Synapses

The synapse is the junction where neurons trade information. It is not a physical component of a cell but rather a name for the gap between two cells: the presynaptic cell (giving the signal) and the postsynaptic cell (receiving the signal). There are two types of possible reactions at the synapse—chemical or electrical. During a chemical reaction, a chemical called a neurotransmitter is released from one cell into another. In an electrical reaction, the electrical charge of one cell is influenced by the charge an adjacent cell.

The electrical response of a neuron to multiple synaptic inputs

Synaptic responses summate in order to bring the postsynaptic neuron to the threshold of excitation, so it can fire an action potential (represented by the peak on the chart).

All synapses have a few common characteristics:

• Presynaptic cell: a specialized area within the axon of the giving cell that transmits information to the dendrite of the receiving cell.
• Synaptic cleft: the small space at the synapse that receives neurotransmitters.
• G-protein coupled receptors: receptors that sense molecules outside the cell and thereby activate signals within it.
• Ligand-gated ion channels: receptors that are opened or closed in response to the binding of a chemical messenger.
• Postsynaptic cell: a specialized area within the dendrite of the receiving cell that contains receptors designed to process neurotransmitters.

The Electrical Synapse

The stages of an electrical reaction at a synapse are as follows:

1. Resting potential. The membrane of a neuron is normally at rest with established concentrations of sodium ions (Na+) and potassium ions (K+) on either side. The membrane potential (or, voltage across the membrane) at this state is -70 mV, with the inside being negative relative to the outside.
2. Depolarization. A stimulus begins the depolarization of the membrane. Depolarization, also referred to as the "upswing," occurs when positively charged sodium ions (Na+) suddenly rush through open sodium gates into a nerve cell. If the membrane potential reaches -55 mV, it has reached the threshold of excitation. Additional sodium rushes in, and the membrane of the stimulated cell actually reverses its polarity so that the outside of the membrane is negative relative to the inside. The change in voltage stimulates the opening of additional sodium channels (called a voltage-gated ion channel), providing what is known as a positive feedback loop. Eventually, the cell potential reaches +40 mV, or the action potential.
3. Repolarization. The "downswing" of repolarization is caused by the closing of sodium ion channels and the opening of potassium ion channels, resulting in the release of positively charged potassium ions (K+) from the nerve cell. This expulsion acts to restore the localized negative membrane potential of the cell.
4. Refractory Phase. The refractory phase is a short period of time after the repolarization stage. Shortly after the sodium gates open, they close and go into an inactive conformation where the cell's membrane potential is actually even lower than its baseline -70 mV. The sodium gates cannot be opened again until the membrane has completely repolarized to its normal resting potential, -70 mV. The sodium-potassium pump returns sodium ions to the outside and potassium ions to the inside. During the refractory phase this particular area of the nerve cell membrane cannot be depolarized; the cell cannot be excited. 

The Chemical Synapse

The process of a chemical reaction at the synapse has some important differences from an electrical reaction. Chemical synapses are much more complex than electrical synapses, which makes them slower, but also allows them to generate different results. Like electrical reactions, chemical reactions involve electrical modifications at the postsynaptic membrane, but chemical reactions also require chemical messengers, such as neurotransmitters, to operate.

Neuron & chemical synapse

This image shows electric impulses traveling between neurons; the inset shows a chemical reaction occurring at the synapse.

A basic chemical reaction at the synapse undergoes a few additional steps:

1. The action potential (which occurs as described above) travels along the membrane of the presynaptic cell until it reaches the synapse. The electrical depolarization of the membrane at the synapse causes channels to open that are selectively permeable, meaning they specifically only allow the entry of positive sodium ions (Na+).
2. The ions flow through the presynaptic membrane, rapidly increasing their concentration in the interior.
3. The high concentration activates a set of ion-sensitive proteins attached to vesicles, which are small membrane compartments that contain a neurotransmitter chemical.
4. These proteins change shape, causing the membranes of some "docked" vesicles to fuse with the membrane of the presynaptic cell. This opens the vesicles, which releases their neurotransmitter contents into the synaptic cleft, the narrow space between the membranes of the pre- and postsynaptic cells.
5. The neurotransmitter diffuses within the cleft. Some of it escapes, but the rest of it binds to chemical receptor molecules located on the membrane of the postsynaptic cell.
6. The binding of neurotransmitter causes the receptor molecule to be activated in some way. Several types of activation are possible, depending on what kind of neurotransmitter was released. In any case, this is the key step by which the synaptic process affects the behavior of the postsynaptic cell.
7. Due to thermal shaking, neurotransmitter molecules eventually break loose from the receptors and drift away.
8. The neurotransmitter is either reabsorbed by the presynaptic cell and repackaged for future release, or else it is broken down metabolically.

Differences Between Electrical and Chemical Synapses

• Electrical synapses are faster than chemical synapses because the receptors do not need to recognize chemical messengers. The synaptic delay for a chemical synapse is typically about 2 milliseconds, while the synaptic delay for an electrical synapse may be about 0.2 milliseconds.
• Because electrical synapses do not involve neurotransmitters, electrical neurotransmission is less modifiable than chemical neurotransmission.
• The response is always the same sign as the source. For example, depolarization of the presynaptic membrane will always induce a depolarization in the postsynaptic membrane, and vice versa for hyperpolarization.
• The response in the postsynaptic neuron is generally smaller in amplitude than the source. The amount of attenuation of the signal is due to the membrane resistance of the presynaptic and postsynaptic neurons.
• Long-term changes can be seen in electrical synapses. For example, changes in electrical synapses in the retina are seen during light and dark adaptations of the retina.