12.2 Properties of Neurons

12.2 Properties of Neurons Expected Learning Outcomes: • Describe three functional properties found in all neurons. • Define the three most basic functional categories of neurons. • Identify the parts of a neuron. • Describe four structural classes of neurons. • Explain how neurons transport materials between the cell body and tips of the axon. 8 12.2a Universal Properties Three properties allow neurons to communicate with other cells: • Excitability—ability to respond to stimuli • Conductivity—produce electrical signals that are conducted to other cells • Secretion—when signal reaches end of axon, the neuron secretes a neurotransmitter that stimulates the next cell 9 12.2b Functional Classes Three functional classes of neurons: • Sensory (afferent) neurons—detect stimuli and transmit information about them toward the CNS • Begin in every organ of body and terminate in CNS • Interneurons—receive signals from other neurons, process this information, and make resulting “decisions” • Lie entirely within CNS connecting motor and sensory pathways; most common functional type (about 90% of all neurons) • Motor (efferent) neurons—send signals out to muscles and gland cells (the effectors) 10 The Three Functional Classes of Neurons Figure 12.3 11 12.2c Structure of a Neuron 1 Principal components of a neuron: • Cell body—also called neurosoma, soma, or perikaryon; contains nucleus and many organelles • Mitochondria, lysosomes, Golgi complex, inclusions, rough ER • Cytoskeleton contains microtubules, and neurofibrils (actin bundles) compartmentalize the rough ER into dark-staining chromatophilic substance • No centrioles; mature neurons cannot undergo mitosis after adolescence • Inclusions may be glycogen granules, lipid droplets, melanin, or lipofuscin (pigment resulting from degradation of organelles by lysosomes) 12 Structure of a Neuron 2 Principal components of a neuron (continued): • Neurons can have many neurites (extensions) reaching out to other cells • Dendrites—most numerous neurites, resemble branching of a tree; primary sites for receiving signals from other neurons • Neurons can have one dendrite or thousands of dendrites • Axon (nerve fiber)—long, cylindrical extension; relatively unbranched but may give off axon collaterals; specialized for rapid conduction of nerve signals • • • • • Originates at axon hillock—mound on one side of cell body Contains axoplasm (its cytoplasm) and its membrane is the axolemma At its end, an axon branches profusely—the terminal arborization Each branch of arborization ends in a bulbous axon terminal (terminal bouton), which forms a synapse with next cell A neuron never has more than one axon; some neurons have none 13 General Structure of a Neuron 1 Figure 12.4 14 General Structure of a Neuron 2 Figure 12.4a 15 General Structure of a Neuron 3 Figure 12.4a 16 General Structure of a Neuron 4 Figure 12.4b 17 General Structure of a Neuron 5 Figure 12.4c 18 General Structure of a Neuron 6 Figure 12.4d,e 19 Structure of a Neuron 3 Four structural classes of neurons, determined by number of cell processes extending from cell body: • Multipolar neuron—one axon and multiple dendrites; most common type in body, most neurons in CNS • Bipolar neuron—one axon and one dendrite; examples include olfactory cells, some neurons of retina, sensory neurons of ear • Unipolar neuron—single process leading away from cell body, splits into peripheral process and central process • Both processes comprise the axon; only short receptive endings of peripheral process are considered dendrites • Anaxonic neuron—many dendrites but no axon; found in brain, retina, and adrenal gland 20 Structural Classes of Neurons 1 Figure 12.5a 21 Structural Classes of Neurons 2 Figure 12.5b 22 Structural Classes of Neurons 3 Figure 12.5c,d 23 12.2d Axonal Transport 1 Axonal transport—two-way passage of materials along an axon • Some materials (enzymes, ion channels) made in cell body must be transported to the axon and terminal • Anterograde transport—movement away from cell body, down the axon; driven by motor protein kinesin • Other materials need to be transported from axon terminals back to cell body for disposal • Retrograde transport—movement up the axon toward the cell body; driven by motor protein dynein 24 Axonal Transport 2 Axonal transport can be fast or slow • Fast axonal transport—200 to 400 mm/day and may be anterograde or retrograde • Fast anterograde transport of organelles, enzymes, synaptic vesicles, small molecules • Fast retrograde transport for recycled materials, some pathogens • Slow axonal transport—0.2 to 0.5 mm/day; “stop-and-go” movement results in slower overall speed; always anterograde • Transport of enzymes, cytoskeletal components, new axoplasm • Damaged nerves regenerate at a speed governed by slow axonal transport 25