Dendrites and dendritic spines.

In this silver-stained preparation of cells in a section of cerebellum, the many dendrites emerging from a single stellate neuron are clearly seen. Each dendrite can be seen further to have many dendritic spines (DS) along its surface. The dendritic spines are sites of synapses with other neurons. Their length and morphology are dependent on actin filaments and are highly plastic. Arrow indicates the cell's axon. X500. Golgi.

Axons

Most neurons have only one axon, a cylindrical process that varies in length and diameter according to the type of neuron. Axons are usually very long processes. For example, axons of the motor cells of the spinal cord that innervate the foot muscles may have a length of up to 100 cm (~40 inches). All axons originate from a pyramid-shaped region, the axon hillock, arising from the perikaryon. The plasma membrane of the axon is often called the axolemma and its

contents are known as axoplasm.

Just beyond the axon hillock, at an area called the initial segment, is the site where various excitatory and inhibitory stimuli impinging on the neuron are algebraically summed, resulting in the decision to propagate—or not to propagate—a nerve impulse. Several types of ion channels are localized in the initial segment and these channels are important in generating the action potential. In contrast to dendrites, axons have a constant diameter and do not branch profusely.

Occasionally, the axon, shortly after its departure from the cell body, gives rise to a branch that returns to the area of the nerve cell body. All axon branches are known as collateral branches. Axoplasm contains mitochondria, microtubules, neurofilaments, and some cisternae of smooth ER. The absence of polyribosomes and rough ER emphasizes the dependence of the axon on the perikaryon for its maintenance. If an axon is severed, its peripheral parts quickly degenerate.

There is a lively bidirectional transport of small and large molecules along the axon. Organelles and macromolecules synthesized in the cell body move by anterograde transport along the axon from the perikaryon to the synaptic terminals. Retrograde transport in the opposite direction carries certain other macromolecules, such as material taken up by endocytosis (including viruses and toxins), from the periphery to the cell body. Retrograde transport can be used to study the pathways of neurons: if peroxidase or another marker is injected into regions with axon terminals, its distribution along the entire axon after a period of time can be followed histochemically.

Axonal transport in both directions utilizes motor proteins attached to microtubules. Kinesin, a microtubule-activated ATPase, attaches to vesicles and allows them to move along microtubules in axons away from the perikarya. Dynein is a similar ATPase that allows retrograde transport in axons, toward the cell bodies.

Anterograde and retrograde transport both occur fairly rapidly, at rates of 50 to 400 mm/day. A much slower anterograde stream (only a few millimeters per day) involves movement of the axonal cytoskeleton itself. This slow transport system corresponds roughly to the rate of axon growth.

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