Axonal Transport

In addition to conducting action potentials, axons transport materials from the cell body to the synaptic terminals (anterograde transport) and from the synaptic terminals to the cell body (retrograde transport). Because ribosomes are not present in the axon, new protein must be synthesized and moved to the axon. This occurs via several types of axonal transport, which differ in terms of the rate and the material transported. Anterograde transport may be fast (up to 400 mm/d) or slow (about 1 mm/d). Retrograde transport is similar to rapid anterograde transport. Fast transport involves microtubules extending through the cytoplasm of the neuronAxons An axon can be injured by being cut or severed, crushed, or compressed. After injury to the axon, the neuronal cell body responds by entering a phase called the axon reaction, or chromatolysis. In general, axons within peripheral nerves can regenerate quickly after they are severed, whereas those within the CNS do not tend to regenerate. The axon reaction and axonal regeneration are further discussed in Chapter 22: Imaging of the Brain.Axons Communication between neurons usually occurs from the axon terminal of the transmitting neuron (presynaptic side) to the receptive region of the receiving neuron (postsynaptic side) (Figs 2–5 and 2–10). This specialized interneuronal complex is a synapse, or synaptic junction. As outlined in Table 2–1, some synapses are located between an axon and a dendrite (axodendritic synapses, which tend to be excitatory), whereas others are located between an axon and a nerve cell body (axosomatic synapses, which tend to be inhibitory). Still other synapses are located between an axon terminal and another axon; these axoaxonic synapses modulate transmitter release by the postsynaptic axon. Synaptic transmission permits information from many presynaptic neurons to converge on a single postsynaptic neuron. Some large cell bodies receive several thousand synapses Axons