Structural Biochemistry/Synapses

Overview

Synaptic transmission occurs from a presynaptic terminal at the end of an axon to the postsynaptic specialization at the end of the dendrite on the desired cell, the two of which are separated by a synaptic cleft. The synaptic cleft contains extracellular proteins involved in the diffusion, binding and degradation of the molecule secreted by the axon. Signals are fired across the synapse then binding to a receptor on the dendrite. The specific receptor that binds the neurotransmitter determines what cation in the extracellular cleft will be allowed to enter the cell at that site. The binding of the neurotransmitter to the dendritic receptor then causes the action potential to fire down the axon, then sending a signal to the next cell.

Types of Synapses

There are two types of synapses where transmission can occur. The first type is an electrical synapse with gap junctions that allow electrical current to flow from one neuron to another. These synapses are responsible for quick and unchanging behavior. The second type are chemical synapses where presynaptic neurons release chemical neurotransmitters that carry information across a synaptic cleft. These synapses allow room for modification in the case that a change in behavioral response is necessary. Behavioral response can be changed by changing the type of receptor and enhancing or changing the number of synaptic vesicles. The majority of synapses are usually chemical synapses.

Process of Chemical Synapse

Inside a synapse, there are many synaptic vesicles located within the terminals of the neuron. These synaptic vesicles are membrane bounded compartments filled with neurotransmitters. They are usually sitting in a synapse waiting to be delivered or taken away. An action potential's journey ends at the synaptic terminal however when the moving depolarization hits the synapse, the Ca2+ voltage gated channels are able to sense the action potential and open. As a result, the Ca2+ ions move into the presynaptic membrane. This action causes the synaptic vesicle to migrate and dock at the bottom of the membrane. The high amount of Ca2+ concentration causes exocytosis where the vesicles fuse with the presynaptic membrane and neurotransmitters are then released into the synaptic cleft. Meanwhile at the post synaptic membrane, there are ligand gated ion channels that are selectively permeable and respond where there is binding of ligand which in this case are the neurotransmitters. The neurotransmitters bind to the ligand gated ion channels and change it's shape causing them to open.

There are also other possibilities for what may occur after neurotransmitters are released from the synaptic vesicle, this includes diffusing out of the synaptic cleft, being taken up by surrounding cells such as astrocytes or getting degraded by enzymes. In the case where neurotransmitters bind to a receptor that is not part of an ion channel such as the example discussed earlier, it undergoes indirect synaptic transmission. This is another form of signaling that can take place at the synapse. The neurotransmitter binding to the receptor activates a signal transduction pathway and while this process is slower, the effects are longer lasting.

Postsynaptic Potential

A postsynaptic potential is a change in the membrane potential generated by the binding of neurotransmitters to ligand gated ion channels in the postsynaptic cell. They are considered graded potentials that rely on the strength of the stimulus and do not regenerate. There are two types of postsynaptic potentials known as the excitatory postsynaptic potential (EPSP) and inhibitory postsynaptic potential (IPSP). EPSP is a depolarization that brings the membrane potential to threshold. On the other hand, IPSP is a hyperpolarization that brings the membrane potential further away from threshold. A single EPSP is too small to trigger an action potential in a postsynaptic neuron. However, when two EPSPS are produced at the same time or within close proximity form the same synapse on the same postsynaptic neuron, it results in a temporal summation. The second EPSP arrives before the depolarization of the first EPSP has a chance to dissipate. When two EPSPS are produced almost simultaneously from two different synapses on the same postsynaptic neuron, they are able to add together and result in a spatial summation. The combination of EPSPS that result from temporal and spatial summation are able to trigger an action potential. Under circumstances where an EPSP and IPSP happen at the same time, they will cancel out and no action potential will arise.

Generation of An EPSP (excitatory postsynaptic potential) -First, there is an impulse arriving in the presynaptic terminal causes the release of neurotransmitter (detailed mechanism is described in Neurotrasmitters page). - Then the neurotrasmitters bind to the trasmitter gated ion channels in the post synaptic membrane. - Na+ enters the post synaptic cell through thte open channels, the membrane will become depolorized (detailed mechanism is explained in the Action Potential page) - The resulting change in membrane potential (Vm) is the EPSP

Generation of an IPSP (inhibitory postsynaptic potential) -First, there is an impulse arriving in the presynaptic terminal causes the release of neurotransmitter (detailed mechanism is described in Neurotrasmitters page). - Then the neurotrasmitters bind to the trasmitter gated ion channels in the post synaptic membrane. - Cl- enters the post synaptic cell through thte open channels, the membrane will become hyperpoloerized (detailed mechanism is explained in the Action Potential page) - The resulting change in membrane potential (Vm) is the IPSP

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Sources

Sources: Purves, Dave, et all. Neuroscience, Fourth Edition. Sunderland, MA: C. 2008, Sinauer Associates, Inc. Text. Purves, Dale, "Principles of Cognitive Neuroscience", Sinauer Associates, Inc., 2008