Why does synapse occur

The response is fast with a receptor linked to an ion channel and is slow with a receptor that leads to a second messenger cascade.

Although slower, second messenger cascades can produce more diverse cellular effects and have the advantage of amplification. Binding of a single molecule of neurotransmitter can produce many molecules of the second messenger. In contrast, if the receptor opens an ion channel, a single molecule of neurotransmitter or sometimes two molecules is needed to open a single ion channel in the postsynaptic cell.

A receptor that produces a second messenger in the postsynaptic cell. Second messengers can lead to a wide range of effects in the postsynaptic cell. An excitatory postsynaptic potential depolarizes the membrane bringing it closer to the threshold potential. An excitatory postsynaptic potential EPSP occurs if the membrane is depolarized by the ion movement.

If, on the other hand, the membrane becomes hyperpolarized when the ions move, an inhibitory postsynaptic potential IPSP is generated. Opening of sodium- or calcium channels leads to depolarization of the membrane. If there is sufficient depolarization, the threshold potential is reached and an action potential will be produced in the postsynaptic membrane. Since an EPSP depolarizes the membrane, it facilitates action potentials.

An inhibitory postsynaptic potential hyperpolarizes the membrane taking it farther from the threshold potential. Opening of potassium- or chloride channels leads to hyperpolarization of the membrane. Since the current is outward for potassium ions, and inward for chloride ions, opening of either of these two channels will cause the postsynaptic membrane to hyperpolarize. A hyperpolarized membrane has moved farther from the threshold potential and has less probability of producing an action potential.

Since an IPSP hyperpolarizes the membrane, it inhibits action potentials. Remember that a neuron synapses with many other neurons. So a postsynaptic neuron can receive signals from many presynaptic neurons simultaneously. Whether or not the postsynaptic cell has an action potential depends on the summation the additive effect of all the incoming signals.

The net effect of all the local potentials on the trigger zone determines whether or not there is an action potential in the postsynaptic cell. There are two different ways that local potentials can sum to excite the postsynaptic cell to have an action potential. Temporal summation occurs when successive EPSPs at a single synapse occur in rapid succession. The successive potentials occur before the previous ones die out producing an increasing membrane depolarization.

Temporal summation occurs when one synapse stimulates the postsynaptic cell very quickly and the EPSPs produced in the postsynaptic cell piggyback on each other causing an increasing level of depolarization.

The effect of temporal summation on membrane voltage at the trigger zone. The threshold voltage is attained and the postsynaptic cell fires an action potential.

Summation can also occur when multiple presynaptic neurons stimulate the postsynaptic neuron at the same time spatial summation. Each individual synapse lets in a limited number of ions and alters the membrane potential a little. The collective effect of all the synapses allows in enough ions to reach the threshold potential and an action potential is triggered.

Spatial summation occurs when the collective effect of multiple synapses depolarizes the postsynaptic neuron to threshold resulting in an action potential. Neurotransmitters are organic molecules that allow neurons to communicate with each other and with target cells. Neurotransmitters fall into four classes based on their chemical makeup. Acetylcholine ACh is a small molecule formed from acetate and choline. It is in a class by itself. Acetylcholine is the sole neurotransmitter used at the neuromuscular junction and is also the neurotransmitters used by the parasympathetic nervous system.

If the number of excitatory postsynaptic events is large enough, they will add to cause an action potential in the postsynaptic cell and a continuation of the "message. Many psychoactive drugs and neurotoxins can change the properties of neurotransmitter release, neurotransmitter reuptake and the availability of receptor binding sites.

Sherrington, in It was probably Charles S. Sherrington who coined the term synapse. The word "synapse" is derived from the Greek words "syn" and "haptein" that mean "together" and "to clasp," respectively. They are who you are. See some synapses "Up Close and Personal". Play the Interactive Word Search Game on the neuron and neurotransmitters. Play an Outside Game to reinforce what you have learned about the synapse.

Table of Contents. What Synapses Do. Parts of the Synapse. Chemical Synapses Gap between: 20 nanometers Speed: Several milliseconds No loss of signal strength Excitatory or inhibitory. Electrical Synapses Gap between: 3. What Dopamine Does for You. Was this page helpful? Thanks for your feedback! Sign Up. What are your concerns? Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

Freberg LA. Discovering Behavioral Neuroscience. Boston: Cengage Learning. Discovering Biological Psychology , Second edition. Related Articles. What Is Acetylcholine?

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