Almost all of the structural and functional protein molecules are synthesized in the nerve cell body. These molecules are distributed to the axons and dendrites via axonal transport systems (described on page 363). However, contrary to the common view that the nerve cell body is the only site of protein synthesis, recent studies indicate that local synthesis of axonal proteins takes place in some large nerve terminals. Some vertebral axon terminals (i.e., from the retina) contain polyribosomes with complete translational machinery for protein synthesis. These discrete areas within the axon terminals, called periaxoplasmic plaques, possess biochemical and molecular characteristics of active protein synthesis. Protein synthesis within the periaxoplasmic plaques is modulated by neuronal activity. These proteins may be involved in the processes of neuronal cell memory.
Neurons communicate with other neurons and with effector cells by synapses.
Synapses are specialized junctions between neurons that facilitate the transmission of impulses from one (presynaptic) neuron to another (postsynaptic) neuron. Synapses also occur between axons and effector (target) cells, such as muscle and gland cells. Synapses between neurons may be classified morphologically as:  axodendritic, occurring between axons and dendrites;  axosomatic, occurring between axons and the cell body; or  axoaxonic, occurring between axons and axons (Fig. 12.5).
Synapses are not resolvable in routine hematoxylin and eosin (H&E) preparations. However, silver precipitation staining methods (e.g., Golgi method) not only demonstrate the overall shape of some neurons but also show synapses as oval bodies on the surface of the receptor neuron. Typically, an axon makes several of these buttonlike contacts with the receptor portion of the neuron. Often, the incoming neuron travels along the surface of the neuron, making several synaptic contacts called boutons en passant (Fr., buttons in passing). The axon then continues, ending finally as a terminal branch with an enlarged tip, a bouton terminal (Fr., terminal button), or end bulb. The number of synapses on a neuron or its processes, which may vary from a few to tens of thousands per neuron (Fig. 12.6), appears to be directly related to the number of impulses that a neuron is receiving and processing.
FIGURE 12.5 • Schematic diagram of different types of synapses. a. Axodendritic or axosomatic. b. Axodendritic, in which an axon terminal synapses with a dendritic spine. c. Axoaxonic. The axoaxonic synapse may enhance or inhibit the axodendritic (or axosomatic) synapse. (Modified from Barr ML. The Human Nervous System. New York: Harper & Row, 1979.)
Synapses are classified as chemical or electrical.
Classification depends on the mechanism of conduction of the nerve impulses and the way the action potential is gener-ated in the target cells. Thus, synapses may also be classified as the following.
FIGURE 12.6 • Scanning electron micrograph of the nerve cell body. This micrograph shows the cell body of a neuron. Axon endings forming synapses are visible as are numerous oval bodies with tail-like appendages. Each oval body represents axon terminal from different neuron making contact with the cell body. 76,000. (Courtesy of Dr. George Johnson.)  Chemical synapses: Conduction of impulses is achieved by the release of chemical substances (neurotransmitters) from the presynaptic neuron. Neurotransmitters then diffuse across the narrow intercellular space that separates the presynaptic neuron from the postsynaptic neuron or target cell.