Types of Synapses
There are numerous variations on the simple form of synapses. The synaptic con-tact between parallel axons and dendrites is called parallel contactor bouton en passant (A1). Many dendrites have thornlike projec-tions (spines) that form a spinous synapse (A2) with the bouton. On the apical den-drites of some pyramidal cells, the terminal swelling of the axon encloses the entire spine, which may be relatively large and branched, bearing numerous synaptic con-tacts (complex synapse) (B). Several axons and dendrites can join to form glomerulus-like complexes in which the different synap-tic elements are closely intertwined. They probably affect each other in terms of fine-tuning (modulating) the transmission of impulses.
Each brain division has characteristic forms of synapses. Gray type I and II synapses are predominantly found in the cerebral cortex, glomerulus-like complexes are found in thecerebellar cortex, in the thalamus, and in the spinal cord.
Adjacent cells can communicate through pores (tunnel proteins), called gap junctions. Cells linked by gap junctions are electrically coupled; this facilitates the transmission of impulses from one cell to another (e.g., in smooth muscles, p. 303, B8). Gap junctions in neurons are therefore also called electri-cal synapses in contradistinction to the chemical synapses, which release neu-rotransmitters. Electrical coupling via gap junctions occurs not only between neurons but also between glial cells.