Otis Singleton
The discovery that the brain is made up of networks of individual cells that generate electrical signals raised the question of how these signals jump from one cell to another. This was one of the most hotly debated questions in neuroscience during the 20th century, with advocates on both sides defending their positions with data-based and theoretical models. It is now recognized that both electrical and chemical modes of communication occur between neurons. Electrical transmission can occur through gap junctions, which are low-resistance intercellular pathways, or through the electric fields generated by neuronal activity. Chemical transmission, on the other hand, involves the release of chemical messengers called neurotransmitters, which carry information from the presynaptic neuron to the postsynaptic neuron.
| Characteristics | Values |
|---|---|
| Basis of electrical signals in neurons | Networks of individual cells that generate electrical signals |
| Nature of signals | Electrical and chemical |
| Electrical signals | Action potentials |
| Chemical signals | Neurotransmitters |
| Nature of electrical communication between neurons | Two different strategies |
| First strategy | Low resistance intercellular pathways, called "gap junctions" |
| Second strategy | Occurs in the absence of cell-to-cell contacts and is a consequence of the extracellular electrical fields generated by the electrical activity of neurons |
| Action potential | A brief (~1 ms) electrical event typically generated in the axon that signals the neuron as "active" |
| Axon | A tube-like structure that propagates the integrated signal to specialized endings called axon terminals |
| Myelin sheath | Acts as an insulator to minimize dissipation of the electrical signal as it travels down the axon |
| Nodes of Ranvier | Gaps in myelin coverage along axons |
| Synapses | A synapse is the location where two neurons are almost in contact with each other, and where a signal is transmitted from one neuron to the next |
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What You'll Learn
- Electrical synapses, or 'gap junctions', allow the spread of electrical currents between neurons
- Extracellular electrical fields generated by neurons can transmit signals without cell-to-cell contact
- Neurotransmitters are chemical messengers that carry signals from one neuron to another
- Action potentials are electrical events that cause neurons to release neurotransmitters
- Myelin acts as an insulator to minimise dissipation of electrical signals as they travel down the axon
Electrical synapses, or 'gap junctions', allow the spread of electrical currents between neurons
Electrical synapses, or gap junctions, are a mechanical and electrically conductive junction between two neighbouring neurons. They are formed at a narrow gap between the pre- and postsynaptic neurons, where the cells approach within about 3.8 nm of each other. This is a much shorter distance than the 20-40 nm distance between cells at a chemical synapse.
Gap junctions are composed of two hemichannels called connexons, with each cell at the synapse contributing one connexon. Connexons are formed by six 7.5 nm long, four-pass membrane-spanning protein subunits called connexins. The connexins may be identical or slightly different from one another.
Gap junction channels contain precisely aligned, paired channels in the membrane of the pre- and postsynaptic neurons, forming a pore. This pore is much larger than the pores of voltage-gated ion channels, allowing a variety of substances to diffuse between the cytoplasm of the pre- and postsynaptic neurons. Ions and molecules with molecular weights as great as several hundred daltons can pass through the gap junction pore, which has a lumen diameter of about 1.2 to 2.0 nm. This permits the transfer of substances such as ATP and other important intracellular metabolites, including second messengers.
Electrical synapses are present throughout the central nervous system and are found in neural systems that require fast responses, such as defensive reflexes and escape mechanisms. They are also involved in creating synchronous firing of neurons, which facilitates processes such as the release of a burst of hormones into circulation.
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Extracellular electrical fields generated by neurons can transmit signals without cell-to-cell contact
The nervous system relies on electrical signaling to react to changes in the environment. While synaptic communication between nerve cells is perceived to be chemically mediated, electrical synaptic interactions also occur. Electrical communication between neurons occurs via two mechanisms. The first mechanism involves low-resistance intercellular pathways, called "gap junctions", which allow electrical currents to spread between the interiors of two cells.
The second mechanism occurs in the absence of cell-to-cell contact and is a result of the extracellular electrical fields generated by neurons during electrical signaling. These electrical fields can modify the excitability of neighboring cells. This form of electrical transmission depends on distinctive structural specializations, such as a dense high-resistance neuropil.
The discovery that the brain is composed of networks of individual cells that generate electrical signals raised questions about how these signals jump from one cell to another. This was a highly debated topic in neuroscience during the 20th century, with advocates on both sides presenting data-based and theoretical models. The debate centered on whether synaptic transmission was mediated electrically or chemically and the potential delay between presynaptic action potential and postsynaptic response.
Electrical transmission can occur as a result of the electric fields generated by neuronal activity. For example, the electric field of an action potential can invade the presynaptic terminal and generate an electric field that causes hyperpolarization at the postsynaptic cell. However, models of electrical transmission must address several challenges, including generating a strong enough postsynaptic signal and explaining how the same presynaptic signal can produce excitation in some sites and inhibition in others.
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Neurotransmitters are chemical messengers that carry signals from one neuron to another
Neurons are specialized cells that can transmit electrical or chemical signals. Electrical signals are action potentials, which are rapid, temporary changes in membrane potential (electrical charge) caused by the movement of sodium and potassium ions. These signals jump from one neuron to another through two mechanisms: "gap junctions" and extracellular electrical fields. "Gap junctions" are low-resistance intercellular pathways that allow the spread of electrical currents between the interiors of two cells. The second mechanism occurs without cell-to-cell contact and involves the extracellular electrical fields generated by neuronal activity.
There are three types of neurotransmitters based on their effects: excitatory, inhibitory, and modulatory. Excitatory neurotransmitters promote the generation of an electrical signal in the receiving neuron, while inhibitory neurotransmitters prevent it. Modulatory neurotransmitters, such as dopamine, can influence large populations of neurons simultaneously and regulate their activity.
Neurotransmitters play a crucial role in human health and development, and medications are often designed to influence these chemical messengers to treat various health conditions, especially brain-related diseases. For example, medications for Alzheimer's disease aim to slow the breakdown of the neurotransmitter acetylcholine, which is associated with memory and cognitive functions.
In summary, neurotransmitters are chemical messengers that carry signals from one neuron to another, playing a vital role in the body's functions and health. They work in conjunction with electrical signals, utilizing "gap junctions" and extracellular electrical fields to facilitate the transmission of information throughout the nervous system.
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Action potentials are electrical events that cause neurons to release neurotransmitters
Neurons are specialised cells that can receive and transmit electrical signals, and they are supported by glial cells. Neurons communicate via electrical signals called action potentials, which transmit information from one neuron to another. Action potentials are rapid, brief, and transitory electrical events that cause neurons to release neurotransmitters. These electrical events are caused by a stimulus with a certain value expressed in millivolts [mV]. Not all stimuli can cause an action potential; only those with sufficient electrical value can reduce the negativity of the nerve cell to the threshold of the action potential.
Action potentials occur when the sum total of all the excitatory and inhibitory inputs makes the neuron's membrane potential reach around -50 mV, a value called the action potential threshold. When an action potential reaches the presynaptic terminal, it causes neurotransmitters to be released from the neuron into the synaptic cleft, a 20-40nm gap between the presynaptic axon terminal and the postsynaptic dendrite. The neurotransmitters can be excitatory or inhibitory, promoting or inhibiting the generation of action potentials in the adjacent neuron.
After being released into the synaptic cleft, the neurotransmitters cross the gap and attach to the postsynaptic membrane, which contains receptors for the neurotransmitters. Once the neurotransmitter binds to the receptor, the ligand-gated ion channels of the postsynaptic membrane open or close, allowing positive or negative ions to flow into and out of the cell. This causes a redistribution of ions in the postsynaptic cell, resulting in a rapid, temporary change in membrane potential (electrical charge) and initiating an action potential in the adjacent neuron.
Thus, action potentials are electrical events that cause neurons to release neurotransmitters, which carry information to the adjacent neuron and initiate an action potential in that neuron. This process of electrical signalling enables the nervous system to react quickly to changes in the environment.
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Myelin acts as an insulator to minimise dissipation of electrical signals as they travel down the axon
Electrical signalling is a key feature of the nervous system, enabling it to react swiftly to changes in the environment. Neurons are specialised cells that can receive and transmit electrical signals, and they are supported by glial cells, which provide various support functions for the neurons. The axon is a tube-like structure that propagates the integrated signal to specialised endings called axon terminals. Neurons usually have one or two axons.
Myelin, a protective membrane produced by glial cells, wraps around certain nerve cells, providing insulation for electrical signals travelling along axons. This insulation minimises the dissipation of electrical signals as they travel down the axon, ensuring signal strength is maintained. Myelin is made up of lipids and proteins, and it is not a solid covering but rather a series of individual sections, each separated by a tiny gap called a node of Ranvier. These nodes are rich in positive sodium ions, which recharge the electrical signal as it jumps from one node to the next.
The presence of myelin greatly increases the speed of conduction, allowing signals to travel quickly and efficiently between nerve cells. Without this insulation, signal transmission slows down and degrades over time, affecting neuronal communication across the nervous system and downstream functions.
Myelin damage, as seen in conditions like multiple sclerosis, can slow or stop electrical signals. Research is focused on protecting, repairing, and regenerating myelin through various interventions, including drugs such as clemastine and metformin.
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Frequently asked questions
Electrical signals jump from neurons through two different strategies: "gap junctions" and extracellular electrical fields. "Gap junctions" are low resistance intercellular pathways that allow for the spread of electrical currents between the interior of two cells. Extracellular electrical fields are generated by the electrical activity of neurons and allow for the transmission of electrical signals in the absence of cell-to-cell contact.
Chemical transmission involves the release of chemical messengers called neurotransmitters. These neurotransmitters carry information from the pre-synaptic (sending) neuron to the post-synaptic (receiving) cell. Neurotransmitters are released as a result of an action potential, which is a rapid, temporary change in membrane potential (electrical charge) caused by the movement of sodium and potassium ions.
Synapses are the locations where two neurons come into close proximity, and where electrical signals are transmitted from one neuron to another. At the synapse, an action potential causes the release of neurotransmitters, which carry the signal to the next neuron. Synapses can be thought of as converting an electrical signal into a chemical signal and then back into an electrical signal in the postsynaptic neuron.
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