Dendrites are receptor processes that receive stimuli from other neurons or from the external environment
The main function of dendrites is to receive information from other neurons or from the external environment and carry that information to the cell body. Generally, dendrites are located in the vicinity of the cell body. They have a greater diameter than axons, are unmyelinated, are usually tapered, and form extensive arborizations called den-dritic trees. Dendritic trees significantly increase the receptor surface area of a neuron. Many neuron types are characterized by the extent and shape of their dendritic trees (see Fig. 11.2). In general, the contents of the cell body and dendrites are similar, with the exception of the Golgi apparatus. Whereas the Golgi network remains close to the nucleus, other organelles characteristic of the cell body proper, including ribosomes and rER, are found in the dendrites, especially in the base of the dendrites.
Axons are effector processes that transmit stimuli to other neurons or effector cells
The main function of the axon is to convey information away from the cell body to another neuron or to an effector cell, such as a muscle cell. Each neuron has only one axon, and it may be extremely long. Axons that originate from neurons in the motor nuclei of the CNS (Golgi type I neurons) may travel more than a meter to reach their effector targets, skeletal muscle. In contrast, interneurons of the CNS (Golgi type II neurons) have a very short axon. Although an axon may give rise to a recurrent branch near the cell body (i.e., one that turns back toward the cell body; see Fig. 11.1) and to other collateral branches, the branching of the axon is most extensive in the vicinity of its targets.
The axon originates from the axon hillock. As mentioned, it usually lacks large cytoplasmic organelles such as Nissl bodies and Golgi cisternae. Microtubules, neurofila-
merits, mitochondria, and vesicles, however, pass through the axon hillock into the axon. The region of the axon between the apex of the axon hillock and the beginning of the myelin sheath (see below) is called the initial segment. The initial segment is the site at which an action potential is generated in the axon. The action potential (described in more detail below) is stimulated by impulses carried to the axon hillock on the membrane of the cell body after other impulses are received on the dendrites or the cell body itself.
Some large axon terminals are capable of local protein synthesis, which may be involved in memory processes
Almost all of the structural and functional protein molecules are synthesized in the perikaryon. These molecules are distributed to the axons and dendrites via axonal transport systems (described on page 293). However, contrary to the common view that the perikaryon is the only site of protein synthesis, recent studies provide evidence of local synthesis of axonal proteins 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.
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