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Lesson Video: Synapses Biology

In this video, we will learn how to describe the structure of a synapse and explain how information is transmitted across synapses.

10:07

Video Transcript

In this video, we’ll learn about the structure of a synapse, explain how information is transmitted across the synapse, and discuss the importance of acetylcholine in synaptic function. We’ll also learn why transmission across the synapse is unidirectional. Then we’ll work through a practice problem and finish up by reviewing some key points.

Neurons are specialized cells which transmit nerve impulses around the body. To do this, they form connections with other neurons, creating an internal communication network. These neuron-to-neuron connections are unique because they meet at junctions called synapses, which include a small space or gap. A synapse is the junction between two neurons or a neuron and an effector cell. Synapses are used to pass chemical or electrical messages from one neuron to the next. Neurons can form synapses with other neurons, muscle fibers, or gland cells. Here we see a synapse formed between two neurons. Our capacity to think, learn, memorize, and move depends crucially on the strength and number of our synapses.

Let’s take a closer look at the basic components of a synapse. There are three basic components to a synapse. First is the neuron that precedes the junction, which is called the presynaptic neuron. The prefix pre- in presynaptic neuron helps to indicate that this neuron is before the synaptic junction. The presynaptic neuron ends in axon terminals, which are also called the synaptic knob. At the synaptic knob, the nerve impulse is converted from an electrical signal to a chemical signal.

After the presynaptic neuron is the synaptic cleft, which is the space between two neurons. This gap is what the chemical messengers will cross to transmit the signal to the next neuron. The neuron following the synaptic cleft is called the postsynaptic neuron. Chemical messengers bind to the receptors located on the dendrites of the postsynaptic neuron. This triggers a new nerve impulse in the form of electrical energy in the postsynaptic neuron.

Now that we know where and what the synapse is, we can discuss how the synapse functions. This is the nerve–muscle synapse. To understand the basic function of a synapse, the nerve–muscle synapse is the perfect example. This is because it was the first synapse to be discovered, and so it is one of the most well-studied examples of a synapse. So, by learning about the nerve–muscle synapse, we can learn about the basic features found in all synapses, including those found within the brain.

In transmission of a nerve impulse, arrival of the action potential at the synaptic knob of the presynaptic neuron causes it to depolarize. Depolarization causes calcium ion channels that are sensitive to changes in voltage to open, making the synaptic knob more permeable to calcium ions. With the calcium channels now open, calcium can now diffuse into the synaptic knob. The influx of calcium ions triggers the movement of synaptic vesicles within the synaptic knob.

Synaptic vesicles are small subcellular components which contain chemical messengers called neurotransmitters that are to be released into the synaptic cleft. They are the perfect container for neurotransmitters as they are made easily within the synaptic knob. Synaptic vesicles are formed from the cell membrane’s phospholipid bilayer. Typically, proteins help to pinch off a small portion of the cell surface membrane, forming the synaptic vesicle. Most nerve muscle cell synapses are cholinergic, which also describes the neurotransmitter that will be found within the synaptic vesicles.

Cholinergic synapses use acetylcholine as the primary neurotransmitter between the presynaptic neuron and the postsynaptic muscle cell. If you’ve noticed the word choline appears both within the word cholinergic and acetylcholine, well done! This is a great way to remember that acetylcholine is the primary neurotransmitter in cholinergic synapses. You will usually see the word acetylcholine abbreviated as Ach.

Following the influx of calcium into the synaptic knob, the synaptic vesicles filled with acetylcholine fuse with the presynaptic membrane closest to the synaptic cleft. When the synaptic vesicle fuses with the presynaptic membrane, the process of exocytosis releases the acetylcholine neurotransmitters into the synaptic cleft. Once in the synaptic cleft, acetylcholine can passively diffuse to the receptors located on the postsynaptic cell. The postsynaptic cell, be it neuron or muscle fiber, has sodium ion channels, which are embedded into the cell surface membrane. The sodium ion channels have a receptor site specific to the neurotransmitter, in this case acetylcholine.

The specificity of the receptor site means only acetylcholine can bind to the sodium ion channels, causing them to open. You may notice that these receptors are only present on the postsynaptic cell surface membrane. This is so the nerve impulse can travel in only one direction. This arrangement means that synapses are unidirectional, which helps to ensure that the nerve impulses do not travel in the wrong direction. So the acetylcholine released into the synaptic cleft travels in one direction towards its receptor binding site located on the postsynaptic cell’s membrane.

When acetylcholine binds to the receptor site on the sodium ion channels, it causes the sodium ion channels to open. This makes the postsynaptic cell more permeable to sodium ions and allows them to freely diffuse into the postsynaptic cell. The influx of sodium causes an action potential to be triggered in the dendrite of the postsynaptic neuron, or in this case the muscle fiber. When the postsynaptic cell is a muscle fiber, the transmission of an action potential causes the muscle cell to contract.

After the action potential has been transmitted to the postsynaptic cell, the sodium channels must be closed, which can only happen if acetylcholine is broken down. The enzyme acetylcholinesterase, which is also embedded in the postsynaptic membrane, hydrolyzes acetylcholine into choline and acetate. The breakdown of acetylcholine allows the sodium ion channels to close and come back to their initial state. Once the broken-down products of acetylcholine are reabsorbed by the presynaptic neuron, the synapse is ready to transmit another action potential.

Now that we’re familiar with the location, structure, and function of the synapse, we can try our hand at a practice problem.

What is the role of synaptic vesicles in the presynaptic neuron? (A) To store neurotransmitters. (B) To store enzymes. (C) To attach to receptor sites on the postsynaptic membrane. (D) To stimulate the influx of sodium ions.

To answer this question, let’s start by reviewing key components of the synapse, which comprises the presynaptic neuron. The junction of two neurons in communication is called the synapse. It is usually composed of three parts: the axon terminals of the presynaptic neuron; the synaptic cleft; and the postsynaptic cell, which can be the dendrites of a postsynaptic neuron, a gland, or a muscle fiber. Our capacity to think, learn, memorize, and move depends crucially on the strength and number of our synapses. This is because synapses play a major role in the transmission of nerve impulses.

To transmit the nerve impulse from one neuron to the next, the presynaptic neuron must release specialized chemical messengers called neurotransmitters, which are shown here in green. These neurotransmitters are able to cross the synaptic cleft and carry the message of the nerve impulse to the next neuron. To make sure that neurotransmitters are ready to transmit on demand, they’re stored in special packages called synaptic vesicles, which are concentrated at the ends of presynaptic neurons.

As the nerve impulse reaches the end of the presynaptic neuron, it stimulates an influx of calcium ions into the presynaptic neuron. The influx of calcium ions causes the synaptic vesicles to fuse with the membrane of the presynaptic neuron and release neurotransmitters into the synaptic cleft. The neurotransmitters will travel unidirectionally to the receptors on the postsynaptic membrane, which open sodium ion channels and generate the next nerve impulse.

With these key facts in mind, let’s take a look at our possible answer choices. The role of synaptic vesicles in the presynaptic neurons is to store neurotransmitters.

Let’s wrap up by taking a moment to review some of the key points we’ve learned in this video. Neurons form synapses with other neurons, muscle fibers, or gland cells. Synapses consist of the presynaptic neuron, the synaptic cleft, and the postsynaptic cell. Cholinergic synapses use acetylcholine as a neurotransmitter. Neurotransmitters binding to the receptors on the postsynaptic cell starts the generation of a new nerve impulse. Acetylcholinesterase breaks down acetylcholine so it can be recycled by the synaptic knob.

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