Chapter 1/Cytology and Organization of Cell Types enable the neuron to respond to environmental- compartments; only after sonication or extremely dependent or activity-dependent fluctuations. This acidic conditions do these tenacious structures dis- apparent contradiction is resolved by the inherent sociate into their component parts nature of cytoskeletal elements that exist in different structural and functional states of assembly and dis- 3. 2. The Components of the Neuronal sembly. Moreover, these structures may be stabilized Cytoskeleton Include Microtubules, and destabilized, providing yet another dimension to Neurofilaments, and Microfilaments he number of possible conformations of cytoskeletal and Their Associated Proteins form The interactions of various cytoskeletal elements The dual nature of the neuronal cytoskeleton, with their associated proteins or with each other con- reflected in its rigidity and plasticity, is a function of tribute to the unique structural and functional iden- three filament types: microtubules, neurofilaments, tity of axons and dendrites and their associated and microfilaments or actin filaments. Each cytoske- dendritic spines. The cytoskeleton also interacts letal element acts in conjunction with a specific set of with the neuronal plasma membrane at specific associated or binding proteins. Some of these cross- sites, including the initial segment of the axon, special link the filaments to each other, the plasma mem- loci along the axon called nodes of Ranvier, and brane, and other intracellular organelles and are complex submembrane filamentous arrays. Such sistency of the cytoskeleton. Other associated and membranous-cytoskeletal associations may restrict binding proteins affect the rate and extent of filament the movement of important membrane proteins, polymerization, providing a mechanism for localized such as receptors, at that site or communicate events plastic changes occurring at the membrane to underlying areas. In Microfilaments consisting of the protein actin are this section, we describe components nts of the neuronal 6 nm in diameter and are prominent in cortical cytoskeleton and how they contribute to the architec- regions, particularly in the highly specialized sub ture of the neuron and confer specificity to each membrane filamentous structures, such as the presy- structural domain haptic and postsynaptic membrane specializations Microtubules are long, tubular structures that are 3. 1. The Neuronal Cytoskeleton Provides 25 nm in diameter and form tracks for the transport arious organelles and molecules, although the Internal Support microtubules are themselves also capable of move- Axons and dendrites emerge from the perikaryon ment(Fig. 6). The microtubules and actin consist of as delicate strands. Axons may be as much as a mil- globular subunits that can assemble and disassemble lion times longer than they are wide. Consequently, with relative ease. Neurofilaments that are 10 nm in hese fragile processes require internal support. The diameter are a subdivision of the ubiquitous class of igidity of the cytoskeletal network is apparent after intermediate filaments found in all cells(Fig. 6) removal of the neuronal membrane with detergents Mammalian neurofilaments consist of three fibrous hat selectively extract membrane lipids and proteins. subunits that have a very high affinity for each other, In experiments using detergent-treated cultured nerve and polymers composed of these subunits are very cells, isolated neuronal processes, and isolated sub- stable. Neurofilament subunits are synthesized and membranous cytoskeletal patches, the cytoskeleton assembled in the cell body and then directed down the remains intact, and its shape is virtually identical to axon, where they contribute to its resiliency and its its original conformation. The cylindrical form of the caliber Neurofilaments are degraded at the entrance axonal cytoskeleton is so cohesive that investigators, to the nerve terminal by Ca-activated proteases using the classic model of the squid giant axon to located at that site. study axonal transport mechanisms, equate the extru- sion of its contents, the axoplasm, to that of tooth- 3.2.1. ACTIN AND TUBULIN POLYMERS paste being squeezed out of its tube. Even isolated Subunits of actin, a 43-k Da globular protein, and submembrane filamentous arrays, such as those microtubules, a heterodimer of two 50-kDa globular found beneath the postsynaptic membrane, appear proteins called a-tubulin and B-tubulin, assemble to retain their curvature after the rigorous processes into polymers that bind to identical subunits at each of homogenization of brain and centrifugation, end of a preexisting polymer. The lengths of the poly lysis, and detergent treatment of isolated synaptic mer are determined by cellular mechanisms thatenable the neuron to respond to environmental￾dependent or activity-dependent fluctuations. This apparent contradiction is resolved by the inherent nature of cytoskeletal elements that exist in different structural and functional states of assembly and dis￾assembly. Moreover, these structures may be stabilized and destabilized, providing yet another dimension to the number of possible conformations of cytoskeletal form. The interactions of various cytoskeletal elements with their associated proteins or with each other con￾tribute to the unique structural and functional iden￾tity of axons and dendrites and their associated dendritic spines. The cytoskeleton also interacts with the neuronal plasma membrane at specific sites, including the initial segment of the axon, special loci along the axon called nodes of Ranvier, and presynaptic and postsynaptic membranes, forming complex submembrane filamentous arrays. Such membranous-cytoskeletal associations may restrict the movement of important membrane proteins, such as receptors, at that site or communicate events occurring at the membrane to underlying areas. In this section, we describe components of the neuronal cytoskeleton and how they contribute to the architec￾ture of the neuron and confer specificity to each structural domain. 3.1. The Neuronal Cytoskeleton Provides Internal Support Axons and dendrites emerge from the perikaryon as delicate strands. Axons may be as much as a mil￾lion times longer than they are wide. Consequently, these fragile processes require internal support. The rigidity of the cytoskeletal network is apparent after removal of the neuronal membrane with detergents that selectively extract membrane lipids and proteins. In experiments using detergent-treated cultured nerve cells, isolated neuronal processes, and isolated sub￾membranous cytoskeletal patches, the cytoskeleton remains intact, and its shape is virtually identical to its original conformation. The cylindrical form of the axonal cytoskeleton is so cohesive that investigators, using the classic model of the squid giant axon to study axonal transport mechanisms, equate the extru￾sion of its contents, the axoplasm, to that of tooth￾paste being squeezed out of its tube. Even isolated submembrane filamentous arrays, such as those found beneath the postsynaptic membrane, appear to retain their curvature after the rigorous processes of homogenization of brain and centrifugation, lysis, and detergent treatment of isolated synaptic compartments; only after sonication or extremely acidic conditions do these tenacious structures dis￾sociate into their component parts. 3.2. The Components of the Neuronal Cytoskeleton Include Microtubules, Neurofilaments, and Microfilaments and Their Associated Proteins The dual nature of the neuronal cytoskeleton, reflected in its rigidity and plasticity, is a function of three filament types: microtubules, neurofilaments, and microfilaments or actin filaments. Each cytoske￾letal element acts in conjunction with a specific set of associated or binding proteins. Some of these cross￾link the filaments to each other, the plasma mem￾brane, and other intracellular organelles and are responsible for the gelatinous and relatively stiff con￾sistency of the cytoskeleton. Other associated and binding proteins affect the rate and extent of filament polymerization, providing a mechanism for localized plastic changes. Microfilaments consisting of the protein actin are 6 nm in diameter and are prominent in cortical regions, particularly in the highly specialized sub￾membrane filamentous structures, such as the presy￾naptic and postsynaptic membrane specializations. Microtubules are long, tubular structures that are 25 nm in diameter and form tracks for the transport of various organelles and molecules, although the microtubules are themselves also capable of move￾ment (Fig. 6). The microtubules and actin consist of globular subunits that can assemble and disassemble with relative ease. Neurofilaments that are 10 nm in diameter are a subdivision of the ubiquitous class of intermediate filaments found in all cells (Fig. 6). Mammalian neurofilaments consist of three fibrous subunits that have a very high affinity for each other, and polymers composed of these subunits are very stable. Neurofilament subunits are synthesized and assembled in the cell body and then directed down the axon, where they contribute to its resiliency and its caliber. Neurofilaments are degraded at the entrance to the nerve terminal by Ca2+-activated proteases located at that site. 3.2.1. ACTIN AND TUBULIN POLYMERS Subunits of actin, a 43-kDa globular protein, and microtubules, a heterodimer of two 50-kDa globular proteins called a-tubulin and b-tubulin, assemble into polymers that bind to identical subunits at each end of a preexisting polymer. The lengths of the poly￾mer are determined by cellular mechanisms that Chapter 1 / Cytology and Organization of Cell Types 11