone or to an effector organ. It is ACh which gives the cholinergic nervous system its name. ACh is only one of the many chemical transmitters in the nervous system, whose function is to act as amplifying relay stations for the nerve impulses from the brain. 

Cotransmission is transmission through a single synapse by means of more than one transmitter. For example, to elicit vasoconstriction, postganglionic sympathetic neurones release their classical transmitter noradrenaline (which acts on smooth muscle a-adrenoceptors) as well as ATP (which acts on smooth muscle P2 receptors) and neuropeptide Y (which acts on smooth muscle Yx receptors). 

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... 

Synaptic Transmission. Figure 1 Synaptic transmission. The presynaptic terminal contains voltage-dependent Na Superscript and Ca2+ channels, vesicles with a vesicular neurotransmitter transporter VNT, a plasmalemmal neurotransmitter transporter PNT, and a presynaptic G protein-coupled receptor GPCR with its G protein and its effector E the inset also shows the vesicular H+ pump. The postsynaptic cell contains two ligand-gated ion channels LGIC, one for Na+ and K+ and one for Cl-, a postsynaptic GPRC, and a PNT. In this synapse, released transmitter is inactivated by uptake into cells. 

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. 

It was generally assumed that it cannot and this became known as Dale s Law. During his studies on antidromic vasodilation he wrote (1935) When we are dealing with two different endings of the same sensory neuron, the one peripheral and concerned with vasodilation and the other at a central synapse, can we suppose that the discovery and identification of a chemical transmitter at axon reflex dilation would furnish a hint as to the nature of the transmission process at a central synapse. The possibility has at least some value as a stimulus to further experiments . 

For further details on the eoneept of synaptie transmission and the morphology of synapses see Shepherd and Erulkar (1997) and Peters and Palay (1996) respeetively. 

In the periphery at the mammalian neuromuscular junction each muscle fibre is generally influenced by only one nerve terminal and the one NT acts on one type of receptor localised to a specific (end-plate) area of the muscle. The system is fitted for the induction of the rapid short postsynaptie event of skeletal muscle fibre contraction and while the study of this synapse has been of immense value in elucidating some basic concepts of neurochemical transmission it would be unwise to use it as a universal template of synaptic transmission since it is atypical in many respects. 

Manipulating the activity of a NT in these ways helps to determine its function either at a synaptic level or in more general behavioural terms. Thus the clearest way of establishing the identity of the NT at a particular synapse is to ascertain which NT receptor antagonist blocks transmission there. 

Certainly, such a complex system for metabolism of noradrenaline (which is shared with the other catecholamines) strongly suggests that its function extends beyond that of merely destroying transmitter sequestered from the synapse. However, as yet, little is known about the regulation of this pathway and any influence it might have on noradrenergic transmission. One crucial, additional role for MAO appears to be the... 

Iproniazid also prevents the reserpine syndrome in rats. Reserpine blocks vesicular uptake of monoamines which, as a consequence, leak from the storage vesicles into the cytosol. Although these monoamines would normally be metabolised by MAO, they are conserved when a MAO inhibitor (MAOI) is present, and so co-administration of reserpine and a MAOI leads to accumulation of monoamines in the neuronal cytosol. It is now known that, when the concentration of cytoplasmic monoamines is increased in this way, they are exported to the synapse on membrane-bound monoamine transporters. The ensuing increase in monoamine transmission, despite the depletion of the vesicular pool, presumably accounts for the effects of iproniazid on the behaviour of reserpine-pretreated rats. 

GC) axon (Jia et al, 1999). The GC are the main inhibitory intemeurones, while the peri-glomerular cells can alter the probability of transmission at the first synapse. The olfactory inputs to the M/TCs have two ways in which they may be connected, via their primary dendrites, to a particular glomerulus. First, they may supply only one functional type as in MOE input [Fig. 5.14(a)], Second, they may supply two or more functional types [Fig. 5.14(b)]. The single connectivity type is found in the MOB, as the primary OR-M/TC... 

Sorensen and Blaustein 1985a). In view of the enhanced neuro-transmitter release that would be expected to accompany block of presynaptic K channels (Llinas et al. 1976), this mechanism could explain the PCP-induced "dopamine storm" (Rappolt et al. 1980). Such altered synaptic transmission at central synapses may underlie the disordered behavior characteristic of PCP intoxication. 

Llinas, R. Walton, K. and Bohr, V. Synaptic transmission in squid giant synapse after potassium conductance block with external 3- and 4 - ami nopyri di nes. Bi ophvs J. 16 83-86, 1976. Mendelsohn, L.G. Kerchner, G.A. Kalra, V. Zimmerman, D.M. and Leander, J.D. Phencyclidine receptors in rat brain cortex. Biochem-Pharmacol 33-3529-3535, 1984. 

Neurotransmitter Chemical intermediary in the nervous system that facilitates transmission of impulses across synapses. 

---