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Details regarding the Schwann cell have been covered within the [PNS Structure - Anatomy & Physiology#Schwann Cell|PNS Structure and Anatomy]].

Details regarding the Schwann cell have been covered within the [PNS Structure - Anatomy & Physiology#Schwann Cell|PNS Structure and Anatomy]].

====Oligodendrocytes====

===Oligodendrocytes===

[[Image:oligodendrocyte.jpg|thumb|right|100px|Oligodendrocyte]]

[[Image:oligodendrocyte.jpg|thumb|right|100px|Oligodendrocyte]]

The term "oligodendrocyte" literally means a "cell with many branches". Oligodendrocyte precursors are found throughout the CNS and these precursors are post-mitotic, meaning oligodendrocytes can be replaced. Oligodendrocytes can [[PNS Structure - Anatomy & Physiology#The Process of Myelination|myelinate]] up to 40 axons at a time. The myelin sheath formed by oligodendrocytes has numerous functions including; decreasing any ion leakage from the axon lowering the capacitance of the cell membrane and increasing impulse speed along the axon by allowing saltatory conduction of action potentials between the nodes of Ranvier.

The term "oligodendrocyte" literally means a "cell with many branches". Oligodendrocyte precursors are found throughout the CNS and these precursors are post-mitotic, meaning oligodendrocytes can be replaced. Oligodendrocytes can [[PNS Structure - Anatomy & Physiology#The Process of Myelination|myelinate]] up to 40 axons at a time. The myelin sheath formed by oligodendrocytes has numerous functions including; decreasing any ion leakage from the axon lowering the capacitance of the cell membrane and increasing impulse speed along the axon by allowing saltatory conduction of action potentials between the nodes of Ranvier.

===Nerve Impulse Propagation===

==Nerve Impulse Propagation==

Nerves are able to create, amplify and propogate electrical impulses that run along their axon. These nerve impulses are a form of action potential that is carried by ions. The nerve impulse is in effect an electrical difference between the inside and outside of the axon and is caused by ion movements across the membrane. Nerve impulses can occur from a number of sources including sensory cells, action potentials from other connected nerves or spontaneous depolarisation of the nerve cell membrane.

Nerves are able to create, amplify and propagate electrical impulses that run along their axon. These nerve impulses are a form of action potential that is carried by ions. The nerve impulse is in effect an electrical difference between the inside and outside of the axon and is caused by ion movements across the membrane. Nerve impulses can occur from a number of sources including sensory cells, action potentials from other connected nerves or spontaneous depolarisation of the nerve cell membrane.

====Unmyelinated Axons====

 

Conduction of an action potential along an unmyelinated axon, i.e. an axon not covered by some form of glial cell, is a much slower form of nerve impulse propagation than that of a myelinated axon.  

===Unmyelinated Axons===

Conduction of an action potential along an unmyelinated axon, i.e. an axon not covered by some form of glial cell, is a '''much slower''' form of nerve impulse propagation than that of a myelinated axon.  

 

At a given moment when the action-potential threshold is reached voltage-gated sodium channels within the membrane of the axon will open resulting in an influx of ions following their electro-chemical gradient. This Na⁺ influx causes the axon to accumulate a positive charge and results in cellular depolarisation. The extracellular area of the axon therefore looses its positive charge, becoming more negative resulting in a current of positive charge that flows  through the tissue towards the axon. The membrane of the axon is not a perfect insulator and at some regions on the axon, particularly within the area of the axon infront of the action potential, the voltage-dependant ion channels have not activated yet. This means that some of the positive charge is able to flow out of the axon membrane in these regions and this positive charge outflow is mainly via potassium. This outward leakage of potassium results in the current within the axon only being able to travel a short distance before the nerve impulse decays. However the effect of the current locally on the voltage-gated channels means that the nerve impulse is able to open voltage-gated channels within it's immediate vicinity and this is enough for the signal to propagate along the nerve.

At a given moment when the action-potential threshold is reached '''voltage-gated sodium channels''' within the membrane of the axon will open resulting in an influx of ions following their electro-chemical gradient. This Na⁺ influx causes the axon to accumulate a positive charge and results in '''cellular depolarisation'''. The '''extracellular area''' of the axon therefore looses its positive charge, becoming '''more negative''' resulting in a current of positive charge that flows  through the tissue towards the axon. The membrane of the axon is not a perfect insulator and at some regions on the axon, particularly within the area of the axon in front of the action potential, the voltage-dependant ion channels have not activated yet. This means that '''some of the positive charge is able to flow out of the axon membrane''' in these regions and this positive charge outflow is mainly via '''potassium'''. This outward leakage of potassium results in the current within the axon only being able to travel a short distance before the nerve impulse decays. However the effect of the current locally on the voltage-gated channels means that the nerve impulse is able to open voltage-gated channels within its immediate vicinity and this is enough for the signal to propagate along the nerve.

The axon membrane at rest has a slightly negative charge and the local current of positive charge during an action potential reduces this negative charge. This process is called '''depolarisation''' and when this has progressed sufficiently, it results in the membrane potential reaching threshold level allowing the voltage-gated sodium channels to open. These channels only stay open for approximately 0.5ms and when they close the membrane is depolarised for a further 0.1-0.5ms. These voltage-gated channels are unable to open again for a short period post depolarisation. This is called the '''refractory period'''.

The axon membrane at rest has a slightly negative charge and the local current of positive charge during an action potential reduces this negative charge. This process is called '''depolarisation''' and when this has progressed sufficiently, it results in the membrane potential reaching threshold level allowing the voltage-gated sodium channels to open. These channels only stay open for approximately 0.5ms and when they close the membrane is depolarised for a further 0.1-0.5ms. These voltage-gated channels are unable to open again for a short period post depolarisation. This is called the '''refractory period'''.

The '''internal electrical resistance''' within the axon is the determining factor regarding how fast the local positive current impulse can pass through. The less the internal resistance, the higher the conduction velocity of the nerve impulses due to the locally positive current being able to depolarise the membrane to threshold level over a greater distance. This internal resistance decreases as a function of axon diameter an therefore '''thicker axons are able to conduct nerve impulses more rapidly'''.

The internal electrical resistance within the axon is the determining factor regarding how fast the local positive current impulse can pass through. The less the internal resistance, the higher the conduction velocity of the nerve impulses due to the locally positive current being able to depolarise the membrane to threshold level over a greater distance. This internal resistance decreases as a function of axon diameter an therefore thicker axons are able to conduct nerve impulses more rapidly.

====Myelinated Axons====

===Myelinated Axons===

The addition of the myelin sheath to nerve axons greatly enhances the speed with which they are able to conduct nerve impulses. Unmyelinated nerves are able to conduct up to speeds to 25m/s whilst myelinated nerves are capable of up to 100m/s. For an unmyelinated nerve to conduct impulses at the same rate, it would have to have a diameter of approximately 100 times that of the myelinated nerve.  

The addition of the myelin sheath to nerve axons greatly '''enhances the speed''' with which they are able to conduct nerve impulses. Unmyelinated nerves are able to conduct up to speeds to 25m/s whilst myelinated nerves are capable of up to 100m/s. For an unmyelinated nerve to conduct impulses at the same rate, it would have to have a diameter of approximately 100 times that of the myelinated nerve.  

As with unmyelinated axons, an area of locally positive charge flows through the cytosol from the activated area within the axon. The major difference with myelinated nerves is that where the current effectively leaks away in unmyelinated nerves, here the current is only able to leak away through '''nodes of Ranvier''' (see above). Therefore even a weak current is able to depolarise the axon membrane to the threshold level. There is a small loss of current but as these nodes are relatively small the local current is able to travel much further in myelinated axons resulting in a much higher conduction velocity.  

As with unmyelinated axons, an area of locally positive charge flows through the cytosol from the activated area within the axon. The major difference with myelinated nerves is that where the current effectively leaks away in unmyelinated nerves, here the current is only able to leak away through '''nodes of Ranvier''' (see above). Therefore even a weak current is able to depolarise the axon membrane to the threshold level. There is a small loss of current but as these nodes are relatively small the local current is able to travel much further in myelinated axons resulting in a much higher conduction velocity.  

The influx of sodium ions via voltage-gated channels is only possible at these nodes and therefore the nerve impulse is able to effectively 'jump' from node to node. This type of impulse propagation is called '''saltatory conduction'''. Myelination of axons in mammals means that the nervous system can sustain a large number of high velocity axons within a relatively small space.

The influx of sodium ions via voltage-gated channels is only possible at these nodes and therefore the nerve impulse is able to effectively 'jump' from node to node. This type of impulse propogation is called '''saltatory conduction'''. Myelination of axons in mammals means that the nervous system can sustain a large number of high velocity axons within a relatively small space.

===Synapses===

==Synapses==

The synapses found at the end of axons are fundamental to the functioning of the nervous system as they facilitate communication between nerves and provide an interconnected network for many of the complex processes required by organisms. Synapses are required as the lipid bi-layer of the cell membrane has a relatively large electrical resistance making electrical impulse propagation directly between cells difficult.  

The synapses found at the end of axons are fundamental to the functioning of the nervous system as they facilitate communication between nerves and provide an interconnected network for many of the complex processes required by organisms. Synapses are required as the lipid bi-layer of the cell membrane has a relatively large electrical resistance making electrical impulse propagation directly between cells difficult.  

The most common form of nerve synapse is the chemical synapse which utilises [[Neurotransmitters - Anatomy & Physiology|neurotransmitters]]. When a nerve impulse reaches a synapse, neurotransmitters are released by the presynaptic terminal of the synapse and these transmitters diffuse to the membrane of the post-synaptic membrane where they bind to receptors. These receptors cause an inhibition or excitement in that nerve resulting in either blocking further electrical impulses or the further propagation of a signal.

The most common form of nerve synapse is the chemical synapse which utilises [[Neurotransmitters_-_Anatomy_%26_Physiology|neurotransmitters]]. When a nerve impulse reaches a synapse, neurotransmitters are released by the pre-synpatic terminal of the synapse and these transmitters diffuse to the membrane of the post-synaptic membrane where they bind to receptors. These receptors cause an inhibition or excitment in that nerve resulting in either blocking further electrical impulses or the further propagation of a signal.

 

====Neuromuscular Synapses====

===Neuromuscular Synapses===

These chemical synapses provide connection between nerves and skeletal muscle cells. These synapses are most commonly found residing within groups of muscle cells where each neuron is in contact with several muscle cells but each muscle cell is only every connected to one neuron. The nerve axon branches out prior to the muscle cells allowing multiple synapses with muscle cells from a single nerve axon. A synaptic cleft of approximately 30-50nm is found between the nerve synapse and the muscle cell.  

These chemical synapses provide connection '''between nerves and skeletal muscle cells'''. These synapses are most commonly found residing within groups of muscle cells where each neuron is in contact with several muscle cells but each muscle cell is only ever connected to one neuron. The nerve axon branches out prior to the muscle cells allowing multiple synapses with muscle cells from a single nerve axon. A '''synaptic cleft''' of approximately 30-50nm is found between the nerve synapse and the muscle cell.  

 

The nerve terminal membrane or pre-synaptic membrane contains numerous vesicles that contain the neurotransmitter, in this case most commonly [[Neurotransmitters - Anatomy & Physiology#Other SMTs|acetylecholine (ACh)]]. Once released from the vesicles the ACh diffuses across the synaptic cleft and binds to receptors in the muscle cell membrane. This binding causes ligand-gated ion channels to open which are permeable to both Na⁺ and K⁺. The movement of potassium is relatively small due to there being only a small electrochemical gradient between the extra and intracellular environment. However there is a large influx of sodium into the cell and consequently this causes depolarisation in the muscle cell thus propagating the impulse into mechanical movement. Within the neuromuscular junction, the release of vesicles is facilitated by an influx of calcium into the pre-synaptic nerve just prior to exocytosis. The calcium enters the pre-synaptic nerve via voltage-gated Ca²⁺ channels. There are several mechanisms that reduce the intracellular concentration of calcium once vesicles begin to be released to ensure that the neurotransmitter release is brief to prevent hyperpolarisation.  

The nerve terminal membrane or pre-synaptic membrane contains numerous vesicles that contain the neurotransmitter, in this case most commonly [[Neurotransmitters_-_Anatomy_%26_Physiology#Other_SMTs|acetylecholine (ACh)]]. Once released from the vesicles the ACh diffuses across the synaptic cleft and binds to receptors in the muscle cell membrane. This binding causes ligand-gated ion channels to open which are permeable to both Na⁺ and K⁺. The movement of potassium is relatively small due to there being only a small electrochemical gradient between the extra and intracellular environment. However there is a large influx of sodium into the cell and consequently this causes depolarisation in the muscle cell thus propagating the impulse into mechanical movement. Within the neuromuscular junction the release of vesicles is facilitated by an influx of calcium into the pre-synaptic nerve just prior to exocytosis. The calcium enters the pre-synaptic nerve via voltage-gated Ca²⁺ channels. There are several mechanisms that reduce the intracellular concentration of calcium once vesicles begin to be released to ensure that the neurotransmitter release is brief to prevent hyperpolarisation.  

 

The particular neurotransmitter ACh is heavily recycled within the synaptic cleft via endocytosis and over time the levels of endocytosis and exocytosis balance one-another resulting in a stable pre-synpatic membrane. Depolarisation within the muscle cell will last as long as the ACh is present in sufficient quantities within the synaptic cleft. In reality this only lasts a few milliseconds as the synaptic cleft also contains the enzyme '''acetylcholinesterase''' which hydrolyses the ACh into acetate and choline. Due to this mechanisms, the calcium restriction and the endocytosis, only one action potential is generated within the muscle fibre.

The particular neurotransmitter ACh is heavily recycled within the synaptic cleft via endocytosis and over time the levels of endocytosis and exocytosis balance one-another resulting in a stable pre-synpatic membrane. Depolarisation within the muscle cell will last as long as the ACh is present in sufficient quantities within the synaptic cleft. In reality this only lasts a few milliseconds as the synaptic cleft also contains the enzyme '''acetylcholinesterase''' which hydrolyses the ACh into acetate and choline. Due to this mechanisms, the calcium restriction and the endocytosis, only one action potential is generated within the muscle fibre.

====Inter-neuron Synapses====

===Inter-neuron Synapses===

The interaction between nerves within the synapse is fairly similar to that within the neuromuscular junction but there are several key differences. Firstly the receiving neuron will be receiving information from multiple other nerves rather than just one nerve as per a muscle cell. Secondly there are many more types of neurotransmitters utilised by inter-neuron synapses than just ACh. There are also a wider range of types of synapses between neurons that include both excitatory which will propagate a nerve impulse, but also inhibitory which will prevent another synapse on the same nerve propagating a nerve impulse.

The interaction between nerves within the synapse is fairly similar to that within the neuromuscular junction but there are several key differences. Firstly the receiving neuron will be receiving information from multiple other nerves rather than just one nerve as per a muscle cell. Secondly there are many more types of neurotransmitters utilised by inter-neuron synapses than just ACh. There are also a wider range of types of synapses between neurons that include both excitatory which will propagate a nerve impulse, but also inhibitory which will prevent another synapse on the same nerve propagating a nerve impulse.

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