The recent works of the present author have revealed that, at least in the cat, the excitatory and inhibitory synapses are clearly distinguished by the shape and size of the synaptic vesicles contained in them. A comparative consideration of the electrophysiological and electron-microscopical findings has namely led him to the hypothesis that the excitatory synapses contain spheroid synaptic vesicles of 500Å in diameter (S-type) whereas the inhibitory synapses contain flattened ones of a smaller size (F-type).
The application of this hypothesis in identifying the different kinds of nerve fibers in the cerebellar cortex of the cat gives the following results:
1. The axon terminals of the Purkinje cells, which are electrophysiologically known as inhibitory neurons, are represented by the synapses of F-type containing flattened vesicles. These fibers are characterized by a conspicuous lamellar structure of the axoplasm in the course.
2. The granule cells are regarded as excitatory neurons, as their axons or parallel fibers contain in their terminals synaptic vesicles of the S-type.
3. The stellate cells are indicated to be of an inhibitory nature as their axon terminals contain F-type synaptic vesicles.
4. The basket cells are inhibitory neurons whose terminal axons embrace the Purkinje cell somata and contain F-type vesicles in their synapses.
5. The axon terminals of the Golgi cells are involved, together with the dendrites of the granule cells and with the axon terminals of the mossy fibers, in the formation of the cerebellar glomeruli. Their synapses contain F-type vesicles indicating the inhibitory nature of these neurons.
6. The climbing fibers go up parallel with the primary or secondary dendrites of the Purkinje cells, making synapses on the short-necked spines of the Purkinje cell dendrites. These synapses contain S-type vesicles in correspondence with the excitatory nature of these fibers established electrophysiologically.
7. The mossy fibers, after entering the granule layer abruptly lose their myelin and swell up in huge synapses. The synaptic vesicles of the S-type contained there indicate the excitatory nature of the fibers.
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