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Neurons use electrical signals to carry information. These signals are called action potentials. There are an estimated 86 billion neurons in the average human brain. Neurons don't act alone. They need to connect to other neurons and pass messages between each other. The electrical signal cannot pass the gap between neurons alone. This is why neurotransmitters are needed to pass signals from one neuron to the next. In this sense they differ from electrical synapses which pass electric signals directly to the next neuron. Chemical synapses can be further classified depending upon function and structure.
Chemical synapses are made up of three parts. The pre-synaptic terminal (the part of the neuron sending a signal); the post-synaptic cell (the neuron getting the signal) and the synaptic cleft (the space in between that is the part that is usually talked about when referring to synapses). The synaptic cleft is typically 20-50 nanometres wide.
Chemicals are released into the synaptic cleft from the pre-synaptic terminal. This process involves small "sacks" called vesicles. Vesicles carry the neurotransmitters inside the neuron. They then open up as they touch the membrane ("skin" of the neuron). This "spills" the contents into the synaptic cleft. The opening of vesicles is controlled by a peptide called a SNARE peptide. Neurotransmitters pass through the synaptic cleft and are absorbed by the post-synaptic membrane.
Some neurotransmitters are still in the synaptic cleft after the message has been sent. These neurotransmitters need to be cleaned-up. This job can be done in a few ways. One way is for the pre-synaptic neuron to reabsorb the neurotransmitter. This process is called reuptake. Another is to simply break down the neurotransmitters in the synaptic cleft. This is done by enzymes that are usually released by the brain's "support cells" which are called Glia. An example of an enzyme that breaks down a neurotransmitter is monoamine oxidase (MAO). MAO breaks down Monoamine neurotransmitters, such as Serotonin. This process is what some drugs act upon. They stop the MAO from working properly, this is called inhibition. The drugs that inhibit the action of MAO are called monoamine oxidase inhibitors (MAOIs). Once broken down, the molecules can be rebuilt into new neurotransmitters. Some may leave the body.
This type of synapse is the most common type of synapse in most mammalian animals. Chemical synapses are common and use chemicals to work. This means that they can targeted by drugs. Many of the effects of drugs are caused by their actions on chemical synapses. These drugs include Cocaine, Heroin and Aspirin.
Structure[change | change source]
The structure of a typical chemical synapse comes in three parts:
- The pre-synaptic terminal is usually located on the axon. This is the structure that releases neurotransmitters into the synaptic cleft. The pre-synaptic terminal is the first part of synaptic transmission and so has a "pre-" suffix.
- The synaptic membrane of the post-synaptic cell is usually located on the dendrite of the next neuron. This is the structure that absorbs neurotransmitters into the post-synaptic neuron (the neuron that is receiving the signal). The post-synaptic cell is the last part of the transmission process and so has a "post-" suffix.
- The synaptic cleft is the bit in the middle of the two membranes. This space is filled with an extracellular (extra- = outside of. cellular = live cell.) matrix of proteins that mainly acts to hold the two neurons together.
Two types of chemical synapse[change | change source]
In the brain there are two types of synapse. They were first described by their structure. Type I were seen to be symmetrical. Because of this, Type I synapses are also called symmetrical synapses. Type II were seen to be not symmetrical (asymmetrical). Because of this, Type II synapses are also called asymmetrical synapses. With better technology scientists could find more differences.
- Type I synapses are the more common chemical synapse in the human brain. These synapses were found to be excitatory in function. That means that they excite the next neuron. The next neuron produces an action potential when this synaptic transmission happens. These synapses are usually found on dendrites of the post-synaptic cell. The pre-synaptic terminal is found on the axon of the neuron that sends the transmission (pre-synaptic cell). Type I synapses are symmetrical in shape.
- Type II synapses are less common in the human brain. These synapses are asymmetrical in shape. They were found to be inhibitory. This is the opposite of excitatory. Instead of causing an action potential in the next neuron, these synapses stop an action potential. They are less common than type I synapses. They occur mainly along the axon or soma of the post-synaptic neuron. The pre-synaptic terminal is still usually on the axon of the pre-synaptic cell.
References[change | change source]
- Neuroscience for kids; Washington University
- Isotropic Fractionator: A simple, rapid method for the quantification of total cell count; The journal of Neuroscience.
- Synapse-A primer; "How many synapses in the human brain?"; The DANA Foundation
- Electrical synapses; NCBI books
- University of Texas, Austin. Synapse web page.
- Neuroscience Third Addition; Lippincott Williams & Wilkins. page 105. ISBN 978-0-7817-6003-4
- Neurons, synapses, action potentials, and neurotransmission; Illinois State University
- Neurotransmitter release; Williams College Website
- Neurotransmission: clamping complexin; Nature Journals
- Monoamine Oxidase and its effects on the brain; Novus Research Inc.
- MAO-B Inhibitors; Parkinson's UK
- Structure of Chemical Synapses; Texas University
- Synapses; Boston University
- Chemical synapses; Encyclopaedia of Life Science; page 5; John Wiley and Sons.
- The Synapse-A primer; DANA Foundation
- Synapses; Boston University