All of the proteins needed by a neuron must be made in the soma, where the protein-synthesizing organelles such as the nucleus, ribosomes, and rough endoplasmic reticulum are located. Yet many of these proteins are needed in the axon, for example to repair and maintain the axolemma, to furnish ion gates in the membrane, or to act in the synaptic knob as enzymes and signaling molecules. Other substances are transported from the axon terminals back to the soma for disposal or recycling. The two-way passage of proteins, organelles, and other materials along an axon is called axonal transport. Movement from the soma down the axon is called anterograde15 transport and movement up the axon toward the soma is called retrograde16 transport.
Materials travel along microtubules of the cytoskele-ton, which act like railroad tracks to guide them to their destination. But what is the "motor" that drives them along the tracks? Anterograde transport employs a motor protein called kinesin, while retrograde transport uses one called dynein (the same protein we encountered earlier in cilia and flagella; see chapter 3). These proteins carry materials "on their backs" while they reach out, like the myosin heads of muscle (see chapter 11), to bind repeatedly to the microtubules and crawl along them.
There are two types of axonal transport, fast and slow.
1. Fast axonal transport occurs at a rate of 20 to 400 mm/day and may be either anterograde or retrograde:
15antero = forward + grad = to walk, to step
• Fast anterograde transport moves mitochondria, synaptic vesicles, other organelles, components of the axolemma, calcium ions, enzymes such as acetylcholinesterase, and small molecules such as glucose, amino acids, and nucleotides.
• Fast retrograde transport returns used synaptic vesicles and other materials to the soma and informs the soma of conditions at the axon terminals. Some pathogens exploit retrograde transport to invade neurons, including tetanus toxin and the herpes simplex, rabies, and polio viruses. In such infections, the delay between infection and the onset of symptoms corresponds to the time needed for the pathogens to reach the somas.
2. Slow axonal transport, also called axoplasmic flow, occurs at a rate of 0.5 to 10 mm/day and is always anterograde. It moves enzymes and cytoskeletal components down the axon, renews worn-out axoplasmic components in mature neurons, and supplies new axoplasm for developing or regenerating neurons. Damaged nerve fibers regenerate at a speed governed by slow axonal transport.
Before You Go On
Answer the following questions to test your understanding of the preceding section:
4. Sketch a multipolar neuron and label its soma, dendrites, axon, terminal arborization, synaptic knobs, myelin sheath, and nodes of Ranvier.
5. Explain the difference between a sensory neuron, motor neuron, and interneuron.
6. What is the functional difference between a dendrite and an axon?
7. How do proteins and other chemicals synthesized in the soma get to the synaptic knobs? By what process can a virus that invades a peripheral nerve fiber get to the soma of that neuron?
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.