External Components of a Neuron

Tutorial 2: External Components of a Neuron

Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test

A neuron is the functioning unit of the nervous system; specialized to receive, integrate, and transmit information. The flow of information moves in the following direction: dendrite to soma to axon to terminal buttons to synapse. Although there are many types of neurons among the estimated ten billion in the human brain, each typically shares the characteristics shown in Tutorial 2.

Neurons are distinguished and categorized according to general function; there are receptor or sensory neurons, motor neurons, and interneurons. Sensory or afferent neurons are specialized to be sensitive to a particular physical stimulation such as light (vision), sound (audition), chemical (olfaction), or pressure (touch), and they carry this information toward the brain. Motor or efferent neurons receive impulses from other neurons and transmit this information away from the brain to muscles or glands. Interneurons or intrinsic neurons form the largest group in the nervous system. They form connections between themselves and sensory neurons for the integration of information before it is passed along to those neurons that will organize a behavioral response.

Neurons also vary according to shape. For example, some have more elaborate dendritic branching while others differ in the placement of the cell body relative to other portions of the neuron. These neuron types will be the topic of discussion in Tutorial 3.

Advanced

Since the time of Ramon y Cajal, the neuron has been extolled as the primary functional unit of the nervous system. This so-called neuron doctrine has met with challenge in the past as well as in the present. The controversy that raged almost a century ago centered on the existence of neurofilaments connecting neurons, suggesting the importance of functional, interconnected neuronal networks (Young, 1992). Cajal was brutal in his reactions to his "reticularists" rivals (Golgi and A. Bethe), calling them "fanatics with haughty minds, inclined toward mysticism"!

One of neuroscience's many paradoxes is that the most widely-studied of synapses, that of the giant squid axon, involves a synaptic fiber connecting the pre- and postsynaptic membranes. We also know that gap junctions allow the passage of ions and small molecules between neurons, and that neurons with long-standing functional relationships actually fuse. In addition, it is now known that dendrites may have synaptic outputs and that axons receive input from other axons. Dendrites may propagate the action potentials thought exclusive to communication along axons, while some axons do not carry action potentials at all. One hundred years of research still support the conclusion that where decisions are to be made, neurons will be separated by synapses. More recent findings, however, draw attention to the need for a better understanding of sub-neuronal mechanisms as well as mechanisms governing the interactions within complex neuronal networks.

Our understanding of neuronal networks has benefited from both technological and theoretical advances in recent years. The use of labeled plant proteins to trace the origins of input (retrograde projections) or targets (anterograde projections) of specific neurons (Parent, 1996). The original tracer, Horseradish peroxidase (a retrograde tracer), when injected is picked up by the axonal endings of neurons and transported back to the cell body. The nervous tissue undergoes special processing after designated periods of time to label the cell bodies that project to the region where the injection of tracer was made. Conversely, other substances may be absorbed by the neuronal cell bodies at the site of injection and transported anterograde to the terminal endings of the axon. More advanced techniques utilize bi-directional tracers (e.g., biotinylated dextran amines), which have simplified somewhat the tedious process of mapping brain circuits (Kaneko, Saeki, Lee & Mizuno, 1996). Recently, neuronal infections with weak viruses are used as transneuronal tracers (Loewy, 1998). A microinjection of live viruses (e.g., pseudorabies virus (PRV) -- a pig herpes virus) is made into a brain region of an experimental animal. Terminal buttons in the region of injection absorb the viruses, and transported them via axon to the cell body. Here the infection begins and the neuron's nuclear material is used to replicate the virus. These progeny are then transferred to neighboring neurons across synapses. If the appropriate length of time occurs between injection and processing of the tissue, first-, second-, third-, and even forth-order neurons in a network may be identified.

Two theories of how complex systems behave have been influential in recent years. The first, Artificial Intelligence, involves a combination of psychology, physiology, computer science, and philosophy. This interdisciplinary approach to the creation of machines that think is sometimes linked with models of how the brain functions (Draghici, 2000; Frenger, 2000). The second theory is based on the nonlinear mathematical model of system dynamics called Chaos Theory (Rabinovich & Abarbanel, 1998). Each of these can be explored further by those interested via the links provided below.

References

Draghici, S. (2000). Neural networks in analog hardware--design implementation. International Journal of Neural Systems, 10(1), 19-42.

Frenger, P. (2000). Human nervous system function emulator. Biomedical Scientific Instrumentation, 36, 289-294.

Kaneko, T., Saeki, K., Lee, T., Mizuno, N. (1996). Improved retrograde axonal transport and subsequent visualization of tetramethylrhodamine (TMR) -- dextran amine by means of an acidic injection vehicle and antibodies against TMR. Journal of Neuroscience Methods, 65(2), 157-165.

Loewy, A.D. (1998). Viruses as transneuronal tracers for defining neural circuits. Neuroscience and Biobehavioral Reviews, 22(6), 679-684.

Parent, A. (1996). Carpenter's human neuroanatomy (9th ed.). London: Williams & Wilkins.

Rabinovich, M.I. & Abarbanel, H.D.I. (1998). The role of chaos in neural systems. Neuroscience 87(1), 5-14.

Young, J.Z. (1992). Nervous starts. Nature, 356.

Suggestions for further study

Tutorial 2: External Components of a Neuron

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