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1.7. The Neuronal Cytoskeleton
Defining the Term ‘Neuronal Cytoskeleton’
The cytoskeleton is a cellular structure made up of filament-like proteins that helps to maintain the shape and organisation of a cell and provides a secure location for cell organelles to attach to. Neurons have a specialised cytoskeleton that also allows them to grow in length and transport substances, such as nerve growth factors, to and from the cell body. Figure 1.7.1. shows the intracellular environment of a neuron, with the different types of proteins in their respective locations.
The Proteins Contributing to the Neuronal Cytoskeleton
There are three main types of proteins (also called protein filaments) that contribute to the neuronal cytoskeleton. These are:
Let’s start by looking at microtubules. Microtubules are made up of lots of smaller units of tubulin which have been joined together, processed and organised in a cellular structure called the centrosome. Microtubules also have a plus end and a minus end. In neurons, the minus ends are described as ‘free’, i.e. they are not anchored down to the centrosome – this allows the neuron to increase the length of its axon. Microtubules also provide a framework for transporting substances along the axon, as described in the section below. Microtubules also help to maintain ‘organelle polarity’ – this means that all the organelles are found where they should relative to each other.
Actin filaments are made up of monomers of the protein actin – the most common protein in eukaryotic cells. Actin tends to lie close to the dendrites and growth cones of developing neurons (see Figure 1.7.1.), and there it helps to maintain the shape of these neuronal projections while they grow. The remainder of actin is found close to the neuronal membrane in the axon. The formation of actin from its monomers is active, i.e. it requires ATP. Moreover, actin is continuously being removed and replaced in a process known as remodelling.
Neurofilaments are three chains wrapped together and are essential for the axon to grow in diameter and for the fast conduction of electrical impulses along the axon. They also form part of the nuclear lamina – this is a network of fibrils that provides structural support to the nucleus and is involved in DNA replication."
- Microtubules
- Actin filaments
- Neurofilaments (a type of intermediate filament)
Let’s start by looking at microtubules. Microtubules are made up of lots of smaller units of tubulin which have been joined together, processed and organised in a cellular structure called the centrosome. Microtubules also have a plus end and a minus end. In neurons, the minus ends are described as ‘free’, i.e. they are not anchored down to the centrosome – this allows the neuron to increase the length of its axon. Microtubules also provide a framework for transporting substances along the axon, as described in the section below. Microtubules also help to maintain ‘organelle polarity’ – this means that all the organelles are found where they should relative to each other.
Actin filaments are made up of monomers of the protein actin – the most common protein in eukaryotic cells. Actin tends to lie close to the dendrites and growth cones of developing neurons (see Figure 1.7.1.), and there it helps to maintain the shape of these neuronal projections while they grow. The remainder of actin is found close to the neuronal membrane in the axon. The formation of actin from its monomers is active, i.e. it requires ATP. Moreover, actin is continuously being removed and replaced in a process known as remodelling.
Neurofilaments are three chains wrapped together and are essential for the axon to grow in diameter and for the fast conduction of electrical impulses along the axon. They also form part of the nuclear lamina – this is a network of fibrils that provides structural support to the nucleus and is involved in DNA replication."
Axonal Transport Using the Cytoskeleton
Axonal transport can occur in one of two directions:
- From the cell body to the presynaptic terminal – this is called anterograde transport. Cargo such as new synaptic vesicles are moved by anterograde transport using a specialised ‘motor protein’ called kinesin. The down side is that this process is slow.
- From the presynaptic terminal to the cell body – this is called retrograde transport. Cargo such as old bits of presynaptic terminal and nerve growth factors are moved by retrograde transport using a specialised ‘motor protein’ called dynein. This process is faster than anterograde transport.
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