Two main types of neuron may be distinguished electron microscopically in the somatic sensory cortex. Pyramidal neurons have a characteristically triangular perikaryon with a high content of ribonucleo-protein consisting mainly of free ribosomes; the nucleus usually shows a single small indentation. Nonpyramidal neurons, which may be large or small, have a higher concentration of all intracytoplasmic organelles and particularly of long cisternae of rough-surfaced endoplasmic reticulum forming Nissl bodies. The nucleus is often deeply indented and crenellated. The two cell types differ also in the nature of their dendritic ramifications and particularly in their synaptic relationships. The majority of axon terminals ending on pyramidal neurons contact dendritic spines and relatively few end on the shafts of dendrites or on the perikaryon. Synapses on spines are typically of the type in which the synaptic thickenings are asymmetrical and the synaptic vesicles spherical. Such synapses, even when they occur on the shafts of pyramidal cell dendrites, are usually associated with a 'spine apparatus'. Most of the few synapses on the dendritic shafts and somata of pyramidal cells are associated with symmetrical membrane contacts and small, flattened or pleomorphic vesicles. Terminals of this type are commonly en passant endings of long, thin unmyelinated axons oriented vertically or transversely within the cortex. The somata and the usually irregular dendrites of non-pyramidal neurons are typically covered in axon terminals most of which contain flattened vesicles and end in symmetrical complexes, but a few may contain spherical vesicles and end asymmetrically. The axon hillocks and initial segments of both types of cell are post-synaptic to axon terminals containing small, flattened vesicles and ending symmetrically.