Anato-Physiology: Chemical Synapses And Neurotransmitters
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✒️ - By Aan Gray
A chemical synapse is a junction at which the presynaptic neuron releases a neurotransmitter to stimulate the postsynaptic cell. The neuromuscular junction (NMJ) is an example of this. The NMJ and many other synapses employ acetylcholine as a neurotransmitter. Postsynaptic neurons of the sympathetic nervous system use norepinephrine.
Some neurotransmitters are excitatory and tend to cause the postsynaptic cell to generate a nerve signal. Some widely used excitatory neurotransmitters in the central nervous system (CNS) are glutamate in the brain and aspartate in the spinal cord. Other neu rotransmitters are inhibitory and suppress responses in the post synaptic cell. The most widely used inhibitory neurotransmitters in the CNS are gamma-aminobutyric acid (GABA) in the brain and glycine in the spinal cord. Some other well-known neurotransmit ters are dopamine, serotonin, histamine, and beta-endorphin. There are over 100 known neurotransmitters.
At a chemical synapse, a terminal branch of the presynaptic nerve fiber ends in a swelling, the synaptic knob. The knob is separated from the next cell by a 20- to 40-nm gap called the synaptic cleft. The knob contains membrane-bounded secretory vesicles called synaptic vesicles, which contain the neurotransmitter. Many of these vesicles are "docked" at release sites on the inside of the plasma membrane, ready to release their neurotransmitter on demand. Neurotransmitter release is achieved by exocytosis. A reserve pool of synaptic vesicles is located a little farther away from the membrane, clustered near the release sites and tethered to the cytoskeleton by protein microfilaments. These vesicles stand by and "step forward" to dock on the membrane and release their neurotransmitter after the previously docked vesicles have expended their contents. Synaptic vesicles are found in a few cells other than neurons, such as the sensory cells of taste, hearing, and equilibrium. They release neurotransmitter to stimulate a nearby nerve cell.
A postsynaptic neuron does not show such conspicuous specializations. At this end, a neuron has no synaptic vesicles and cannot release neurotransmitters. Its membrane does, however, contain proteins that function as neurotransmitter receptors, and it may be folded to increase its receptor-laden surface area and, therefore, its sensitivity to the neurotransmitter. A signal always travels in only one direction across a chemical synapse, from the presynaptic cell with synaptic vesicles to the postsynaptic cell with neurotransmitter receptors. This one-way transmission ensures the precise routing of nerve signals in the body.
Synaptic transmission begins when a nerve signal arrives at the end of the presynaptic neuron and triggers the exocytosis of synaptic vesicles. Neurotransmitter is released into the synaptic cleft, diffuses across to the postsynaptic cell, and binds to receptors on that cell's membrane. Depending on the neurotransmitter and the type of receptor, this may either stimulate or inhibit the postsynaptic cell. The postsynaptic cell "decides" whether or not to initiate a new nerve signal based on the composite effects of excitatory and inhibitory input through the many synapses on its dendrites and soma.
Tags: #ScienceWithGray #ScienceOnBuzz #Anatomy #Science
A chemical synapse is a junction at which the presynaptic neuron releases a neurotransmitter to stimulate the postsynaptic cell. The neuromuscular junction (NMJ) is an example of this. The NMJ and many other synapses employ acetylcholine as a neurotransmitter. Postsynaptic neurons of the sympathetic nervous system use norepinephrine.Some neurotransmitters are excitatory and tend to cause the postsynaptic cell to generate a nerve signal. Some widely used excitatory neurotransmitters in the central nervous system (CNS) are glutamate in the brain and aspartate in the spinal cord. Other neu rotransmitters are inhibitory and suppress responses in the post synaptic cell. The most widely used inhibitory neurotransmitters in the CNS are gamma-aminobutyric acid (GABA) in the brain and glycine in the spinal cord. Some other well-known neurotransmit ters are dopamine, serotonin, histamine, and beta-endorphin. There are over 100 known neurotransmitters.
At a chemical synapse, a terminal branch of the presynaptic nerve fiber ends in a swelling, the synaptic knob. The knob is separated from the next cell by a 20- to 40-nm gap called the synaptic cleft. The knob contains membrane-bounded secretory vesicles called synaptic vesicles, which contain the neurotransmitter. Many of these vesicles are "docked" at release sites on the inside of the plasma membrane, ready to release their neurotransmitter on demand. Neurotransmitter release is achieved by exocytosis. A reserve pool of synaptic vesicles is located a little farther away from the membrane, clustered near the release sites and tethered to the cytoskeleton by protein microfilaments. These vesicles stand by and "step forward" to dock on the membrane and release their neurotransmitter after the previously docked vesicles have expended their contents. Synaptic vesicles are found in a few cells other than neurons, such as the sensory cells of taste, hearing, and equilibrium. They release neurotransmitter to stimulate a nearby nerve cell.
A postsynaptic neuron does not show such conspicuous specializations. At this end, a neuron has no synaptic vesicles and cannot release neurotransmitters. Its membrane does, however, contain proteins that function as neurotransmitter receptors, and it may be folded to increase its receptor-laden surface area and, therefore, its sensitivity to the neurotransmitter. A signal always travels in only one direction across a chemical synapse, from the presynaptic cell with synaptic vesicles to the postsynaptic cell with neurotransmitter receptors. This one-way transmission ensures the precise routing of nerve signals in the body.Synaptic transmission begins when a nerve signal arrives at the end of the presynaptic neuron and triggers the exocytosis of synaptic vesicles. Neurotransmitter is released into the synaptic cleft, diffuses across to the postsynaptic cell, and binds to receptors on that cell's membrane. Depending on the neurotransmitter and the type of receptor, this may either stimulate or inhibit the postsynaptic cell. The postsynaptic cell "decides" whether or not to initiate a new nerve signal based on the composite effects of excitatory and inhibitory input through the many synapses on its dendrites and soma.
Tags: #ScienceWithGray #ScienceOnBuzz #Anatomy #Science
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