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Imagine having to share the Nobel prize for your life’s work with your intellectual arch-nemesis. This is exactly what happened with early neuroscientists Golgi and Cajal in 1906. They had radically different ideas about how the nervous system worked - Golgi believed that neurones are part of one connected network similar to the bloodstream, called the reticular theory. Cajal thought that the nervous system was made of separate units, known as the neuron theory.
What separates the two theories is the mechanism of how neurones connect. The development of microscopy has confirmed Cajal’s theory - neurones are separate cells that don’t connect directly; there are gaps between them. It’s via these gaps or synapses that information is exchanged. This exchange of information is known as synaptic transmission.
A synapse is the contact site where a neurone and another neurone or other cell meet. Being tiny structures on the cell, specialised electron microscopes visualise synapses. Through these, we know that the average neurone has 1000 synapses, but estimates go from one up to 200,000 synapses per cell. The cortex (the outermost layer of the brain) has around 125 trillion (125,000,000,000,000) synapses alone, which means there are many more synapses in every brain than stars exist in our entire galaxy.
There’s usually a small gap between neurones called the synaptic cleft. In the most common type of synapse, the chemical synapse neurones don’t touch except via specific protein structures that act like scaffolding. It’s here at the synapse that neurones and cells communicate via chemical molecules called neurotransmitters. The synapse converts electrical signals into chemical information through its unique mechanism, which is again converted into electrical signals. Another type of synapse, the electrical synapse, works a bit differently. Let’s have a closer look at the two types of synapses.
There are two types of synapses; electrical synapses and chemical synapses. There are more chemical synapses in the human body than electrical, but both have essential functions.
An electrical synapse features a channel made of proteins called a gap junction, connexons or pore. The gap junction directly connects a neurone and another cell, bridging the synaptic cleft. Although electrical synapses are more frequent in animals such as squid and zebrafish, they can also be found in humans’ central nervous system, retina and olfactory bulbs, where it’s most important to have optimal synchronisation and fast coordination of neurones.
Charged ions and messenger proteins can pass through gap junctions uninhibited. This direct connection makes the transmission of information in electrical synapses faster than in chemical synapses. In contrast to chemical synapses, the charge and the protein molecules can flow back and forth between the cells in some electrical synapses, making it bidirectional.Chemical synapses are the most common synapses in the human body. The chemical synapse uses chemical messenger molecules to generate an electrical signal, and it includes:
The axon terminal of the presynaptic neurone, meaning the neurone that is sending information.
The synaptic cleft is a tiny 20-30 nanometre wide gap between the two neurones filled with the interstitium fluid.
The postsynaptic membrane of a second receiving cell would usually be another neurone, but it might also be a gland, organ or muscle. The postsynaptic membrane has protein channels called receptors, and they are more abundant here than in other parts of the cell. We’ll have a closer look at receptors later.
Now we know what a chemical synapse is, but let’s look at it in action- during synaptic transmission.
Study tip: The chemical synapse is what most people refer to when speaking about synapses.
Synaptic transmission or neurotransmission is when a neurone communicates with another neurone or cell by releasing neurotransmitters into the synaptic cleft. When action potential (electrical charge firing along the axon) arrives in the axon terminal, neurotransmitters are released into the synaptic cleft. These then bind to receptors, allowing only negatively charged or only positively charged ions to enter into the next cell and depolarise it.
Each synapse usually specialises in one type of neurotransmitter. These are specific messenger molecules produced in the cell body and transported along the cytoskeleton (a network of protein strings and tubes that act like scaffolding for the cell) to the end of the axon. Once they arrive in the axon terminal, they are wrapped in membrane sacs called vesicles and gather at the presynaptic end of the axon, ready to be released.
A receptor is a protein molecule in the cell membrane that reacts to a specific neurotransmitter, hormone or other molecules.
You can think of it as a gate or door that opens when unlocked by one specific molecule. When a neurotransmitter binds to a receptor, the gate opens to let other specific molecules in, often either ions with a positive charge or ions with a negative charge.
Synaptic transmission can either be excitatory or inhibitory, depending on the neurotransmitter released. The impulse received on the postsynaptic membrane is either called excitatory postsynaptic potential (EPSP) or inhibitory postsynaptic potential (IPSP), depending on whether the neurotransmitter is excitatory or inhibitory.
Excitatory: This means that the gates opened by the neurotransmitters let positive ions such as Na+ or K+ flow into the cell, resulting in depolarisation of the cell membrane (the inside of the cell becomes positively charged). This makes it more likely for an action potential to be produced. Examples of excitatory neurotransmitters include glutamate and dopamine.
Inhibitory: This means that the gates opened by these neurotransmitters let negative ions such as Cl- into the cell, resulting in hyperpolarisation of the cell membrane (the inside of the cell becomes more negatively charged than usual). This makes it less likely for an action potential to be produced. Examples of inhibitory neurotransmitters include GABA and glycine.
A third possibility is that the neurotransmitter released doesn’t open an ion channel but rather that it sets off a protein chain reaction that has more long-term consequences. These are called g-protein cascades or second-messenger cascades.
Action potentials or the electrical impulses that travel along the axon can only be initiated if a certain voltage threshold is reached (usually -55mV). Action potentials follow the all-or-nothing principle and only travel in one direction. One incoming impulse is usually not enough to initiate the action potential, which starts transmission to the next cell via the axon. The addition of a few incoming signals is needed. This process is called summation. Two types of summation can lead to depolarisation/action potential:
Spatial summation: When enough excitatory impulses arrive on one cell from different locations.
Temporal summation: When enough excitatory impulses arrive on one cell from one other cell in quick succession.
In synaptic transmission, electrical charge is converted to chemicals that bridge a gap between the two cells. These chemicals react with the cell membrane to create an electrical charge in the receiving cell.
Let’s go step by step to see how the process of synaptic transmission works:
Action potential (electrical current) arrives in the axon terminal from the cell body. The electrical charge opens Ca++ channels in the axon terminal. These Ca++ channels are voltage-gated, meaning they open up in response to electrical current. Ca++ is more abundant outside the cell and is attracted to the negative charge in the cell, so as soon as the gates open, Ca++ rushes into the cell. Ca++ enters the axon terminal, enabling exocytosis. Exocytosis means that the vesicles’ membrane containing the neurotransmitters fuse with the presynaptic membrane. Vesicles open up, and neurotransmitters diffuse into the synaptic cleft. Neurotransmitters diffuse across the synaptic cleft and bind with receptors on the postsynaptic membrane. The ion channels open up, and either negative ions or positive ions flow into the postsynaptic cell. The remaining neurotransmitters are recycled by the presynaptic cell.
There are a variety of types of synaptic connections:
Axodendritic: The axon of one neurone connects to the dendrites of another - by far the most common synapse in the human body.
Axo-somatic: The axon of one neurone connects to the cell membrane of the body or soma of another cell.
Axo-axonal: The axon of one neurone connects to the axon of another. Usually, these are inhibitory synapses.
Neuromuscular junction: The axon of one neurone connects to a muscle. Usually, these are large synapses and lead to muscle contraction.
Various others: Neurones connect to all parts of the body, including dendrites to dendrites, axons into the interstitial spaces or to a blood vessel, etc.
Synaptic transmission is when a neurone communicates with another neurone or cell by releasing neurotransmitters into the synaptic cleft.
Synaptic transmission is the main mechanism by which the nervous system communicates.
During synaptic transmission electrical charge is converted to chemicals that bridge a fluid-filled gap between the two cells, and these chemicals react with the cell membrane to create an electrical charge in the receiving cell.
The synaptic transmission allows electrical signals to be passed on throughout the nervous system.
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