| Line 42: |
Line 42: |
| | | | |
| | ==The Nerve Fibre== | | ==The Nerve Fibre== |
| − | The nerve fibre consists of the: the '''[[#The Axon|axon]]''' which carries impulses, '''[[#The Schwann cell|Schwann cells]]''' which ensheath the axon and the '''basal lamina''' which surrounds the Schwann cells.
| |
| − | <br />
| |
| − | <br />
| |
| − | ===The Axon===
| |
| − | The axon consists of an outer membrane, called the '''axolemma'''. The '''axoplasm''' is contained within the axolemma and is continuous with the cytoplasm of the neuron. There are no ribosomes (free or attached to the ER) and therefore no protein synthesis can take place in the axon. All protein required for the maintenance of the axon depends on proteins being imported from the cell body.The '''cytoskeleton''' is a key feature of the axon, which consists of two key elements:
| |
| − | ***#'''Neurofilaments'''
| |
| − | ***#* Neurofilaments are the axon's intermediate filaments.
| |
| − | ***#** 10 nm diameter.
| |
| − | ***#** Formed from three polypeptide subunits, which tend to be heavily phosphorylated.
| |
| − | ***#* Neurofilaments are more numerous than microtubules, especially in large diameter axons.
| |
| − | ***#** The have a pivotal role in determining axon diameter.
| |
| − | ***#* Neurofilaments are formed in the cell body, transported down the axon and degraded in the terminals by Ca<sup>2+</sup> activated proteases.
| |
| − | ***#** I.e there is a constant turnover of neurofilaments.
| |
| − | ***# '''Microtubules'''
| |
| − | ***#* Axonal microtubules are similar to microtubules elsewhere.
| |
| − | ***#** 28nm diameter.
| |
| − | ***#** Consist of polymerised dimers of alpha and beta tubulin arranged as a hollow tube.
| |
| − | ***#* Relatively abundant in smaller diameter axons.
| |
| − | ***#* Synthesised in the cell body and transported down the axon.
| |
| − | ***#* '''Microtubule associated proteins (MAPs)''' and the '''tau protein''' are important components of the cytoskeleton.
| |
| − | ***#** These proteins contribute to microtubule assembly and stability. They:
| |
| − | ***#*** Form cross links between adjacent microtubules.
| |
| − | ***#*** Connect microtubules to neurofilaments and actin microfilaments.
| |
| − | ***#**** This implies complex interactions between the components of the axon cytoskeleton exist.
| |
| − | ***#** The classes of MAPs present differ between the dendrites and the axons.
| |
| − | ***#*** This may account for the different ultrastructural features that distinguish these two types of neuronal process.
| |
| − |
| |
| − | ====Axoplasmic Transport====
| |
| − |
| |
| − | * Neurons are very large cells.
| |
| − | ** A high proportion of a neurons cytoplasm is present in its processes.
| |
| − | ** However, the cell's protein producing machinery (the Nissl substance) is located in the cell body.
| |
| − | * To overcome these issues, neurons have evolved mechanisms to transport large macromolecules and organelles up and down their processes.
| |
| − |
| |
| | =====Anterograde Transport===== | | =====Anterograde Transport===== |
| | | | |