There are multiple different types of synapses that each have specific functions. Check out these articles to learn more about synapses and neurotransmitters to get a better understanding of the structure and function of neurons.
There are different types of synapses that can be broadly categorized into ion channels, electrical synapses, and enzymatic synapses.
Chemical Synapses
Excitatory ion channel synapses are based on sodium channels. The sodium-potassium pump is powered by ATP and lets potassium in and sodium out. When these channels are opened, ions enter the cell which leads to depolarization within the cell. The difference in potential triggers the action potential. Normally, acetylcholine, glutamate, or aspartate are used in these synapses.
In inhibitory ion channel synapses, chlorine channels are targeted. When they open, negative ions flood into the neuron causing hyperpolarization which reduces the chance of having an action potential. This is typically triggered by glycine or GABA.
Voltage-gated calcium synapses require an influx of calcium into the cell to release neurotransmitters that cause depolarization and the subsequent action potential in the postsynaptic cell. They can be found in the cortex, thalamus, hippocampus, etc. They are also found in glial cells (see this article for more information on how glial cells function).
Neuromuscular junctions are found between neurons and muscle cells. The action potential from the neuron to the motor neuron is then transmitted to a muscle fiber which contracts. Acetylcholine is always used in these synapses to trigger excitatory reactions in the muscle cells. Myasthenia Gravis is an auto-immune disease that results in the degradation of neuromuscular junctions. Acetylcholine receptors are attacked by the body which causes symptoms such as weakness in muscles and vision issues.
These 4 forms of synapses are chemical synapses that rely on neurotransmitters moving across the gap between two neurons (the synaptic cleft) in order to trigger or suppress an action potential.
Electrical Synapses
Electrical synapses are specialized types of synapses that are only found in the heart and in the eye. They are extremely fast and are activated when proteins are shared between two neurons that are in contact with each other. They are connected by proteins known as connexins at sites known as gap junctions. These junctions are found where pores between two cells align. Unlike other synapses, the signal can be transferred bidirectionally. These synapses are often found in parts of the brain that need to be highly synchronized, such as the hippocampus or spinal cord.
Metabotropic Synapses
There are even synapses that do not rely on protein channels at all. Instead, they use membrane-bound enzymes that are activated by neurotransmitters. They are known as metabotropic receptors. When the neurotransmitter binds to the enzyme it triggers a reaction within the cell to produce a messenger chemical. This molecule in turn leads to a series of reactions within the cell such as opening and closing ion channels or producing chemicals. The effects can alter the number and sensitivity of receptors on the neuron and are especially important in processes such as learning and memory wherein information is stored within a neuron. Epinephrine, dopamine, endorphin are examples of the type of neurotransmitters often seen in such synapses.
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