Changes

==Nerve Fibre==

==Nerve Fibre==

*The nerve fibre consists of the impulse-carrying axon, which is surrounded by an ensheathing cell, the [[#The Schwann cell|Schwann cell]], which in turn is surrounded by an acellular basal lamina that is continuous along the length of the nerve.

The nerve fibre consists of the impulse-carrying axon, which is surrounded by an ensheathing cell, the [[#The Schwann cell|Schwann cell]], which in turn is surrounded by an acellular basal lamina that is continuous along the length of the nerve. Nerve fibres come in various discrete diameter groups, which are reflected in their conduction velocities. The larger the diameter the more rapid the rate of impulse conduction. Particular targets or receptors are associated with axons of a particular diameter. Axons connected to muscles spindles have a large diameter (20 um) and conduct at 120 m/s whilst the smallest myelinated fibres are about 1um and conduct at around 6 m/s. The smallest fibres of all are the unmyelinated fibres (the high-threshold sensory afferents, or C-fibres, and post-ganglionic autonomies) and have a diameter of between 1 and 0.1 um. These fibres do not conduct by saltatory conduction and have very slow conduction rates of around 0.5 m/s.

*Nerve fibres come in various discrete diameter groups, which are reflected in their conduction velocities.  

*The larger the diameter the more rapid the rate of impulse conduction.  

*Particular targets or receptors are associated with axons of a particular diameter, for example:

**Those connected to muscles spindles have a large diameter (20 um) and conduct at 120 m/s

**The smallest myelinated fibres are about 1um and conduct at around 6 m/s.  

**The smallest fibres of all are the unmyelinated fibres (the high-threshold sensory afferents, or C-fibres, and post-ganglionic autonomies) and have a diameter of between 1 and 0.1 um. These fibres do not conduct by saltatory conduction and have very slow conduction rates of around 0.5 m/s.

 

===The Axon===

===The Axon===

*An outer membrane called the axolemma,

Axons have an outer membrane called the '''axolemma''' and within this there is the '''axoplasm''' which is continuous with the cytoplasm of the neuron. There are no ribosomes, either free or attached to endoplasmic reticulum in axons and therefore, no protein synthesis.  

*Within this there is the axoplasm which is continuous with the cytoplasm of the neuron.  

Protein synthesis takes place within the cell body and some dendrites and all protein replacement required for the maintenance of the axon depends on proteins being imported from the cell body. A critical feature of the axon is its '''cytoskeleton''', which consists of two key elements; '''neurofilaments''' and '''microtubules'''. '''Neurofilaments''' are intermediate filaments of about 10 nm diameter, and belong to the same class as other cytoskeletal proteins such as keratin, desmin, vimentin, or GFAP of astrocytes. Neurofilaments are formed from a triplet of polypeptide subunits of heavy (~ 200 kD), medium (~ 150 kD) and low (~ 60 kD) molecular weights. Typically, these subunits are heavily phosphorylated and are more numerous than microtubules, especially in large diameter axons, having a pivotal role in determining axon diameter. They are formed in the cell body, transported down the axon by axoplasmic transport and degraded in the terminals by Ca²⁺ activated proteases. In other words, there is a constant turnover of neurofllament within the healthy axon. '''Microtubules''' within axons are similar to microtubules elsewhere, consisting of polymerised dimers of alpha and beta tubulin arranged as a hollow tube of about 28 nm. They are relatively abundant in smaller diameter axons, and are also synthesised in the cell body. An important component of the cytoskeleton are the '''microtubule associated proteins''' or MAP's and the tau proteins. These proteins are important in microtubule assembly and stability. Different classes of MAP's occur in the dendrites and the axons, and to some extent account for the different ultrastructural features that distinguish these two types of neuronal process. They form cross links between adjacent microtubules but also connect to neurofilaments and actin microfilaments, implying complex interactions between the various components of the axon skeleton.

*There are no ribosomes, either free or attached to endoplasmic reticulum in axons and therefore, no protein synthesis.  

*''Protein synthesis takes place within the cell body and some dendrites.''

**All protein replacement required for the maintenance of the axon depends on proteins being imported from the cell body.

*A critical feature of the axon is its '''cytoskeleton''', which consists of two key elements:

#'''''The Neurofilaments'''''

#*Neurofilaments are intermediate filaments of about 10 nm diameter, and belong to the same class as other cytoskeletal proteins such as keratin, desmin, vimentin, or GFAP of astrocytes.  

#*Neurofilaments are formed from a triplet of polypeptide subunits of heavy (~ 200 kD), medium (~ 150 kD) and low (~ 60 kD) molecular weights.  

#*Typically, these subunits are heavily phosphorylated and are more numerous than microtubules, especially in large diameter axons, having a pivotal role in determining axon diameter.

#*They are formed in the cell body, transported down the axon by axoplasmic transport and degraded in the terminals by Ca²⁺ activated proteases.

#*In other words, there is a constant turnover of neurofllament within the healthy axon.

#'''''The microtubules'''''

#*Micro tubules within axons are similar to microtubules elsewhere, consisting of polymerised dimers of alpha and beta tubulin arranged as a hollow tube of about 28 nm.

#*They are relatively abundant in smaller diameter axons, and are also synthesised in the cell body.

#*An important component of the cytoskeleton are the '''''microtubule associated proteins''''' or MAP's and the tau protein.  

#**These proteins are important in microtubule assembly and stability.  

#**Different classes of MAP's occur in the dendrites and the axons, and to some extent account for the different ultrastructural features that distinguish these two types of neuronal process.  

#**They form cross links between adjacent microtubules but also connect to neurofilaments and actin microfilaments, implying complex interactions between the various components of the axon skeleton.

 

 

===The Schwann Cell===

===The Schwann Cell===

*Myelination in the PNS is achieved by the Schwann cell, a derivative of neural crest cells, which bud off from the neuroepithelium at a very early stage of neurogenesis.  

*Myelination in the PNS is achieved by the Schwann cell, a derivative of neural crest cells, which bud off from the neuroepithelium at a very early stage of neurogenesis.  

*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.

==WikiPath Neurolical Information==

==WikiPath Neurolical Information==