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Synapses are junctions between cells that allow for the transmission of chemical messages. The chemicals used to send messages are known as neurotransmitters, and one important type of neurotransmitter is acetylcholine. A cholinergic synapse is a gap where a neuron that produces acetylcholine sends messages to other neurons, or to skeletal muscle cells.
Messages may be conveyed at a cholinergic synapse through two types of receptors: ionotropic and metabotropic. Ionotropic receptors bind acetylcholine released into the synapse, and open ion channels in the receiving cell's membrane. The resulting change in polarization causes the recipient to fire or refrain from firing, depending on which type of ion is received.
Metabotropic receptors function using proteins. After acetylcholine binds a receptor at a cholinergic synapse, another protein is activated. This protein, called a second messenger, can have a variety of effects on the cell. It may cause an ion channel to open, or it can induce changes in the cell itself.
This type of synapse is essential to muscle movement. Any neuron sending messages to muscle does so through a cholinergic synapse. Acetylcholine that is released opens ion channels in the muscle cell, and causes it to contract. Some bacterial toxins can prevent the release of acetylcholine, and can have life-threatening effects by preventing muscle contractions that assist in breathing.
In order to prevent the continuous activation of cells, this chemical must be cleared from the synapse after use. The recipient cell in the synapse takes acetylcholine inside of its cell membrane. Enzymes like acetylcholinesterase then break down this neurotransmitter. Neurons continually produce acetylcholine to ensure that it can be released as needed.
The brain and central nervous system also contain neurons that release acetylcholine. Here, the cholinergic synapse can influence a variety of processes. Memory is one important process that these synapses are involved in. Learning seems to strengthen the connection of these synapses, by making the postsynaptic, or recipient, cell more sensitive to acetylcholine. Alzheimer's disease is thought to cause memory problems by damaging cells that produce this neurotransmitter, thereby weakening synaptic connections.
Synapses involving acetylcholine help to modulate signals sent by other neurons. Many neurons can receive chemical messages from several different synapses. As a modulator, acetylcholine may prevent cells from firing and conducting messages, instead of encouraging firing. Activity at the cholinergic synapse, in these cases, provides input that can regulate signals from other cells. Inhibitory signals that are strong enough at these synapses will override other excitatory transmissions.
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