The Synapse
The role of the neuron is to communicate, and the role of the nervous system is to generate behavior, both by virtue of interneuronal connections. Neurons communicate with neurons at sites called synapses or synaptic junctions. The structural details of these communication links are of central importance in understanding how the brain works.
A synapse is a junction found at the tips or the terminal ends of the axons to another nerve cell via a button-like appendage called a synaptic bouton. It is the region of functional contact between two nerve cells or between a nerve cell and an effector cell. It consists of a minute gap across which impulses pass by diffusion of a neurotransmitter. The synaptic terminals transmit nerve impulses from one neuron to the next, thereby providing one of the primary means of intercommunication in the nervous systems.
That specialized site of the target neuron or other cell type (example: muscle) upon which the terminal impinges constitutes one of the most important anatomical and function entities in the entire nervous system; the synapse. Typically, transmission is unidirectional and synapses are functionally polarized. Thus one of the apposed cell membranes may be called presynaptic and the other postsynaptic. The presynaptic membranes are the output regions (to other cells) while the post-synaptic areas provide the input to the cell (from other cells)
Every synaptic junction is made up of a part of a neuron that conducts an impulse to the synapse and a part of another neuron that receives the stimulus at the synapse. The stimulus must cross a narrow (approximately 20mm) gap, the synaptic cleft, which separates pre and postsynaptic structures. Since the synapse does not involve a physical contact between the neurons, a chemical carrier (called a neurotransmitter) is generally required to bridge the gap. Such synapses, characterized by the release of neurotransmitters from presynaptic terminals, are the most common type in mammalian nervous systems.
Chemical synapses are recognized with the electron microscope by the presence of many small, bubble-like synaptic vesicles in the presynaptic terminals. Frequently shown to contain neurotransmitters, synaptic vesicles come in many sizes (20 - 120 mm in diameter) and shapes. Vesicles of a certain size or shape are sometimes associated with specific transmitters. energy for the release of the transmitter is generated in the mitochondria of the presynaptic terminal. Binding of the neurotransmitter to receptors of the postsynaptic membrane produces change in permeability of that membrane. Depending on the nature of the neurotransmitter and of the postsynaptic receptor, the effect may be either excitatory or inhibitory. (Mitochondria (cell power) are also found in the postsynaptic structure.)
Chemical synapses are generally named for the neuronal elements of which they are composed and are generally considered to be asymmetrical or symmetrical. Asymmetrical synapses are characterized by a difference in the density of the presynaptic and postsynaptic membranes, the postsynaptic density being the thicker. This density consists of a protein material that is applied to the postsynaptic membrane and may be associated with postsynaptic receptors. In symmetrical synapses, the opposed presynaptic and postsynaptic membranes are of the same thickness.
Synapses can be formed between axon and soma; axon and dendrite, axon and dendritic spine; axon and axon; and soma and soma. More complex combinations of several of these elements, often surrounded by a neuroglial sheath, are called glomeruli. The electro-tonic synapse, pre- and postsynaptic processes are contiguous, and the stimulus is thus able to pass directly from one cell to the next without chemical mediation.
Synapses are transmitters of neural information and they also produce and discharge chemical substances called neurotransmitters. Further, they may gather up the remnants of this material and reprocess and repackage it for future use. We consider the dynamics of the movement, discharge, uptake and reformation (resynthesis) of a neurotransmitter, such as acetylcholine, as a part of the process that conveys a stimulus from one neuron to another.
Some neurotransmitters, such as peptides are produced in the cell body, packaged within vesicles that migrate down the axon by axoplasmic flow to the presynaptic terminal, where they are called synaptic vesicles. In an alternative process, the transmitter substance, such as acetylcholine may be produced from precursor substances in the immediate vicinity of the presynaptic terminal. In either event, it is critical for the synaptic vesicles to be adjacent to the presynaptic membrane.
When an impulse reaches the presynaptic terminal, it is accompanied by the entry of calcium ions into the neuronal cytoplasm from the tissue fluids outside the neuron. calcium ions, which have passed through the cell membrane, bind to a carrier molexule called calmodulin. The Ca++ enhances the migration of some of the synaptic vesicles toward the presynaptic membrane. The membrane of each vesicle undergoes fusion with the presynaptic membrane, followed by rapid expulsion of free neurotransmitter into the synaptic cleft. recent data indicate that acetylcholine may also exist un-encapsulated in the cytoplasm of the presynaptic terminal.
The free neurotransmitter released into the synaptic cleft interacts directly with the receptor molecules in the postsynaptic membrane. By such an interaction, a number of ion-specific channels are believed to be opened. This permits an electric current, carried by charged ions, to flow through the postsynaptic membrane affecting the electrochemical status of the membrane in the immediate area of the channel. in this way, the electrical excitability of that tiny patch of membrane can be increases or decreased (through electrical depolarization or hyperpolarization of the membrane). Individual electrical disturbances in the post-syanptic membrane exert an effect on the membrane potential of the neuron, which can lead to the generation of a nerve impulse.
Once released into the synaptic cleft, the neurotransmitter is, in some cases, quickly inactivated (broken down) by specific enzymes. The resulting neurotransmitter fragments are either washed away or recycled through a process called endocytosis in which case the fragments are incorporated into a new vesicle formed from the presynaptic membrane. Such a structure is aclled a coated vesicle and is somewhat different in appearance from the synaptic vesicle. It is interesting to note that the venom of the black widow spider, Lactrodectus mactans, causes rapid fusion of synaptic vesicles to the symaptic membrans and prevents the re-formation of coated vesicles.
Neurotransmitter fragments taken up by endocytosis are ultimately re-synthesized as the complete neurotransmitter. In other cases, as with amine-containing transmitters such as norepinephrine, the neurotransmitter in the synaptic cleft is not fragmented but is reclaimed, in his entirety, through the presynaptic membrane into the presynaptic terminal.
Neurons communicate with each other at synapses/synaptic bouton and receive information at the dendrites/dendritic spines:
Chemical synapses are the most common form of communication between neurons. They consist of a presynaptic axon terminal and a postsynaptic element, which can be a dendrite, cell body or axon of the target neuron. The presynaptic terminal contains the chemical transmitter, which is stored in synaptic vesicles. The arrival of an action potential produces an influx of calcium ions into the presynaptic terminal and this triggers the vesicular release of the chemical transmitter, an example of the process known as exocytosis.
Neurochemical transmitters act on different types of postsynaptic receptors to produce two different types of responses: 1 the classic responses (referred to as classic neurotransmission) or the postsynaptic potentials described above and 2 neurochemicals may also produce. hangs in the excitability and responses of the postsynaptic neuron to other neurotransmitters, a process called neuromodulation.
A synapse is a junction found at the tips or the terminal ends of the axons to another nerve cell via a button-like appendage called a synaptic bouton. It is the region of functional contact between two nerve cells or between a nerve cell and an effector cell. It consists of a minute gap across which impulses pass by diffusion of a neurotransmitter. The synaptic terminals transmit nerve impulses from one neuron to the next, thereby providing one of the primary means of intercommunication in the nervous systems.
That specialized site of the target neuron or other cell type (example: muscle) upon which the terminal impinges constitutes one of the most important anatomical and function entities in the entire nervous system; the synapse. Typically, transmission is unidirectional and synapses are functionally polarized. Thus one of the apposed cell membranes may be called presynaptic and the other postsynaptic. The presynaptic membranes are the output regions (to other cells) while the post-synaptic areas provide the input to the cell (from other cells)
Every synaptic junction is made up of a part of a neuron that conducts an impulse to the synapse and a part of another neuron that receives the stimulus at the synapse. The stimulus must cross a narrow (approximately 20mm) gap, the synaptic cleft, which separates pre and postsynaptic structures. Since the synapse does not involve a physical contact between the neurons, a chemical carrier (called a neurotransmitter) is generally required to bridge the gap. Such synapses, characterized by the release of neurotransmitters from presynaptic terminals, are the most common type in mammalian nervous systems.
Chemical synapses are recognized with the electron microscope by the presence of many small, bubble-like synaptic vesicles in the presynaptic terminals. Frequently shown to contain neurotransmitters, synaptic vesicles come in many sizes (20 - 120 mm in diameter) and shapes. Vesicles of a certain size or shape are sometimes associated with specific transmitters. energy for the release of the transmitter is generated in the mitochondria of the presynaptic terminal. Binding of the neurotransmitter to receptors of the postsynaptic membrane produces change in permeability of that membrane. Depending on the nature of the neurotransmitter and of the postsynaptic receptor, the effect may be either excitatory or inhibitory. (Mitochondria (cell power) are also found in the postsynaptic structure.)
Chemical synapses are generally named for the neuronal elements of which they are composed and are generally considered to be asymmetrical or symmetrical. Asymmetrical synapses are characterized by a difference in the density of the presynaptic and postsynaptic membranes, the postsynaptic density being the thicker. This density consists of a protein material that is applied to the postsynaptic membrane and may be associated with postsynaptic receptors. In symmetrical synapses, the opposed presynaptic and postsynaptic membranes are of the same thickness.
Synapses can be formed between axon and soma; axon and dendrite, axon and dendritic spine; axon and axon; and soma and soma. More complex combinations of several of these elements, often surrounded by a neuroglial sheath, are called glomeruli. The electro-tonic synapse, pre- and postsynaptic processes are contiguous, and the stimulus is thus able to pass directly from one cell to the next without chemical mediation.
Synapses are transmitters of neural information and they also produce and discharge chemical substances called neurotransmitters. Further, they may gather up the remnants of this material and reprocess and repackage it for future use. We consider the dynamics of the movement, discharge, uptake and reformation (resynthesis) of a neurotransmitter, such as acetylcholine, as a part of the process that conveys a stimulus from one neuron to another.
Some neurotransmitters, such as peptides are produced in the cell body, packaged within vesicles that migrate down the axon by axoplasmic flow to the presynaptic terminal, where they are called synaptic vesicles. In an alternative process, the transmitter substance, such as acetylcholine may be produced from precursor substances in the immediate vicinity of the presynaptic terminal. In either event, it is critical for the synaptic vesicles to be adjacent to the presynaptic membrane.
When an impulse reaches the presynaptic terminal, it is accompanied by the entry of calcium ions into the neuronal cytoplasm from the tissue fluids outside the neuron. calcium ions, which have passed through the cell membrane, bind to a carrier molexule called calmodulin. The Ca++ enhances the migration of some of the synaptic vesicles toward the presynaptic membrane. The membrane of each vesicle undergoes fusion with the presynaptic membrane, followed by rapid expulsion of free neurotransmitter into the synaptic cleft. recent data indicate that acetylcholine may also exist un-encapsulated in the cytoplasm of the presynaptic terminal.
The free neurotransmitter released into the synaptic cleft interacts directly with the receptor molecules in the postsynaptic membrane. By such an interaction, a number of ion-specific channels are believed to be opened. This permits an electric current, carried by charged ions, to flow through the postsynaptic membrane affecting the electrochemical status of the membrane in the immediate area of the channel. in this way, the electrical excitability of that tiny patch of membrane can be increases or decreased (through electrical depolarization or hyperpolarization of the membrane). Individual electrical disturbances in the post-syanptic membrane exert an effect on the membrane potential of the neuron, which can lead to the generation of a nerve impulse.
Once released into the synaptic cleft, the neurotransmitter is, in some cases, quickly inactivated (broken down) by specific enzymes. The resulting neurotransmitter fragments are either washed away or recycled through a process called endocytosis in which case the fragments are incorporated into a new vesicle formed from the presynaptic membrane. Such a structure is aclled a coated vesicle and is somewhat different in appearance from the synaptic vesicle. It is interesting to note that the venom of the black widow spider, Lactrodectus mactans, causes rapid fusion of synaptic vesicles to the symaptic membrans and prevents the re-formation of coated vesicles.
Neurotransmitter fragments taken up by endocytosis are ultimately re-synthesized as the complete neurotransmitter. In other cases, as with amine-containing transmitters such as norepinephrine, the neurotransmitter in the synaptic cleft is not fragmented but is reclaimed, in his entirety, through the presynaptic membrane into the presynaptic terminal.
Neurons communicate with each other at synapses/synaptic bouton and receive information at the dendrites/dendritic spines:
Chemical synapses are the most common form of communication between neurons. They consist of a presynaptic axon terminal and a postsynaptic element, which can be a dendrite, cell body or axon of the target neuron. The presynaptic terminal contains the chemical transmitter, which is stored in synaptic vesicles. The arrival of an action potential produces an influx of calcium ions into the presynaptic terminal and this triggers the vesicular release of the chemical transmitter, an example of the process known as exocytosis.
Neurochemical transmitters act on different types of postsynaptic receptors to produce two different types of responses: 1 the classic responses (referred to as classic neurotransmission) or the postsynaptic potentials described above and 2 neurochemicals may also produce. hangs in the excitability and responses of the postsynaptic neuron to other neurotransmitters, a process called neuromodulation.