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Mansa Musa was the ruler of the Mali Empire in West Africa. Details recorded here should be sourced; unknown information is left blank.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
| Electrical synapse | |
|---|---|
 Diagram of a gap junction | |
| Identifiers | |
| MeSH | D054351 |
| TH | H1.00.01.1.02024 |
| FMA | 67130 |
| Anatomical terminology | |
An electrical synapse is a synapse in which electrical current flows directly from one cell to another via a gap junction. It is one of the two major classes of synapses, the other being the chemical synapse. At a gap junction, the two neurons are separated by a very narrow gap of about 2–4 nm of each other,[1] a distance much smaller than the 20- to 40-nanometer distance that separates cells at a chemical synapse.[2] Electrical synapses co-exist with chemical synapses in the nervous system of many animals, from nematodes to arthropods to vertebrates, including mammals.[3] Most synapses in the brain are chemical, but electrical synapses are an important minority with a distinct purpose.[4]
Compared to chemical synapses, electrical synapses conduct nerve impulses faster and provide continuous-time bidirectional coupling via linked cytoplasm.[5][6][7] As such, the notion of signal directionality across these synapses is not always defined.[7] They are known to produce synchronization of network activity in the brain[8] and can create chaotic network level dynamics.[9][10] In situations where a signal direction can be defined, they lack gain (unlike chemical synapses)—the signal in the postsynaptic neuron is the same or smaller than that of the originating neuron.[citation needed] The fundamental bases for perceiving electrical synapses comes down to the connexons that are located in the gap junction between two neurons. Electrical synapses are often found in neural systems that require the fastest possible response, such as defensive reflexes. An important characteristic of electrical synapses is that they are mostly bidirectional, allowing impulse transmission in either direction.[11]
Each gap junction (sometimes called a nexus) contains numerous gap junction channels that cross the plasma membranes of both cells.[12] With a lumen diameter of about 1.2 to 2.0 nm,[2][13] the pore of a gap junction channel is wide enough to allow ions and even medium-size molecules like signaling molecules to flow from one cell to the next,[2][14] thereby connecting the two cells' cytoplasm. Thus when the membrane potential of one cell changes, ions may move through from one cell to the next, carrying positive charge with them and depolarizing the postsynaptic cell.
Gap junction channels are composed of two hemichannels called connexons in vertebrates, one contributed by each cell at the synapse.[2][13][15] Connexons are formed by six 7.5 nm long, four-pass membrane-spanning protein subunits called connexins, which may be identical or slightly different from one another.[13]
An autapse is an electrical (or chemical) synapse formed when the axon of one neuron synapses with its own dendrites.
They are found in many regions in animal and human body. The simplicity of electrical synapses results in synapses that are fast, but more importantly the bidirectional coupling can produce very complex behaviors at the network level.[16]
The relative speed of electrical synapses also allows for many neurons to fire synchronously.[12][13][18] Because of the speed of transmission, electrical synapses are found in escape mechanisms and other processes that require quick responses, such as the response to danger of the sea hare Aplysia, which quickly releases large quantities of ink to obscure enemies' vision.[19]
Normally, current carried by ions could travel in either direction through this type of synapse.[2] However, sometimes the junctions are rectifying synapses,[2] containing voltage-gated ion channels that open in response to depolarization of an axon's plasma membrane, and prevent current from traveling in one of the two directions.[18] Some channels may also close in response to increased calcium (Ca2+
) or hydrogen (H+
) ion concentration, so as not to spread damage from one cell to another.[18]
There is also evidence of synaptic plasticity where the electrical connection established can either be strengthened or weakened as a result of activity, or during changes in the intracellular concentration of magnesium.[20][21]
Electrical synapses are present throughout the central nervous system and have been studied specifically in the neocortex, hippocampus, thalamic reticular nucleus, locus coeruleus, inferior olivary nucleus, mesencephalic nucleus of the trigeminal nerve, olfactory bulb, retina, and spinal cord of vertebrates.[22][23] Other examples of functional gap junctions detected in vivo are in the striatum, cerebellum, and suprachiasmatic nucleus.[24][25]
The model of a reticular network of directly interconnected cells was one of the early hypotheses for the organization of the nervous system at the beginning of the 20th century. This reticular hypothesis was considered to conflict directly with the now predominant neuron doctrine, a model in which isolated, individual neurons signal to each other chemically across synaptic gaps. These two models came into sharp contrast at the award ceremony for the 1906 Nobel Prize in Physiology or Medicine, in which the award went jointly to Camillo Golgi, a reticularist and widely recognized cell biologist, and Santiago Ramón y Cajal, the champion of the neuron doctrine and the father of modern neuroscience. Golgi delivered his Nobel lecture first, in part detailing evidence for a reticular model of the nervous system. Ramón y Cajal then took the podium and refuted Golgi's conclusions in his lecture. Modern understanding of the coexistence of chemical and electrical synapses, however, suggests that both models are physiologically significant; it could be said that the Nobel committee acted with great foresight in awarding the Prize jointly.[26]
There was substantial debate on whether the transmission of information between neurons was chemical or electrical in the first decades of the twentieth century, but chemical synaptic transmission was seen as the only answer after Otto Loewi's demonstration of chemical communication between neurons and heart muscle. Thus, the discovery of electrical communication was surprising.
Electrical synapses were first demonstrated between escape-related giant neurons in crayfish in the late 1950s,[27] and were later found in vertebrates.[28]
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Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
An electrical synapse is a synapse in which electrical current flows directly from one cell to another via a gap junction. It is one of the two major classes of synapses, the other being the chemical synapse. At a gap junction, the two neurons are separated by a very narrow gap of about 2–4 nm of each other, a distance much smaller than the 20- to 40-nanometer distance that separates cells at a chemical synapse. Electrical synapses co-exist with chemical synapses in the nervous system of many animals, from nematodes to arthropods to vertebrates, including mammals. Most synapses in the brain are chemical, but electrical synapses are an important minority with a distinct purpose. Compared to chemical synapses, electrical synapses conduct nerve impulses faster and provide continuous-time bidirectional coupling via linked cytoplasm. As such, the notion of signal directionality across these synapses is not always defined. They are known to produce synchronization of network activity in the brain and can create chaotic network level dynamics. In situations where a signal direction can be defined, they lack gain (unlike chemical synapses)—the signal in the postsynaptic neuron is the same or smaller than that of the originating neuron. The fundamental bases for perceiving electrical synapses comes down to the connexons that are located in the gap junction between two neurons. Electrical synapses are often found in neural systems that require the fastest possible response, such as defensive reflexes. An important characteristic of electrical synapses is that they are mostly bidirectional, allowing impulse transmission in either direction.
An excitatory synapse is a synapse in which an action potential in a presynaptic neuron depolarizes the membrane of the postsynaptic cell, and thus increases the probability of triggering an action potential in that cell. The postsynaptic cell—a muscle cell, a glandular cell or another neuron—typically receives input signals through many excitatory and many inhibitory synapses. If the total of excitatory influences exceeds that of the inhibitory influences and the resulting depolarization exceeds the threshold level, the postsynaptic cell will be activated. If the postsynaptic cell is a neuron it will generate a new action potential at its axon hillock, thus transmitting the information to yet another cell. If it is a muscle cell, it will contract. If it is a gland cell, it will release its product (e.g., hormone). In an excitatory synapse, the electric response of the postsynaptic membrane to a single action potential in the presynaptic neuron is known as an excitatory postsynaptic potential (EPSP). It may occur via direct contact between cells (i.e., via gap junctions), as in an electrical synapse, but most commonly occurs via the vesicular release of neurotransmitters from the presynaptic axon terminal into the synaptic cleft, as in a chemical synapse. The excitatory neurotransmitters, the most common of which is glutamate, then migrate via diffusion to the dendritic spine of the postsynaptic neuron and bind a specific transmembrane receptor protein that triggers the depolarization of that cell. Depolarization, a deviation from a neuron's resting membrane potential towards its threshold potential, increases the likelihood of an action potential and normally occurs with the influx of positively charged sodium (Na+) ions into the postsynaptic cell through ion channels activated by neurotransmitter binding.
In the nervous system, a synapse is a structure that allows a neuron to exchange (receive or send) signals with another cell in its immediate vicinity. Synapses can be classified as either chemical or electrical, depending on the mechanism of signal transmission between neurons. In the case of electrical synapses, neurons are coupled bidirectionally with each other through gap junctions and have a connected cytoplasmic milieu. These types of synapses are known to produce synchronous network activity in the brain, but can also result in complicated, chaotic network level dynamics. Therefore, signal directionality cannot always be defined across electrical synapses. Synapses are essential for the transmission of neuronal impulses from one neuron to the next, playing a key role in enabling rapid and direct communication by creating circuits. In addition, a synapse serves as a junction where both the transmission and processing of information occur, making it a vital means of communication between neurons. At the synapse, the plasma membrane of the signal-passing neuron (the presynaptic neuron) comes into close apposition with the membrane of the target (postsynaptic) cell. Both the presynaptic and postsynaptic sites contain extensive arrays of molecular machinery that link the two membranes together and carry out the signaling process. In many synapses, the presynaptic part is located on the terminals of axons and the postsynaptic part is located on a dendrite or soma. Astrocytes also exchange information with neurons, responding to synaptic activity and, in turn, regulating neurotransmission. Synapses (at least chemical synapses) are stabilized in position by synaptic adhesion molecules (SAMs)[1] projecting from both the pre- and post-synaptic neuron and sticking together where they overlap; SAMs may also assist in the generation and functioning of synapses. Moreover, SAMs coordinate the formation of synapses, with various types working together to achieve the remarkable specificity of synapses. In essence, SAMs function in both excitatory and inhibitory synapses, likely serving as the mediator for signal transmission.
Gap junctions are membrane channels between adjacent cells that allow the direct exchange of cytoplasmic substances, such as small molecules, substrates, and metabolites. Gap junctions were first described as close appositions alongside tight junctions, but based on electron microscopy studies, they were later renamed gap junctions to distinguish them from tight junctions. They bridge a 2-4 nm gap between cell membranes. Gap junctions use protein complexes known as connexons, composed of connexin proteins to connect one cell to another. Gap junction proteins include the more than 26 types of connexin, as well as at least 12 non-connexin components that make up the gap junction complex or nexus, including the tight junction protein ZO-1—a protein that holds membrane content together and adds structural clarity to a cell, sodium channels, and aquaporin. More gap junction proteins have become known due to the development of next-generation sequencing. Connexins were found to be structurally homologous between vertebrates and invertebrates but different in sequence. As a result, the term innexin is used to differentiate invertebrate connexins. There are more than 20 known innexins, along with unnexins in parasites and vinnexins in viruses. An electrical synapse is a gap junction that can transmit action potentials between neurons. Such synapses create bidirectional continuous-time electrical coupling between neurons. Connexon pairs act as generalized regulated gates for ions and smaller molecules between cells. Hemichannel connexons form channels to the extracellular environment. A gap junction is different from an ephaptic coupling that involves electrical signals external to the cells.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Joe Louis.
