Transmission electron micrograph of a frog myoneural junction prepared by freeze substitution. The nerve terminal is covered by a portion of Schwann cell (above), and is separated from the muscle fiber (below) by a synaptic cleft containing a conspicuous external lamina. The nerve ending's cytoplasm contains a few microtubules, several mitochondria, a large aggregation of synaptic vesicles, and a number of neurofilaments.