The mode of neurotransmission in electrical synapses is quite different from that in chemical synapses. While electrical synapses are fewer in number than chemical synapses, they are found in all nervous systems and play important and unique roles. Generally an excitatory neurotransmitter, memory Voluntary muscle movements, cognition, reward pathways Intestinal movement, mood regulation, sleep Once released, the acetylcholine stays in the cleft and can continually bind and unbind to postsynaptic receptors. This inhibition of the enzyme essentially increases neurotransmission at synapses that release acetylcholine. For example, some drugs that are given to Alzheimer’s patients work by inhibiting acetylcholinesterase, the enzyme that degrades acetylcholine. Several drugs act at this step of neurotransmission. This can be accomplished in three ways: the neurotransmitter can diffuse away from the synaptic cleft, it can be degraded by enzymes in the synaptic cleft, or it can be recycled (sometimes called reuptake) by the presynaptic neuron. Once neurotransmission has occurred, the neurotransmitter must be removed from the synaptic cleft so the postsynaptic membrane can “reset” and be ready to receive another signal. Cl – ions enter the cell and hyperpolarizes the membrane, making the neuron less likely to fire an action potential. For example, when the neurotransmitter GABA (gamma-aminobutyric acid) is released from a presynaptic neuron, it binds to and opens Cl – channels. Release of neurotransmitter at inhibitory synapses causes inhibitory postsynaptic potentials (IPSPs), a hyperpolarization of the presynaptic membrane. This depolarization is called an excitatory postsynaptic potential (EPSP) and makes the postsynaptic neuron more likely to fire an action potential. Na + enters the postsynaptic cell and causes the postsynaptic membrane to depolarize. For example, when acetylcholine is released at the synapse between a nerve and muscle (called the neuromuscular junction) by a presynaptic neuron, it causes postsynaptic Na + channels to open. There are several examples of well known neurotransmitters detailed in Table 1. Neurotransmitters can either have excitatory or inhibitory effects on the postsynaptic membrane. The binding of a specific neurotransmitter causes particular ion channels, in this case ligand-gated channels, on the postsynaptic membrane to open. The neurotransmitter diffuses across the synaptic cleft and binds to ligand-gated ion channels in the postsynaptic membrane, resulting in a localized depolarization or hyperpolarization of the postsynaptic neuron. The calcium entry causes synaptic vesicles to fuse with the membrane and release neurotransmitter molecules into the synaptic cleft. When the presynaptic membrane is depolarized, voltage-gated Ca2 + channels open and allow Ca2 + to enter the cell. The neurotransmitter diffuses across the synaptic cleft and binds to receptor proteins on the postsynaptic membrane.įigure 2. Communication at chemical synapses requires release of neurotransmitters. Synaptic vesicles are shown in Figure 1, which is an image from a scanning electron microscope.įusion of a vesicle with the presynaptic membrane causes neurotransmitter to be released into the synaptic cleft, the extracellular space between the presynaptic and postsynaptic membranes, as illustrated in Figure 2. Calcium ions entering the cell initiate a signaling cascade that causes small membrane-bound vesicles, called synaptic vesicles, containing neurotransmitter molecules to fuse with the presynaptic membrane. This depolarization causes voltage-gated Ca 2+ channels to open. Na + ions enter the cell, further depolarizing the presynaptic membrane. When an action potential reaches the axon terminal it depolarizes the membrane and opens voltage-gated Na + channels. (credit: modification of work by Tina Carvalho, NIH-NIGMS scale-bar data from Matt Russell) Figure 1. This pseudocolored image taken with a scanning electron microscope shows an axon terminal that was broken open to reveal synaptic vesicles (blue and orange) inside the neuron.
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