Axo-axonal synapse

The axon terminals of one neuron synapse with other neurons either on the dendrites (axo-dendritic synapse) or soma (axo-somatic synapse). Synapses on another axon... 

Figure 1.6 Presynaptic inhibition of the form seen in the dorsal horn of the spinal cord, (a) The axon terminal (i) of a local neuron is shown making an axo-axonal contact with a primary afferent excitatory input (ii). (b) A schematic enlargement of the synapse, (c) Depolarisation of the afferent terminal (ii) at its normal resting potential by an arriving action potential leads to the optimal release of neurotransmitter, (d) When the afferent terminal (ii) is already partially depolarised by the neurotransmitter released onto it by (i) the arriving acting potential releases less transmitter and so the input is less effective...

At the ultrastructural level the majority of the release sites (65-70%) appear as a single cluster of synaptic vesicles accumulated at a single active zone (Fig. ID), which may contain specializations of pure chemical or mixed synapses (Pfenninger and Rovainen, 1974 Ringham, 1975 Christensen, 1976 Rovainen, 1979). The latter type thus contains gap junctions as well as active zones (Fig. ID). Most of the synapses are established on dendritic shafts. A distinct class of intemeuron, however, receives axo-somatic synapses (OS and LB unpublished). The majority of the postsynaptic cells receive multiple chemical or mixed synaptic contacts from a single axon. Therefore most of the EPSPs recorded in postsynaptic neurons are composed of two components, an electrotonic and a chemical component. 

Fig. 10. Electron micrograph from a 300- xm neostriatal slice incubated for 2 hr before fixation. (Top) Electron micrograph shows a well-preserved neostriatal medium-sized cell and surrounding neurophil. N, nucleus No, nucleolus G, golgi M, mitochondria D and d, dendrite AX, axon. (Bottom left) An electron micrograph taken from the middle layer of the slice demonstrating well-preserved neurophil with many axo-spinous synapses and clear...

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