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PNS Structure - Anatomy & Physiology (view source)

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*The motor molecule for retrograde transport is '''''dynein''''' which is a microtubule-associated ATPase.
 
*The motor molecule for retrograde transport is '''''dynein''''' which is a microtubule-associated ATPase.
 
*The retrograde transport system is important not only for returning material to the cell body, but also provides the means whereby target-derived trophic factors, such as nerve growth factor (NGF) for dorsal root ganglion neurons, are conveyed to the cell body where they promote cell survival.
 
*The retrograde transport system is important not only for returning material to the cell body, but also provides the means whereby target-derived trophic factors, such as nerve growth factor (NGF) for dorsal root ganglion neurons, are conveyed to the cell body where they promote cell survival.
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==Dump of information from WikiPath Below==
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'''Needs to be integrated with above content'''
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==The Nerve Fibre==
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* The nerve fibre consists of the:
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** [[PNS Gross Anatomy#The Axon|Axon]]
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*** Carries impulses
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** [[PNS Gross Anatomy#The Schwann cell|Schwann cell]]
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*** Ensheaths the axon
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** Basal lamina
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*** Surrounds the Schwann cell
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===<u>The Axon</u>===
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* The axon consists of:
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** An outer membrane, called the axolemma.
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** The axoplasm.
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*** This is contained within the axolemma.
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*** Itis continuous with the cytoplasm of the neuron.
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** '''NO''' ribosomes (free of attached to the ER).
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*** Therefore no protein synthesis can take place in the axon.
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*** All protein required for the maintenance of the axon depends on proteins being imported from the cell body.
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** The cytoskeleton.
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*** This is a key feature of the axon, which consists of two key elements:
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***#'''Neurofilaments'''
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***#* Neurofilaments are the axon's intermediate filaments.
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***#** 10 nm diameter.
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***#** Formed from three polypeptide subunits, which tend to be heavily phosphorylated.
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***#* Neurofilaments are more numerous than microtubules, especially in large diameter axons.
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***#** The have a pivotal role in determining axon diameter.
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***#* Neurofilaments are formed in the cell body, transported down the axon and degraded in the terminals by Ca<sup>2+</sup> activated proteases.
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***#** I.e there is a constant turnover of neurofilaments.
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***# '''Microtubules'''
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***#* Axonal microtubules are similar to microtubules elsewhere.
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***#** 28nm diameter.
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***#** Consist of polymerised dimers of alpha and beta tubulin arranged as a hollow tube.
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***#* Relatively abundant in smaller diameter axons.
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***#* Synthesised in the cell body and transported down the axon.
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***#* '''Microtubule associated proteins (MAPs)''' and the '''tau protein''' are important components of the cytoskeleton.
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***#** These proteins contribute to microtubule assembly and stability. They:
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***#*** Form cross links between adjacent microtubules.
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***#*** Connect microtubules to neurofilaments and actin microfilaments.
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***#**** This implies complex interactions between the components of the axon cytoskeleton exist.
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***#** The classes of MAPs present differ between the dendrites and the axons.
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***#*** This may account for the different ultrastructural features that distinguish these two types of neuronal process.
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====Axoplasmic Transport====
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* Neurones are very large cells.
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** A high proportion of a neurons cytoplasm is present in its processes.
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** However, the cell's protein producing machinery (the Nissl substance) is located in the cell body.
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* To overcome these issues, neurons have evolved mechanisms to transport large macromolecules and organelles up and down their processes.
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=====Anterograde Transport=====
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* Anterograde transport moves substances from the cell body to the axon.
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* Two forms of anterograde transport are recognised:
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# '''Fast anterograde transport'''
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#* All membranous organelles are transported by fast anterograde transport.
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#* Movement occurs at around 400mm/day.
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#* Microtubules act as a static track along which the organelles can move, driven by the ATPase '''kinesin''' which acts as a "motor" molecule.
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#* Fast anterograde transport depends on oxidative metabolism.
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#** However, it is independent of the cell body.
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#* Anything which interfering with with energy supply or cytoskeleton necessary for fast anterograde transport has profound effects on the health of the axon.
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#** Agents such as colchicine or vincristine block microtubule assembly, disrupting fast anterograde transport.
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#*** They also block the microtubules of the mitotic spindle, having an antimitotic effect. This makes them useful in anticancer therapy.
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# '''Slow anterograde transport'''
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#* This transports cytoskeletal elements and large soluble proteins.
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#* There are two components so slow anterograde transport
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#** A slow component.
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#*** Transport occurs at around 2mm/day.
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#*** Neurofilaments, rubulin and actin actin are transported in this manner.
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#** A fast component.
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#*** Movement occurs at around 4 mm/day.
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#*** All other proteins are transported this way, for example myosin and clathrin.
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=====Retrograde Transport=====
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* Retrograde transport returns materials from the axon terminal to the cell body.
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** The purpose of this is either for degradation or for restoration and reuse.
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* Particles move along microtubules, as for fast anterograde transport.
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** '''Dynein''', a microtubule-associated ATPase, is the motor molecule for retrograde transport.
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* Apart from returning material to the cell body, target-derived trophic factors are conveyed by retrograde transport to the cell body where they promote cell survival.
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** An example of such a trophic factor is nerve growth factor (NGF), which promotes the growth of dorsal root ganglion neurons.
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** Neurons are particularly dependent on a supply of trophic factors during development.
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** Research is being undertaken into the use of trophic factors to promote cell survival during degenerative pathology.
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* The retrograde transport system can be "hijacked" by harmful substances to gain entry to the peripheral neuron and ultimately the CNS. For example:
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** Viruses: [[Herpesviridae|herpes simplex virus]], [[Rhabdoviridae|rabies]].
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** Toxins: [[Tremors and Movement Disorders (Nervous System) - Pathology#Tetanus|tetanus]], heavy metals.
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===<u>The Schwann Cell</u>===
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* Schwann cells provide myelination in the PNS.
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** They are derived from  neural crest cells.
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* During development of the nervous system, Schwann cells interact with many small axons.
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** Schwann cells eventually relate to only one axon, as axonal diameter increases with maturation of the system.
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***[[Microscopic Anatomy of the CNS#Oligodendrocytes|Oligodendrocytes]], that myelinate the CNS, differ from Schwann cells in that they interact with many axons.
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====The Process of Myelination====
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# Initially, the single axon to be myelinated by the Schwann cell sits in a trough formed by the Schwann cell processes.
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# The processes come together to enclose the axon, forming an inner mesaxon.
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# The leading-edge process continues to move over the axon, creating a spiral of Schwann cell around it.
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# Cytoplasm within the proces is extruded, meaning the axon is wrapped in Schwann cell membrane (myelin) alone.
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#* The internal surfaces of the membrane, now vacated of cytoplasm, come together as the major dense line.
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#* The outer membrane forms the intraperiod line.
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# *The alternating pattern of these major dense line and the intraperiod line  form the lamellae of compacted myelin.
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# The myelin sheath remains attached to, and is an integral part of, the Schwann cell.
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#* The myelin is therefore dependent on the Schwann cell for its maintenance.
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====Relationship With Axons====
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* A single Schwann cell forms a single myelin sheath or internode.
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** One cell does not myelinate the whole length of an axon!
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* Myelin thickness is related to internodal length, which in turn is associated with axon calibre:
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** Large axons have long, thick myelin sheaths, and therefore also conduct more rapidly.
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* The internodes do not abut one another but are separated by an exposed area of axon called the '''Node of Ranvier'''.
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** Action potentials are able to leap between Nodes of Ranvier in saltatory conduction. This increases conduction speed.
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* If the axon diameter remains small, then a Schwann cell will continue to associate with many axons, although none of them are fully myelinated.
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** Thus, even unmyelinated axons retain a Schwann cell ensheathment.
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** These non-myelinating Schwann cells are sometimes referred to as '''Remak cells'''.
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===<u>Fibre Types</u>===
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* Nerve fibres can be assigned to different fibre types depending on their diameter and conduction velocity.
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** The larger the fibre diameter, the more rapid the rate of impulse conduction.
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** Particular targets or receptors are associated with axons of a particular diameter, for example:
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*** Those connected to muscles spindles have a large diameter (20 um) and conduct at 120 m/s
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*** The smallest myelinated fibres are about 1um and conduct at around 6 m/s.
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*** The smallest fibres of all are the unmyelinated fibres.
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**** These are the high-threshold sensory afferents, or C-fibres, and post-ganglionic autonomic fibres.
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**** They have a diameter of between 1 and 0.1 um.
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**** These fibres do not conduct by saltatory conduction and have very slow conduction rates of around 0.5 m/s.
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==Connective Tissue==
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* Peripheral nerves have a three-tiered hierarchical arrangement of connective tissue.
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** A nerve fibre is surrounded by a connective tissue matrix called the '''endoneurium'''.
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** Bundles, or '''fasicles''' of neurons are enclosed in a second connective tissue layer called the '''perineurium'''.
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** Groups of fascicles are then gathered together in a third connective tissue layer called the '''epineurium'''.
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* Renaut bodies may be present in some species.
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** They are loose, cigar-shaped whorls of extracellular matrix within fascicles.
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** Common in horse nerve.
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** May also occur in human and rat nerves at points of stress or compression.
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==Blood Supply==
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* The epineurium is penetrated by the vascular supply to the nerve.
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** This blood supply is known as the '''vasa nervorum'''.
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* Only capillaries occur within the endoneurial compartment.
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** The capillaries of the endoneurium are joined by tight junctions and provide a barrier to large macromolecules.
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*** This forms the basis of the blood-nerve barrier (BNB), which has similarities to the [[Blood Brain Barrier (BBB)|blood-brain barrier]] of the CNS.
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**** The BNB appears to be relatively weak in the sensory ganglia because fenestrations occur between endothelial cells in this location.
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***** Sensory ganglia are therefore more vulnerable to blood-borne agents.
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* A further "barrier" is provided by the perineurium.
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** This consists of sheets of flattened cells, connected by tight junctions, and covered on both sides by a basal lamina.
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*** The only route across this structure is trans- rather than inter-cellular.
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