Wiktor Wise
Neurons are the key players in the nervous system's activity, conveying information both electrically and chemically. Neurons transmit electrical impulses to other neurons through their axons, which are long, thin fibers. The mechanism underlying this transmission is based on voltage differences between the inside and outside of the cell, created by the uneven distribution of electrically charged particles or ions. These ions enter and exit the cell through specific protein channels in the cell membrane, which open and close in response to neurotransmitters or changes in the cell's membrane potential. This redistribution of electric charge alters the voltage difference across the membrane, resulting in a decrease called depolarization. If this depolarization exceeds a certain threshold, an impulse or action potential will travel along the neuron. Neurotransmitters, such as dopamine, play a crucial role in initiating and modulating electrical signals. The nervous system relies on this electrical signaling to perform rapid computations that underlie animal behavior.
| Characteristics | Values |
|---|---|
| How do neurons transmit electrical impulses? | Neurons transmit electrical impulses by using the Action Potential, which is generated through the flow of positively charged ions across the neuronal membrane. |
| What is the mechanism underlying signal transmission within neurons? | Signal transmission within neurons is based on voltage differences (potentials) that exist between the inside and outside of the cell. |
| What creates the membrane potential? | The membrane potential is created by the uneven distribution of electrically charged particles, or ions, such as sodium (Na+), potassium (K+), chloride (Cl-), and calcium (Ca2+). |
| How do ions enter and exit the cell? | Ions enter and exit the cell through specific protein channels in the cell's membrane, which open or close in response to neurotransmitters or changes in the cell's membrane potential. |
| What happens when there is a redistribution of electric charge? | A decrease in the voltage difference is called depolarization. If depolarization exceeds a threshold, an impulse (action potential) travels along the neuron. |
| What are second messengers? | Second messengers (e.g., G proteins) are molecules that help relay signals from the cell's surface to its interior. |
| How do neurotransmitters work? | Neurotransmitters like dopamine can excite or inhibit further electrical signals. They may also attach to receptors on the transmitting cell's presynaptic sites, affecting future communication. |
| How do neurons communicate? | Neurons communicate across microscopic gaps called synaptic clefts. A presynaptic neuron releases neurotransmitters that bind to receptors on the postsynaptic neuron. |
| What is the role of dendrites? | Dendrites are specialized to receive neuronal signals, although receptors may be located elsewhere on the cell. |
| How many neurotransmitters exist, and how many does each neuron produce? | Approximately 100 different neurotransmitters exist. Each neuron produces and releases only one or a few types but can carry receptors for several types. |
| What is the relationship between electrical and chemical communication? | Chemical communication is an earlier evolutionary strategy, but it is controlled by and capable of generating electrical signals. Electrical signaling is faster and more prevalent in the nervous system. |
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What You'll Learn
- Neurons transmit electrical impulses via the flow of positively charged ions
- Neurotransmitters are released from presynaptic terminals
- Neurotransmitters can bind to a receptor on the receiving neuron
- The neuron has three main components: dendrites, the cell body, and the axon
- The nervous system relies on electrical signaling to perform fast computations
Neurons transmit electrical impulses via the flow of positively charged ions
Neurons are essentially electrical devices. They transmit electrical impulses via the flow of positively charged ions, a phenomenon known as the Action Potential. This occurs when positively charged sodium and potassium ions flow in and out of the neuron, creating a voltage fluctuation in the form of a rapid upward (positive) spike followed by a rapid fall. This up-and-down cycle is the action potential.
The neuronal membrane contains specialised proteins called channels, which form pores in the membrane. These channels are permeable to positive or negative ions, allowing them to flow into and out of the cell. Under resting conditions, the potassium channel is more permeable to potassium ions than the sodium channel is to sodium ions. This results in a slow outward leak of potassium ions that is larger than the inward leak of sodium ions. The membrane, therefore, has a charge on the inside face that is negative relative to the outside, as more positively charged ions flow out of the neuron than in.
However, when the sodium channels open in response to a small depolarisation of the membrane potential, positively charged sodium ions flood into the neuron, causing depolarisation. This, in turn, opens the potassium channels, allowing potassium ions to leave the cell. This cycle of depolarisation and repolarisation is extremely rapid, taking only about 2 milliseconds (0.002 seconds). The frequency at which a neuron elicits action potentials is referred to as its firing rate or neural firing rate.
The axon is the long, thin structure where electrical impulses from the neuron travel away to be received by other neurons. Axons are much longer than the rest of the cell and are insulated by a fatty substance called myelin, which helps to send signals over long distances. The speed of transmission is usually directly related to the size of the axon, with big axons resulting in fast transmission rates.
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Neurotransmitters are released from presynaptic terminals
Neurons communicate with each other via electrical events called "action potentials" and chemical neurotransmitters. At the junction between two neurons, known as the synapse, an action potential causes neuron A to release a chemical neurotransmitter. This neurotransmitter is released from the presynaptic terminal of neuron A into the synaptic cleft, a 20-40 nanometre-wide gap between the presynaptic axon terminal and the postsynaptic dendrite.
The process begins with the synthesis of the neurotransmitter in the cell body, where it is then transmitted down the microtubules of the axon to the presynaptic terminal. Alternatively, the neurotransmitter can be synthesized directly in the presynaptic terminal from recycled neurotransmitters. The neurotransmitter is stored in presynaptic vesicles until its release, which occurs in a regulated manner.
As the action potential propagates down the presynaptic neuron, the membrane depolarizes. This depolarization allows voltage-dependent calcium channels to open, permitting the rapid influx of calcium into the presynaptic terminal. The influx of calcium triggers the activation of SNARE proteins, which are essential for binding the vesicles to the membrane and facilitating the release of their contents.
Once released into the synaptic cleft, the neurotransmitter binds to postsynaptic receptors, producing a response in the postsynaptic neuron. This response involves the movement of particular positive or negative ions through channels that span the membrane. The synapse thus converts an electrical signal (the action potential) into a chemical signal (neurotransmitter release) and then back into an electrical signal as charged ions flow into or out of the postsynaptic neuron.
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Neurotransmitters can bind to a receptor on the receiving neuron
Neurons transmit electrical impulses to other neurons through their axons. Axons are long cables that snake away from the main part of the neuron. At the end of these axons are synaptic terminals, which are separated from the next neuron by a 20-40 nanometre-wide gap called the synaptic cleft or synaptic junction.
In this gap, electrical signals are converted into chemical signals through the release of neurotransmitters. Neurotransmitters are molecules that carry messages from one neuron to another. They are released from the axon terminal of the transmitting neuron into the synaptic cleft. Each type of neurotransmitter then binds to a specific receptor on the receiving neuron, like a key fitting into its partner lock. This process is called synapsing, and it is how neurons communicate.
There are at least 100 known neurotransmitters, which can be grouped into types based on their chemical nature. One example of a neurotransmitter is acetylcholine, which is released by most neurons in the autonomic nervous system. It regulates heart rate, blood pressure, and gut motility and plays a role in muscle contractions, memory, motivation, sexual desire, sleep, and learning. Other examples include serotonin, norepinephrine, dopamine, glutamate, and GABA.
After delivering their message, neurotransmitters must be cleared from the synaptic cleft through one of three processes: diffusion, reuptake, or degradation. Problems with the neurotransmitters or their receptors can lead to diseases such as Alzheimer's, autism spectrum disorders, seizures, and mania.
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The neuron has three main components: dendrites, the cell body, and the axon
Neurons transmit electrical impulses to other cells via specialised projections called axons. Neurons can also receive these electrical impulses through root-like extensions known as dendrites.
The axon is a long, tail-like structure that joins the cell body at a specialised junction called the axon hillock. Axons are much longer than the rest of the cell, and they are usually insulated by a fatty substance called myelin, which helps them to conduct electrical signals. Myelin is produced by Schwann cells in the peripheral nervous system and by oligodendrocytes in the central nervous system.
The cell body is the main part of the neuron, from which the axon and dendrites extend. It receives signals from other cells and houses the nucleus and other organelles.
Together, these three components enable neurons to transmit and receive electrical impulses, facilitating communication between different parts of the brain and the body.
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The nervous system relies on electrical signaling to perform fast computations
Each neuron typically has one axon, although some have multiple. These axons vary in length, with the longest ones extending over a meter, such as those traveling from the brain down the spinal cord. To facilitate communication with multiple target cells, axons often develop side branches called axon collaterals, which further divide into smaller extensions called terminal branches. These terminal branches form connections with other cells through structures called synapses.
Synapses are the junctions or contact points between the axon terminals of one neuron and the dendrites (receiving branches) of another neuron or target cell. When an electrical signal, known as an action potential, reaches the axon terminal, it triggers the release of chemical neurotransmitters into the synaptic cleft, a small gap between the neurons. These neurotransmitters carry the signal across the synapse to the receiving cell, where it is converted back into an electrical signal, continuing the propagation of the signal.
The speed and efficiency of electrical signaling in the nervous system are enhanced by a fatty substance called myelin, which acts as an insulator. Myelin encases many axons, facilitating the long-distance transmission of signals. It enables the signal to jump between nodes of Ranvier, which are gaps in the myelin sheath, significantly increasing the rate of transmission. This insulation is crucial for proper nerve function, and its damage can lead to neurodegenerative diseases or conditions like Multiple Sclerosis.
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Frequently asked questions
Neurons transmit electrical impulses to other neurons through the flow of positively charged ions across the neuronal membrane. This is known as the Action Potential.
The Action Potential is the transient switch in membrane potential. It is caused by the difference in concentrations of ions on either side of the membrane, which creates a voltage difference. This voltage difference is known as the membrane potential.
The membrane potential is created by the uneven distribution of electrically charged particles, or ions, specifically sodium, potassium, chloride, and calcium ions. These ions enter and exit the cell through specific protein channels in the cell membrane, which open and close in response to neurotransmitters or changes in the cell's membrane potential.
Neurotransmitters are chemicals that are released by a neuron sending a signal (a presynaptic neuron). They bind to receptors on the surface of the receiving neuron (the postsynaptic neuron).
Neurons receive electrical impulses through their dendrites, which are thin fibers that extend from the cell in branched tendrils.
