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THE SYNAPSE

Gap junctions are characteristically associated with which of the following?

a. Axodendritic synapses

b. Axoaxonic synapses

c. Axosomatic synapses

d. Dendrodendritic synapses

e. Electrical synapses

The answer is E. (Kandel, pp 178-180. Purves, pp 85-87. Siegel and Sapru, pp 95-96.) Electrical synapses are less common than chemical synapses but can be found in the nervous systems of different species. A unique feature of electrical synapses is that two neurons communicate with each other by having the membranes of each neuron lie very close together. The contact between the neurons is called a gap junction. These junctions contain aligned paired channels so that each paired channel forms a pore larger than that observed in ligand-gated channels and which allows for bidirectional transmission.

In treating a patient with a movement disorder, you administer a newly developed drug whose functions are mediated at axosomatic synapses. Which of the following statements best characterizes axosomatic synapses?

a. It is referred to as a type I synapse.

b. They have an electrical continuity linking the pre- and postsynaptic cells.

c. They are typically inhibitory.

d. Synaptic transmission is mediated by glutamate.

e. They form the predominant synapse of cortical projections to the neostriatum.

The answer is C. (Kandel, pp 209-217. Purves, pp 5-9, 85, 117.) Axon terminals that make synaptic contact with the soma of postsynaptic cells are frequently observed to be inhibitory and are referred to as type II synapses. A classic example of this is in the cerebellar cortex, where an interneuron (basket cell) makes synaptic contact with the soma of the Purkinje cell. These are chemical and not electrical synapses, and their actions are frequently mediated by GABA. Activation of the basket cell results in subsequent inhibition of the Purkinje cell. The overwhelming number of excitatory synapses is observed to be axodendritic. They are referred to as type I synapses and are frequently characterized by specialized extensions of the dendrites called spines. These synapses also display a dense basement membrane and a prominent presynaptic density. Cortical projections to the neostriatum have been shown to be excitatory and their functions mediated by glutamate.

Which of the following best describes a basic property of synapses in the central nervous system (CNS)?

a. Synaptic vesicles constitute important features for transmission in both chemical and electrical synapses.

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