Dendrites

Dendrites are nerve cell fibers that receive signals from other cells.

The nervous system relies on both chemicals and electrical impulses to transmit sensory and command signals. Neurons, or nerve cells, have a specialized structure. One end, called the dendrite, consists of branched projections. The dendrite enables the neuron to process chemical signals from other neurons as well as endocrine-produced hormones from other areas of the body.

KEY TERMS

Dendrite—
A portion of a nerve cell that carries nerve impulses toward the cell body.
Synapse—
The narrow gap between the terminal of one neuron and the dendrites of the next.

If the signals received at the dendrite are properly timed and of sufficient strength, they are transformed into electrical action potentials that sweep down the neural cell body (axon) from the dendrite end to the presynaptic portion of the axon, called the axon terminal. The axon terminal ends at the synapse, an extracellular gap between neurons. The arrival of the action potential at the presynaptic terminus causes the release of ions and neurotransmitter chemicals that travel across the synapse. These substances act as the stimulus to initiate another action potential in the next neuron, thereby perpetuating the neural impulse through the neural pathway.

Dendrites are one of two types of short, threadlike fibers that extend from the cell body of a nerve cell, or neuron. Fibers of the other type are called axons. Dendrites receive electrochemical signals, which are known as postsynaptic potentials, from the axons of other neurons. The information contained in these signals is fired across a synaptic gap or cleft, which is about 0.02 microns (8 millionths of an inch) wide, and transmitted toward the cell body. The signals fade as they approach their destination. A single neuron can have many dendrites, each composed of numerous branches; together, they comprise the greater part of the neuron's receptive surface.

Neurotransmitters can excite or inhibit the postsynaptic neuron. In excitation, they create an action potential. Excitation results from neurotransmitter-driven shifts in ion balance that result in depolarization. Specifically, excitatory neurotransmitters achieve their effect by causing changes in sodium ion balance. If this stimulus is strong enough (i.e., sufficient amounts of neurotransmitter bind to dendritic receptors), it results in the postsynaptic neuron reaching the threshold potential and creates an electrical action potential. Excitation can also result from a combination of neurotransmitters released from several presynaptic neurons that terminate on one postsynaptic neuron. In addition to spatial control mechanisms, excitation also involves time-dependent mechanisms (temporal controls). Neurotransmitters remain bound to their receptors for a period of time, so excitation can also result from an increased rate of neurotransmitter release in the presynaptic neuron.

Inhibitory neurotransmitters generally achieve their effect by inducing a state of hyperpolarization. They cause membrane changes that result in a movement of ions across the postsynaptic neural cell membrane, moving the electrical potential away from the threshold potential.

The number of axons and dendrites increases dramatically during infancy and childhood, possibly to facilitate the rapid development experienced during this period, and decreases in early adolescence. A child of six or seven has more dendrites than an adult.

See also Nervous system ; Neuron ; Neurotransmitter ; Synapse .

Resources

BOOKS

Bennett, M. R. Neuroscience and Philosophy: Brain, Mind, and Language. New York: Columbia University Press, 2007.

Clark, David L., et al. The Brain and Behavior: An Introduction to Behavioral Neuroanatomy. New York: Cambridge University Press, 2010.

Cuntz, Hermann, et al. The Computing Dendrite: From Structure to Function. New York: Springer, 2014.

Frank, Lone. Mindfield: How Brain Science Is Changing Our World. Oxford: Oneworld, 2009.

Girard, Ines C., and Jade S. Andre. Brain Mapping Research Progress. New York: Nova Biomedical Books, 2009.

Lynch, Gary, and Richard Granger. Big Brain: The Origins and Future of Human Intelligence. New York: Palgrave Macmillan, 2008.

Lynch, Zack, and Byron Laursen. The Neuro Revolution: How Brain Science Is Changing Our World. New York: St. Martin's Press, 2009.

Pickel, Virginia M., and Menahem Segal. The Synapse Structure and Function. Oxford: Academic Press, 2013.

Woolsey, Thomas A., et al. The Brain Atlas: A Visual Guide to the Human Central Nervous System. Hoboken, NJ: Wiley, 2008.

Zeman, Adam. A Portrait of the Brain. New Haven, CT: Yale University Press, 2008.

WEB SITES

National Geographic Society. “Beyond the Brain.” http://science.nationalgeographic.com/science/health-andhuman-body/human-body/mind-brain.html (accessed September 18, 2015).

National Geographic Society. “Brain.” http://science.nationalgeographic.com/science/health-and-human-body/human-body/brain-article.html (accessed September 18, 2015).