a) opening of voltage-gated Na+ channels.
b) opening of voltage-gated Ca2+ channels.
c) reuptake of neurotransmitters into the neuron.
d) opening of membrane channels in the post-synaptic cell.
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Synapses are the junctions between two neurons, which allow neurons to pass signals to one another or to target cells. There are two different types of synapses: electrical synapses and chemical synapses. There are several significant differences between the two that are important to know for the MCAT.
In electrical synapses (see Figure 1), the cytoplasm of two cells is connected by gap junctions. These gap junctions allow molecules to move freely from one cell to another, thereby allowing for direct cell-to-cell communication.
The fact that molecules can flow freely from cell to cell in an electrical synapse has three major consequences:
(1) It means that the postsynaptic response is always the same as the presynaptic input. For example, if one cell participating in the synapse experiences depolarization, the positive charge from that cell will move to the adjacent cell and also result depolarization. Likewise, if one cell becomes hyperpolarized, then the adjacent cell will also become hyperpolarized. So, depolarization cannot turn into a hyperpolarization, and vice versa.
(2) It means electrical synapses are bidirectional, i.e. signals can be sent in either direction. The first neuron can send signals to the second neuron, and the second neuron can send signals back to the first.
(3) It makes electrical synapses much simpler than their chemical counterparts and less subject to regulation.
In chemical synapses (see Figure 2), the cytoplasm of the two cells is not continuous. Thus, in order to transmit signals, the presynaptic neuron releases neurotransmitters via exocytosis into the synaptic cleft, the narrow area between the presynaptic and postsynaptic neuron. These neurotransmitters diffuse across the synaptic cleft and bind to receptors on the membrane of the postsynaptic neuron. This can cause various effects in the postsynaptic neuron, including the opening or closing of membrane channels.
Chemical synapses are more complicated than electrical synapses. Thus, it is worth examining signal transmission across chemical synapses in more detail.
The first step in signal transmission across a chemical synapse is the firing of an action potential by the presynaptic neuron. The action potential travels down the axon and eventually reaches the axon terminal. The flow of positive charge into the axon terminal opens voltage-gated calcium channels, causing calcium to rush into the cell. The rapidly increasing calcium concentration then activates synaptic vesicles in the axon terminal, which contain chemical neurotransmitters. Once activated, the synaptic vesicles fuse to the membrane of the presynaptic cell, releasing neurotransmitters into the synaptic cleft. The neurotransmitters then diffuse across the synaptic cleft and bind to receptors on the postsynaptic neuron. This leads to the opening or closing of membrane channels in the postsynaptic neuron.
Afterwards, the synaptic neurotransmitters are either degraded by extracellular enzymes or else transported back into the presynaptic cell by reuptake transporters in the presynaptic membrane. The processes of neurotransmitter degradation and reuptake are shown in Figure 3.
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