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Neurons - Anatomy & Physiology (view source)
Revision as of 10:18, 25 August 2011
, 10:18, 25 August 2011→Synapses
===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.
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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.
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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 post-synaptic membrane. This binding causes ligand-gated ion channels to open which are permeable to both Na<sup>+</sup> and K<sup>+</sup>. 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 nerve thus propagating the impulse. 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 Ca<sup>2+</sup> 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.
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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 post-synaptic nerve will last as long as the ACh is present in sufficient quantities within the synaptic cleft.
====Inter-neuron Synapses====
====Inter-neuron Synapses====