Synapses in the Nervous System

Where Nerve Impulses Are Passed from Neuron to Neuron

Synapse
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In the central nervous system, a synapse is a small gap at the end of a neuron that allows a signal to pass from one neuron to the next. Synapses are found where nerve cells connect with other nerve cells. Synapses are key to the brain's function, especially when it comes to memory.

What Do Synapses Do?

When a nerve signal reaches the end of the neuron, it cannot simply continue to the next cell. Instead, it must trigger the release of neurotransmitters which can then carry the impulse across the synapse to the next neuron.

Once a nerve impulse has triggered the release of neurotransmitters, these chemical messengers cross the tiny synaptic gap and are taken up by receptors on the surface of the next cell. These receptors act much like a lock, while the neurotransmitters function much like keys. Neurotransmitters may excite the neuron they bind to or inhibit it.

Think of the nerve signal like the electrical current, and the neurons like wires. Synapses would be the outlets or junction boxes that connect the current to a lamp (or other electrical appliance of your choosing), allowing the lamp to light.

The Parts of the Synapse

Synapses are composed of three main parts:

  • The presynaptic ending that contains neurotransmitters
  • The synaptic cleft between the two nerve cells
  • The postsynaptic ending that contains receptor sites

An electrical impulse travels down the axon of a neuron and then triggers the release of tiny vesicles containing neurotransmitters.

These vesicles will then bind to the membrane of the presynaptic cell, releasing the neurotransmitters into the synapse. These chemical messengers cross the synaptic cleft and connect with receptor sites in the next nerve cell, triggering an electrical impulse known as an action potential.

Different Types of Synapses

There are two main types of synapses:

Chemical Synapse: The first is the chemical synapse in with the electrical activity in the presynaptic neuron triggers the release of chemical messengers, the neurotransmitters. The neurotransmitters diffuse across the synapse and bind to the specialized receptors of the postsynaptic cell. The neurotransmitter then either excites or inhibits the postsynaptic neuron. Excitation leads to the firing of an action potential while inhibition prevents the propagation of a signal.

Electrical Synapses: In this type, two neurons are connected by specialized channels known as gap junctions. Electrical synapses allow electrical signals to travel quickly from the presynaptic cell to the postsynaptic cell, rapidly speeding up the transfer of signals. The gap between electrical synapses is much smaller than that of a chemical synapse (about 3.5 nanometers compared to 20 nanometers). The special protein channels that connect the two cells make it possible for the positive current from the presynaptic neuron to flow directly into the postsynaptic cell.

Electrical synapses transfer signals much faster than chemical synapses. While the speed of transmission in chemical synapses can take up to several milliseconds, the transmission at electrical synapses is nearly instantaneous.

Where chemical synapses can be excitatory or inhibitory, electrical synapses are excitatory only.

While electrical synapses have the advantage of speed, the strength of a signal diminishes as it travels from one cell to the next. Because of this loss of signal strength, it requires a very large presynaptic neuron to influence much smaller postsynaptic neurons. Chemical synapses may be slower, but they can transmit a message without any loss in signal strength. Very small presynaptic neurons are also able to influence even very large postsynaptic cells.

History

The term synapse was first introduced in 1897 by physiologist Michael Foster in his "Textbook of Physiology" and is derived from the GreekĀ synapsis, meaning "conjunction."

Sources:

Freberg LA. Discovering Behavioral Neuroscience. Boston: Cengage Learning. 2016.

Freberg LA. Discovering Biological Psychology, Second edition. Belmont, CA: Wadsworth, Cengage Learning. 2010

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