Synapse anatomy

FIGURE 2.2 The anatomy of the neuron. Communication between two neurons occurs at the synapse. The presynaptic neuron produces and releases the neurotransmitter into the synaptic cleft. Four mechanisms (1 ) are important to understand the function of most neurotransmitter systems. The release of neurotransmitter can be modulated via presynaptic receptors (1). The amount of neurotransmitter in the synaptic cleft can be decreased by reuptake into the presynaptic neuron (2) or via enzymatic degradation. Neurotransmitter effects at the target neuron are relayed via fast-acting ion channel—coupled receptors (3) or via slower-acting G protein—coupled receptors (4). Down-stream effects of postsynaptic receptors include the phosphorylation (P) of nuclear proteins. 

The anatomy of autonomic synapses and junctions determines the localization of transmitter effects around nerve endings. Classic synapses such as the mammalian neuromuscular junction and most neuron-neuron synapses are relatively "tight" in that the nerve terminates in small boutons very close to the tissue innervated, so that the diffusion path from nerve terminal to postsynaptic receptors is very short. The effects are thus relatively rapid and localized. In contrast, junctions between autonomic neuron terminals and effector cells (smooth muscle, cardiac muscle, glands) differ from classic synapses in that transmitter is released from a chain of varicosities in the postganglionic nerve fiber in the region of the smooth muscle cells rather than boutons, and autonomic junctional clefts are wider than somatic synaptic clefts. Effects are thus slower in onset and often involve many effector cells. 

With these thoughts in mind, we examined the effects of various drugs on GABA-mediated synapses in the crayfish. While there are a number of GABAergic neuromuscular junction preparations in invertebrates which are accessible for physiological research, the crayfish stretch receptor is one of very few invertebrate preparations which can be used for studies of synapses between nerves. The anatomy and physiology of these receptor organs from the abdomen of the crayfish has been extensively studied (6) ... 

Sotelo C, Gotow T, Wassef M (1986) Localization of glutamic-acid-decarboxylase-immunoreactive axon terminals in the inferior olive of the rat, with special emphasis on anatomieal relations between GABAergic synapses and dendrodendritic gap junctions. J. Comp. Neurol, 252, 32-50. 

It s estimated that each cortical neuron has an average of 10,000 synapses with other neurons near and far. But these connections do not develop all at once. They are formed in the brain during fetal development, infancy, childhood, and even adulthood. The simple fact that the gross anatomy of a child s nervous system is not completely developed until about n or 12 years of age is another refutation of the idea that neurological wiring is complete at birth and therefore uninfluenced by environment. 

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